Diffusional conductances to CO2 as a target for increasing photosynthesis and photosynthetic water-use efficiency

Partial root-zone drying irrigation improves intrinsic water-use efficiency and maintains high photosynthesis by uncoupling stomatal and mesophyll conductance in cotton leaves

Partial root-zone drying irrigation (PRD) can improve water-use efficiency (WUE) without reductions in photosynthesis; however, the mechanism by which this is attained is unclear. To amend that, PRD conditions were simulated by polyethylene glycol 6000 in a root-splitting system and the effects of PRD on cotton growth were studied. Results showed that PRD decreased stomatal conductance (gs) but increased mesophyll conductance (gm). Due to the contrasting effects on gs and gm, net photosynthetic rate (AN) remained unaffected, while the enhanced gm/gs ratio facilitated a larger intrinsic WUE. Further analyses indicated that PRD-induced reduction of gs was related to decreased stomatal size and stomatal pore area in adaxial and abaxial surface which was ascribed to lower pore length and width. PRD-induced variation of gm was ascribed to the reduced liquid-phase resistance, due to increases in chloroplast area facing to intercellular airspaces and the ratio of chloroplast surface area to total mesophyll cell area exposed to intercellular airspaces, as well as to decreases in the distance between cell wall and chloroplast, and between adjacent chloroplasts. The above results demonstrate that PRD, through alterations to stomatal and mesophyll structures, decoupled gs and gm responses, which ultimately increased intrinsic WUE and maintained AN.

Effect of light-induced changes in leaf anatomy on intercellular and cellular components of mesophyll resistance for CO2 in Fagus sylvatica

Mesophyll resistance for CO2 diffusion (rm) is one of the main limitations for photosynthesis and plant growth. Breeding new varieties with lower rm requires knowledge of its distinct components. We tested new method for estimating the relative drawdowns of CO2 concentration (c) across hypostomatous leaves of Fagus sylvatica. This technique yields values of the ratio of the internal CO2 concentrations at the adaxial and abaxial leaf side, cd/cb, the drawdown in the intercellular air space (IAS), and intracellular drawdown between IAS and chloroplast stroma, cc/cbd. The method is based on carbon isotope composition of leaf dry matter and epicuticular wax isolated from upper and lower leaf sides. We investigated leaves from tree-canopy profile to analyse the effects of light and leaf anatomy on the drawdowns and partitioning of rm into its inter- (rIAS) and intracellular (rliq) components. Validity of the new method was tested by independent measurements of rm using conventional isotopic and gas exchange techniques. 73% of investigated leaves had adaxial epicuticular wax enriched in 13C compared to abaxial wax (by 0.50‰ on average), yielding 0.98 and 0.70 for average of cd/cb and cc/cbd, respectively. The rIAS to rliq proportion were 5.5:94.5% in sun-exposed and 14.8:85.2% in shaded leaves. cc dropped to less than half of the atmospheric value in the sunlit and to about two-thirds of it in shaded leaves. This method shows that rIAS is minor but not negligible part of rm and reflects leaf anatomy traits, i.e. leaf mass per area and thickness.

The response of mesophyll conductance to short-term CO2 variation is related to stomatal conductance

The response of mesophyll conductance (gm) to CO2 plays a key role in photosynthesis and ecosystem carbon cycles under climate change. Despite numerous studies, there is still debate about how gm responds to short-term CO2 variations. Here we used multiple methods and looked at the relationship between stomatal conductance to CO2 (gsc) and gm to address this aspect. We measured chlorophyll fluorescence parameters and online carbon isotope discrimination (Δ) at different CO2 mole fractions in sunflower (Helianthus annuus L.), cowpea (Vigna unguiculata L.), and wheat (Triticum aestivum L.) leaves. The variable J and Δ based methods showed that gm decreased with an increase in CO2 mole fraction, and so did stomatal conductance. There were linear relationships between gm and gsc across CO2 mole fractions. gm obtained from A-Ci curve fitting method was higher than that from the variable J method and was not representative of gm under the growth CO2 concentration. gm could be estimated by empirical models analogous to the Ball-Berry model and the USO model for stomatal conductance. Our results suggest that gm and gsc respond in a coordinated manner to short-term variations in CO2, providing new insight into the role of gm in photosynthesis modelling.

Restricted internal diffusion weakens transpiration-photosynthesis coupling during heatwaves: Evidence from leaf carbonyl sulphide exchange

Increasingly frequent and intense heatwaves threaten ecosystem health in a warming climate. However, plant responses to heatwaves are poorly understood. A key uncertainty concerns the intensification of transpiration when heatwaves suppress photosynthesis, known as transpiration-photosynthesis decoupling. Field observations of such decoupling are scarce, and the underlying physiological mechanisms remain elusive. Here, we use carbonyl sulphide (COS) as a leaf gas exchange tracer to examine potential mechanisms leading to transpiration-photosynthesis decoupling on a coast live oak in a southern California woodland in spring 2013. We found that heatwaves suppressed both photosynthesis and leaf COS uptake but increased transpiration or sustained it at non-heatwave levels throughout the day. Despite statistically significant decoupling between transpiration and photosynthesis, stomatal sensitivity to environmental factors did not change during heatwaves. Instead, midday photosynthesis during heatwaves was restricted by internal diffusion, as indicated by the lower internal conductance to COS. Thus, increased evaporative demand and nonstomatal limitation to photosynthesis act jointly to decouple transpiration from photosynthesis without altering stomatal sensitivity. Decoupling offered limited potential cooling benefits, questioning its effectiveness for leaf thermoregulation in xeric ecosystems. We suggest that adding COS to leaf and ecosystem flux measurements helps elucidate diverse physiological mechanisms underlying transpiration-photosynthesis decoupling.

Plant responses to co-occurring heat and water deficit stress: A comparative study of tolerance mechanisms in old and modern wheat genotypes

Global climate change increases the likelihood of co-occurrence of hot and dry spells with increased intensity, frequency, and duration. Studying the impact of the two stresses provide a better understanding of tolerance mechanisms in wheat, and our study was focused on revealing plant stress responses to different severities of combined stress at two phenophases in old and modern wheat genotypes. During the stem elongation and anthesis stages, plants were exposed to four treatments: control, deficit irrigation, combined heat, and deficit irrigation at 31 °C (HD31) and 37 °C (HD37). The modern genotypes were less affected by deficit irrigation at stem elongation as they maintained higher photosynthesis, stomatal conductance, and leaf cooling than old genotypes. When the HD37 stress was imposed during anthesis, the modern genotypes exhibited superior performance compared to the old, which was due to their higher photosynthetic rates resulting from improved biochemical regulation and a higher chlorophyll content. The plant responses varied during two phenophases under the combined stress exposure. Genotypes subjected to HD37 stress during stem elongation, photosynthesis was mainly controlled by stomatal regulation, whereas at anthesis it was predominated by biochemical regulation. These findings contribute to a deeper comprehension of plant tolerance mechanisms in response to different intensities of co-occurring hot and dry weather conditions.

Greater mesophyll conductance and leaf photosynthesis in the field through modified cell wall porosity and thickness via AtCGR3 expression in tobacco

Mesophyll conductance (gm) describes the ease with which CO2 passes from the sub-stomatal cavities of the leaf to the primary carboxylase of photosynthesis, Rubisco. Increasing gm is suggested as a means to engineer increases in photosynthesis by increasing [CO2] at Rubisco, inhibiting oxygenation and accelerating carboxylation. Here, tobacco was transgenically up-regulated with Arabidopsis Cotton Golgi-related 3 (CGR3), a gene controlling methylesterification of pectin, as a strategy to increase CO2 diffusion across the cell wall and thereby increase gm. Across three independent events in tobacco strongly expressing AtCGR3, mesophyll cell wall thickness was decreased by 7%-13%, wall porosity increased by 75% and gm measured by carbon isotope discrimination increased by 28%. Importantly, field-grown plants showed an average 8% increase in leaf photosynthetic CO2 uptake. Up-regulating CGR3 provides a new strategy for increasing gm in dicotyledonous crops, leading to higher CO2 assimilation and a potential means to sustainable crop yield improvement.

Characteristics of photosynthetic rates in different vegetation types at high-altitude in mountainous regions

Mountains play an important role in the carbon cycle of the terrestrial ecosystem and are one of the most sensitive ecosystems to climate change. However, our current knowledge regarding the physiological responses of alpine plants to environmental changes remains limited due to the severe climatic conditions prevailing in these high-altitude regions. Therefore, this study quantified the variations in photosynthetic rates (An) and identified their driving factors of herbaceous plants, shrubs, and trees along an elevation gradient (2200 m asl to 3200 m asl) on Mount Gongga. Elevation emerged as a significant determinant of An, with a general increase observed, albeit followed by a decline above 3000 m asl. In high-altitude regions, trees displayed more significant fluctuations in An compared to herbaceous plants and shrubs. The lower levels of atmospheric carbon dioxide concentration (eCO2) and temperature in high-altitude regions resulted in a 16 % increase in An for herbaceous plants, 60 % increase for shrubs, and 43 % increase for trees compared to the low-altitude areas. Structural equation modeling (SEM) analyses underscored the considerable impact of environmental factors on An. Notably, photosynthetically active radiation, eCO2, and stomatal conductance were identified as positive influencers, while other factors exerted negative effects. Our results further highlighted that trees were subject to greater constraints from multiple factors compared to herbs and shrubs, aligning with the outcomes of our variance analysis. In summary, our study presents a comprehensive assessment of vegetation responses to environmental factors along elevational gradients. The significance of An in plants at high altitude to external factors suggests the potential adaptability of alpine plants, and also indicates that changes in photosynthetic physiological functions at high altitude should be paid more attention to in the study of climate change.

Altered cell wall hydroxycinnamate composition impacts leaf- and canopy-level CO2 uptake and water use in rice

Cell wall properties play a major role in determining photosynthetic carbon uptake and water use through their impact on mesophyll conductance (CO2 diffusion from substomatal cavities into photosynthetic mesophyll cells) and leaf hydraulic conductance (water movement from xylem, through leaf tissue, to stomata). Consequently, modification of cell wall (CW) properties might help improve photosynthesis and crop water use efficiency (WUE). We tested this using 2 independent transgenic rice (Oryza sativa) lines overexpressing the rice OsAT10 gene (encoding a "BAHD" CoA acyltransferase), which alters CW hydroxycinnamic acid content (more para-coumaric acid and less ferulic acid). Plants were grown under high and low water levels, and traits related to leaf anatomy, CW composition, gas exchange, hydraulics, plant biomass, and canopy-level water use were measured. Alteration of hydroxycinnamic acid content led to statistically significant decreases in mesophyll CW thickness (-14%) and increased mesophyll conductance (+120%) and photosynthesis (+22%). However, concomitant increases in stomatal conductance negated the increased photosynthesis, resulting in no change in intrinsic WUE (ratio of photosynthesis to stomatal conductance). Leaf hydraulic conductance was also unchanged; however, transgenic plants showed small but statistically significant increases in aboveground biomass (AGB) (+12.5%) and canopy-level WUE (+8.8%; ratio of AGB to water used) and performed better under low water levels than wild-type plants. Our results demonstrate that changes in CW composition, specifically hydroxycinnamic acid content, can increase mesophyll conductance and photosynthesis in C3 cereal crops such as rice. However, attempts to improve photosynthetic WUE will need to enhance mesophyll conductance and photosynthesis while maintaining or decreasing stomatal conductance.

Increased stomatal conductance and leaf biochemical capacity, not mesophyll conductance, contributing to the enhanced photosynthesis in Oryza plants during domestication

MAIN CONCLUSION

Leaf biochemical capacity and the ratio of leaf biochemical capacity to stomatal conductance are promising to enhance leaf photosynthetic rate and water use efficiency in rice plants, respectively. Domestication may have great impact on crop photosynthetic rate, which has not been fully understood, especially from the perspective of stomatal conductance, mesophyll conductance, and leaf biochemical capacity simultaneously. In this study, we constructed a database consisting of 141 and 92 sets of data from wild and cultivated rice, respectively, including leaf gas exchange parameters, hydraulic conductance, structural traits, and nitrogen content. We found that, compared to wild rice, enhanced leaf photosynthetic rate in cultivated rice was mainly resulted by the increased stomatal conductance and leaf biochemical capacity, rather than mesophyll conductance. The unchanged mesophyll conductance observed during domestication suggested that it might have been optimized during plant evolution in wild rice. Additionally, the positive relationship between stomatal density and stomatal conductance disappeared in Oryza plants during domestication, suggesting that stomatal conductance in cultivated rice is less restricted by stomatal density, compared to wild rice. Moreover, in both wild and cultivated rice, leaf photosynthetic rate was mainly determined by leaf biochemical capacity, rather than stomatal conductance and mesophyll conductance. This study highlighted the important role of stomatal conductance and leaf biochemical capacity in improvement of leaf photosynthetic rate in rice during domestication. Leaf biochemical capacity and the ratio of leaf biochemical capacity to stomatal conductance should be further investigated to enhance leaf photosynthetic rate and water use efficiency in rice plants, respectively.

Improving photosynthetic efficiency toward food security: Strategies, advances, and perspectives

Photosynthesis in crops and natural vegetation allows light energy to be converted into chemical energy and thus forms the foundation for almost all terrestrial trophic networks on Earth. The efficiency of photosynthetic energy conversion plays a crucial role in determining the portion of incident solar radiation that can be used to generate plant biomass throughout a growth season. Consequently, alongside the factors such as resource availability, crop management, crop selection, maintenance costs, and intrinsic yield potential, photosynthetic energy use efficiency significantly influences crop yield. Photosynthetic efficiency is relevant to sustainability and food security because it affects water use efficiency, nutrient use efficiency, and land use efficiency. This review focuses specifically on the potential for improvements in photosynthetic efficiency to drive a sustainable increase in crop yields. We discuss bypassing photorespiration, enhancing light use efficiency, harnessing natural variation in photosynthetic parameters for breeding purposes, and adopting new-to-nature approaches that show promise for achieving unprecedented gains in photosynthetic efficiency.

Exploring through the use of physiological and isotopic techniques the potential of a PGPR-based biofertilizer to improve nitrogen fertilization practices efficiency in strawberry cultivation

The use of microorganisms as a biofertilizer in strawberry has focused mainly on pathogen biocontrol, which has led to the underestimation of the potential of microorganisms for the improvement of nutritional efficiency in this crop. A study was established to investigate the impact of a plant growth-promoting rhizobacteria (PGPR) based biofertilizer integrated by self-compatible stress tolerant strains with multiple PGP properties, including atmospheric nitrogen fixation, on strawberry (Fragaria × ananassa cv. Rociera) tolerance to N deficiency in terms of growth and physiological performance. After 40 days of nitrogen fertilization shortage, inoculated plants were able to maintain root development and fertility structures (i.e. fruits and flowers) at a level similar to plants properly fertilized. In addition, inoculation lessened the negative impact of nitrogen deficiency on leaves' dry weight and relative water content. This effect was mediated by a higher root/shoot ratio, which would have allowed them to explore larger volumes of soil for the acquisition of water. Moreover, inoculation was able to buffer up to 50% of the reduction in carbon assimilation capacity, due to its positive effect on the diffusion efficiency of CO2 and the biochemical capacity of photosynthesis, as well as on the activity of photosystem II light harvesting. Furthermore, the higher leaf C/N ratio and the maintained δ15N values close to control plants were related to positive bacterial effects at the level of the plant nutritional balance. Despite these positive effects, the application of the bacterial inoculum was unable to completely counteract the restriction of fertilization, being necessary to apply a certain amount of synthetic fertilizer for the strawberry nutrition. However, according to our results, the complementary effect of this PGPR-based biofertilizer could provide a higher efficiency in environmental and economic yields on this crop.

Genetic modification strategies for enhancing plant resilience to abiotic stresses in the context of climate change

Enhancing the resilience of plants to abiotic stresses, such as drought, salinity, heat, and cold, is crucial for ensuring global food security challenge in the context of climate change. The adverse effects of climate change, characterized by rising temperatures, shifting rainfall patterns, and increased frequency of extreme weather events, pose significant threats to agricultural systems worldwide. Genetic modification strategies offer promising approaches to develop crops with improved abiotic stress tolerance. This review article provides a comprehensive overview of various genetic modification techniques employed to enhance plant resilience. These strategies include the introduction of stress-responsive genes, transcription factors, and regulatory elements to enhance stress signaling pathways. Additionally, the manipulation of hormone signaling pathways, osmoprotectant accumulation, and antioxidant defense mechanisms is discussed. The use of genome editing tools, such as CRISPR-Cas9, for precise modification of target genes related to stress tolerance is also explored. Furthermore, the challenges and future prospects of genetic modification for abiotic stress tolerance are highlighted. Understanding and harnessing the potential of genetic modification strategies can contribute to the development of resilient crop varieties capable of withstanding adverse environmental conditions caused by climate change, thereby ensuring sustainable agricultural productivity and food security.

Natural variation in photosynthesis and water use efficiency of locally adapted Persian walnut populations under drought stress and recovery

Persian walnut is a drought-sensitive species with considerable genetic variation in the photosynthesis and water use efficiency of its populations, which is largely unexplored. Here, we aimed to elucidate changes in the efficiency of photosynthesis and water content using a diverse panel of 60 walnut families which were submitted to a progressive drought for 24 days, followed by two weeks of re-watering. Severe water-withholding reduced leaf relative water content (RWC) by 20%, net photosynthetic rate (Pn) by 50%, stomatal conductance (gs) by 60%, intercellular CO2 concentration (Ci) by 30%, and transpiration rate (Tr) by 50%, but improved water use efficiency (WUE) by 25%. Severe water-withholding also inhibited photosystem II functionality as indicated by reduced quantum yield of intersystem electron transport (φEo) and transfer of electrons per reaction center (ET0/RC), also enhanced accumulation of QA (VJ) resulted in the reduction of the photosynthetic performance (PIABS) and maximal quantum yield of PSII (FV/FM); while elevated quantum yield of energy dissipation (φDo), energy fluxes for absorption (ABS/RC) and dissipated energy flux (DI0/RC) in walnut families. Cluster analysis classified families into three main groups (tolerant, moderately tolerant, and sensitive), with the tolerant group from dry climates exhibiting lesser alterations in assessed parameters than the other groups. Multivariate analysis of phenotypic data demonstrated that RWC and biophysical parameters related to the chlorophyll fluorescence such as FV/FM, φEo, φDo, PIABS, ABS/RC, ET0/RC, and DI0/RC represent fast, robust and non-destructive biomarkers for walnut performance under drought stress. Finally, phenotype-environment association analysis showed significant correlation of some photosynthetic traits with geoclimatic factors, suggesting a key role of climate and geography in the adaptation of walnut to its habitat conditions.

Leaf physiological and morphological constraints of water-use efficiency in C3 plants

The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants' WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization.

Within-branch photosynthetic gradients are more related to the coordinated investments of nitrogen and water than leaf mass per area

Nitrogen (N) and water are key resources for leaf photosynthesis and the growth of whole plants. Within-branch leaves need different amounts of N and water to support their differing photosynthetic capacities according to light exposure. To test this scheme, we measured the within-branch investments of N and water and their effects on photosynthetic traits in two deciduous tree species Paulownia tomentosa and Broussonetia papyrifera. We found that leaf photosynthetic capacity gradually increased from branch bottom to top (i.e. from shade to sun leaves). Concomitantly, stomatal conductance (g_s) and leaf N content gradually increased, owing to the symport of water and inorganic mineral from root to leaf. Variation of leaf N content led to large gradients of mesophyll conductance, maximum velocity of Rubisco for carboxylation, maximum electron transport rate and leaf mass per area (LMA). Correlation analysis indicated that the within-branch difference in photosynthetic capacity was mainly related to g_s and leaf N content, with a relatively minor contribution of LMA. Furthermore, the simultaneous increases of g_s and leaf N content enhanced photosynthetic N use efficiency (PNUE) but hardly affected water use efficiency. Therefore, within-branch adjustment of N and water investments is an important strategy used by plants to optimize the overall photosynthetic carbon gain and PNUE.

Variation in Photosynthetic Efficiency under Fluctuating Light between Rose Cultivars and its Potential for Improving Dynamic Photosynthesis

Photosynthetic efficiency under both steady-state and fluctuating light can significantly affect plant growth under naturally fluctuating light conditions. However, the difference in photosynthetic performance between different rose genotypes is little known. This study compared the photosynthetic performance under steady-state and fluctuating light in two modern rose cultivars (Rose hybrida), "Orange Reeva" and "Gelato", and an old Chinese rose plant Rosa chinensis cultivar, "Slater's crimson China". The light and CO₂ response curves indicated that they showed similar photosynthetic capacity under steady state. The light-saturated steady-state photosynthesis in these three rose genotypes was mainly limited by biochemistry (60%) rather than diffusional conductance. Under fluctuating light conditions (alternated between 100 and 1500 μmol photons m^-2 m^-1 every 5 min), stomatal conductance gradually decreased in these three rose genotypes, while mesophyll conductance (g_m) was maintained stable in Orange Reeva and Gelato but decreased by 23% in R. chinensis, resulting in a stronger loss of CO₂ assimilation under high-light phases in R. chinensis (25%) than in Orange Reeva and Gelato (13%). As a result, the variation in photosynthetic efficiency under fluctuating light among rose cultivars was tightly related to g_m. These results highlight the importance of g_m in dynamic photosynthesis and provide new traits for improving photosynthetic efficiency in rose cultivars.

Leaf anatomy does not explain the large variability of mesophyll conductance across C3 crop species

Increasing mesophyll conductance of CO₂ (g_m ) is a strategy to improve photosynthesis in C₃ crops. However, the relative importance of different anatomical traits in determining g_m in crops is unclear. Mesophyll conductance measurements were performed on 10 crops using the online carbon isotope discrimination method and the 'variable J' method in parallel. The influences of crucial leaf anatomical traits on g_m were evaluated using a one-dimensional anatomical CO₂ diffusion model. The g_m values measured using two independent methods were compatible, although significant differences were observed in their absolute values. Quantitative analysis showed that cell wall thickness and chloroplast stroma thickness are the most important elements along the diffusion pathway. Unexpectedly, the large variability of g_m across crops was not associated with any investigated leaf anatomical traits except chloroplast thickness. The g_m values estimated using the anatomical model differed remarkably from the values measured in vivo in most species. However, when the species-specific effective porosity of the cell wall and the species-specific facilitation effect of CO₂ diffusion across the membrane and chloroplast stoma were taken into account, the model could output g_m values very similar to those measured in vivo. These results indicate that g_m variation across crops is probably also driven by the effective porosity of the cell wall and effects of facilitation of CO₂ transport across the membrane and chloroplast stroma in addition to the thicknesses of the elements.

Estimation of intrinsic water-use efficiency from δ13C signature of C3 leaves: Assumptions and uncertainty

Carbon isotope composition (δ¹³C) has been widely used to estimate the intrinsic water-use efficiency (iWUE) of plants in ecosystems around the world, providing an ultimate record of the functional response of plants to climate change. This approach relies on established relationships between leaf gas exchange and isotopic discrimination, which are reflected in different formulations of ¹³C-based iWUE models. In the current literature, most studies have utilized the simple, linear equation of photosynthetic discrimination to estimate iWUE. However, recent studies demonstrated that using this linear model for quantitative studies of iWUE could be problematic. Despite these advances, there is a scarcity of review papers that have comprehensively reviewed the theoretical basis, assumptions, and uncertainty of ¹³C-based iWUE models. Here, we 1) present the theoretical basis of ¹³C-based iWUE models: the classical model (iWUE_sim), the comprehensive model (iWUE_com), and the model incorporating mesophyll conductance (iWUE_mes); 2) discuss the limitations of the widely used iWUE_sim model; 3) and make suggestions on the application of the iWUE_mes model. Finally, we suggest that a mechanistic understanding of mesophyll conductance associated effects and post-photosynthetic fractionation are the bottlenecks for improving the ¹³C-based estimation of iWUE.

Impacts of Dutch elm disease-causing fungi on foliage photosynthetic characteristics and volatiles in Ulmus species with different pathogen resistance

Global warming affects the abiotic and biotic growth environment of plants, including the spread of fungal diseases such as Dutch elm disease (DED). Dutch elm disease-resistance of different Ulmus species varies, but how this is reflected in leaf-level physiological pathogen responses has not been investigated. We studied the impacts of mechanical injury alone and mechanical injury plus inoculation with the DED-causing pathogens Ophiostoma novo-ulmi subsp. novo-ulmi and O. novo-ulmi subsp. americana on Ulmus glabra, a more vulnerable species, and U. laevis, a more resistant species. Plant stress responses were evaluated for 12 days after stress application by monitoring leaf net CO2 assimilation rate (A), stomatal conductance (gs), ratio of ambient to intercellular CO2 concentration (Ca/Ci) and intrinsic water-use efficiency (A/gs), and by measuring biogenic volatile (VOC) release by plant leaves. In U. glabra and U. laevis, A was not affected by time, stressors or their interaction. Only in U. glabra, gs and Ca/Ci decreased in time, yet recovered by the end of the experiment. Although the emission compositions were affected in both species, the stress treatments enhanced VOC emission rates only in U. laevis. In this species, mechanical injury especially when combined with the pathogens increased the emission of lipoxygenase pathway volatiles and dimethylallyl diphosphate and geranyl diphosphate pathway volatiles. In conclusion, the more resistant species U. laevis had a more stable photosynthesis, but stronger pathogen-elicited volatile response, especially after inoculation by O. novo-ulmi subsp. novo-ulmi. Thus, stronger activation of defenses might underlay higher DED-resistance in this species.

Genotype-dependent changes of cell wall composition influence physiological traits of a long and a non-long shelf-life tomato genotypes under distinct water regimes

Water shortage strongly affects plants' physiological performance. Since tomato (Solanum lycopersicum) non-long shelf-life (nLSL) and long shelf-life (LSL) genotypes differently face water deprivation, we subjected a nLSL and a LSL genotype to four treatments: control (well watering), short-term water deficit stress at 40% field capacity (FC) (ST 40% FC), short-term water deficit stress at 30% FC (ST 30% FC), and short-term water deficit stress at 30% FC followed by recovery (ST 30% FC-Rec). Treatments promoted genotype-dependent elastic adjustments accompanied by distinct photosynthetic responses. While the nLSL genotype largely modified mesophyll conductance (g_m ) across treatments, it was kept within a narrow range in the LSL genotype. However, similar g_m values were achieved under ST 30% FC conditions. Particularly, modifications in the relative abundance of cell wall components and in sub-cellular anatomic parameters such as the chloroplast surface area exposed to intercellular air space per leaf area (S_c /S) and the cell wall thickness (T_cw ) regulated g_m in the LSL genotype. Instead, only changes in foliar structure at the supra-cellular level influenced g_m in the nLSL genotype. Even though further experiments testing a larger range of genotypes and treatments would be valuable to support our conclusions, we show that even genotypes of the same species can present different elastic, anatomical, and cell wall composition-mediated mechanisms to regulate g_m when subjected to distinct water regimes.

Volatile compounds of Bacillus pseudomycoides induce growth and drought tolerance in wheat (Triticum aestivum L.)

The plant growth-boosting biofilm-forming bacteria Bacillus pseudomycoides is able to promote growth and drought stress tolerance in wheat by suppressing the MYB gene, which synthesizes Myb protein (TaMpc1-D4) through secreted volatile compounds. In the present study, Triticum aestivum seeds were inoculated with five distinct bacterial strains. The growth, germination rate, root-shoot length, RWC, and chlorophyll content of seedlings were investigated. Furthermore, the levels of soluble sugars, proteins, H₂O₂, NO, cell death, and antioxidant enzymes (CAT, SOD, POD, and APX) were observed throughout the growth stage. All of the results showed that B. pseudomycoides had a substantially higher ability to form biofilm and promote these traits than the other strains. In terms of molecular gene expression, B. pseudomycoides inoculation strongly expressed the Dreb1 gene by silencing the expression of MYB gene through secreted volatile compounds. For identifying the specific volatile compound that silenced the MYB gene, molecular docking with Myb protein was performed. Out of 45 volatile compounds found, 2,6-ditert-butylcyclohexa-2,5-diene-1,4-dione and 3,5-ditert-butylphenol had a binding free energy of - 6.2 and - 6.5, Kcal/mol, respectively, which predicted that these compounds could suppress this protein's expression. In molecular dynamics simulations, the RMSD, SASA, Rg, RMSF, and hydrogen bonding values found assured the docked complexes' binding stability. These findings suggest that these targeted compounds may be suppressing Myb protein expression as well as the expression of Dreb1 and other drought response genes in wheat. More research (field trial) into plant growth and drought stress is needed to support the findings of this study.

Leaf nitrogen from the perspective of optimal plant function

Leaf dry mass per unit area (LMA), carboxylation capacity (V _cmax) and leaf nitrogen per unit area (N_area) and mass (N_mass) are key traits for plant functional ecology and ecosystem modelling. There is however no consensus about how these traits are regulated, or how they should be modelled. Here we confirm that observed leaf nitrogen across species and sites can be estimated well from observed LMA and V _cmax at 25°C (V _cmax25). We then test the hypothesis that global variations of both quantities depend on climate variables in specific ways that are predicted by leaf-level optimality theory, thus allowing both N_area to be predicted as functions of the growth environment.A new global compilation of field measurements was used to quantify the empirical relationships of leaf N to V _cmax25 and LMA. Relationships of observed V _cmax25 and LMA to climate variables were estimated, and compared to independent theoretical predictions of these relationships. Soil effects were assessed by analysing biases in the theoretical predictions.LMA was the most important predictor of N_area (increasing) and N_mass (decreasing). About 60% of global variation across species and sites in observed N_area, and 31% in N_mass, could be explained by observed LMA and V _cmax25. These traits, in turn, were quantitatively related to climate variables, with significant partial relationships similar or indistinguishable from those predicted by optimality theory. Predicted trait values explained 21% of global variation in observed site-mean V _cmax25, 43% in LMA and 31% in N_area. Predicted V _cmax25 was biased low on clay-rich soils but predicted LMA was biased high, with compensating effects on N_area. N_area was overpredicted on organic soils. Synthesis. Global patterns of variation in observed site-mean N_area can be explained by climate-induced variations in optimal V _cmax25 and LMA. Leaf nitrogen should accordingly be modelled as a consequence (not a cause) of V _cmax25 and LMA, both being optimized to the environment. Nitrogen limitation of plant growth would then be modelled principally via whole-plant carbon allocation, rather than via leaf-level traits. Further research is required to better understand and model the terrestrial nitrogen and carbon cycles and their coupling.

Overestimated gains in water-use efficiency by global forests

Increases in terrestrial water-use efficiency (WUE) have been reported in many studies, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. However, gains in WUE are of uncertain magnitude over longer (i.e. >10 years) periods of time largely owing to difficulties in accounting for structural and physiological acclimation. ¹³ C signatures (i.e. δ¹³ C) of plant organic matter have long been used to estimate WUE at temporal scales ranging from days to centuries. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO₂ to sites of carboxylation. Here we apply new knowledge of mesophyll conductance to 464 δ¹³ C chronologies in tree-rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, gains in WUE during the 20th century (0.15 ppm year^-1 ) were considerably smaller than those estimated from conventional modelling (0.26 ppm year^-1 ). Across the globe, mean sensitivity of WUE to atmospheric CO₂ was 0.15 ppm ppm^-1 . Ratios of internal-to-atmospheric CO₂ (on a mole fraction basis; c_i /c_a ) in leaves were mostly constant over time but differed among biomes and plant taxa-highlighting the significance of both plant structure and physiology. Together with synchronized responses in stomatal and mesophyll conductance, our results suggest that ratios of chloroplastic-to-atmospheric CO₂ (c_c /c_a ) are constrained over time. We conclude that forest WUE may have not increased as much as previously suggested and that projections of future climate forcing via CO₂ fertilization may need to be adjusted accordingly.

Overexpression of cytoplasmic C4 Flaveria bidentis carbonic anhydrase in C3 Arabidopsis thaliana increases amino acids, photosynthetic potential, and biomass

An important method to improve photosynthesis in C₃ crops, such as rice and wheat, is to transfer efficient C₄ characters to them. Here, cytosolic carbonic anhydrase (CA: βCA3) of the C₄ Flaveria bidentis (Fb) was overexpressed under the control of ³⁵ S promoter in Arabidopsis thaliana, a C₃ plant, to enhance its photosynthetic efficiency. Overexpression of CA resulted in a better supply of the substrate HCO3- for the endogenous phosphoenolpyruvate carboxylase in the cytosol of the overexpressers, and increased its activity for generating malate that feeds into the tricarboxylic acid cycle. This provided additional carbon skeleton for increased synthesis of amino acids aspartate, asparagine, glutamate, and glutamine. Increased amino acids contributed to higher protein content in the transgenics. Furthermore, expression of FbβCA3 in Arabidopsis led to a better growth due to expression of several genes leading to higher chlorophyll content, electron transport, and photosynthetic carbon assimilation in the transformants. Enhanced CO₂ assimilation resulted in increased sugar and starch content, and plant dry weight. In addition, transgenic plants had lower stomatal conductance, reduced transpiration rate, and higher water-use efficiency. These results, taken together, show that expression of C₄ CA in the cytosol of a C₃ plant can indeed improve its photosynthetic capacity with enhanced water-use efficiency.

When time is not of the essence: constraints to the carbon balance of bryophytes

The data available so far indicate that the photosynthetic and relative growth rates of bryophytes are 10% of those reported for tracheophytes. By examining the existing literature and reanalysing data published in over 100 studies, this review examines the ecophysiological, biochemical, and structural reasons behind this phenomenon. The limiting Rubisco content and surface for gas exchange are the internal factors that can explain the low photosynthetic and growth rates of bryophytes. The role of the thicker cell walls of bryophytes in limiting CO2 diffusion is unclear, due to the current uncertainties regarding their porosity and permeability to CO2. From this review, it is also evident that, despite bryophytes having low photosynthetic rates, their positive carbon balance is tightly related to their capacity to deal with extreme conditions. Contributing factors include their capacity to deal with large daily temperature oscillations, and their capacity to delay the cessation of photosynthesis under water deficit (or to tolerate desiccation in extreme situations). Although further studies on bryophytes are needed before more solid conclusions can be drawn, it seems that their success relies on their remarkable tolerance to a highly variable environment, possibly at the expense of their maximum photosynthetic rate.

The structural correlations and the physiological functions of stomatal morphology and leaf structures in C3 annual crops

This study suggests that stomatal and leaf structures are highly correlated, and mesophyll cell size is an important anatomical trait determining the coordination between stomatal size and mesophyll porosity. A comprehensive study of the correlations between the structural traits and on their relationships with gas exchange parameters may provide some useful information into leaf development and improvement in efficiencies of photosynthetic CO₂ fixation and transpirational water loss. In the present study, nine plant materials from eight crop species were pot grown in a growth chamber. Leaf structural traits, gas exchange, and leaf nitrogen content were measured. We found that stomatal size, mesophyll cell size (MCS), and mesophyll porosity were positively correlated and that the surface areas of mesophyll cells and chloroplasts facing intercellular air spaces were positively correlated with both stomatal density and stomatal area per leaf area (SA). These results suggested that the developments of stomata and mesophyll cells are highly correlated among different crop species. Additionally, MCS was positively correlated with leaf thickness and negatively correlated with leaf density and leaf mass per area, which indicated that MCS might play an important role in leaf structural investments and physiological functions among species. In summary, this study illustrates the correlations between stomatal and mesophyll structures, and it highlights the importance of considering the covariations among leaf traits with the intent of improving photosynthesis and iWUE.

Ecophysiological Responses of Tall Wheatgrass Germplasm to Drought and Salinity

Tall wheatgrass (Thinopyrum ponticum (Podp.) Barkworth and D.R. Dewey) is an important, highly salt-tolerant C3 forage grass. The objective of this work was to learn about the ecophysiological responses of accessions from different environmental origins under drought and salinity conditions, to provide information for selecting superior germplasm under combined stress in tall wheatgrass. Four accessions (P3, P4, P5, P9) were irrigated using combinations of three salinity levels (0, 0.1, 0.3 M NaCl) and three drought levels (100%, 50%, 30% water capacity) over 90 days in a greenhouse. The control treatment showed the highest total biomass, but water-use efficiency (WUE), δ¹³C, proline, N concentration, leaf length, and tiller density were higher under moderate drought or/and salinity stress than under control conditions. In tall wheatgrass, K⁺ functions as an osmoregulator under drought, attenuated by salinity, and Na⁺ and Cl^- function as osmoregulators under salinity and drought, while proline is an osmoprotector under both stresses. P3 and P9, from environments with mild/moderate stress, prioritized reproductive development, with high evapotranspiration and the lowest WUE and δ¹³C values. P4 and P5, from more stressful environments, prioritized vegetative development through tillering, showing the lowest evapotranspiration, the highest δ¹³C values, and different mechanisms for limiting transpiration. The δ¹³C value, leaf biomass, tiller density, and leaf length had high broad-sense heritability (H²), while the Na⁺/K⁺ ratio had medium H². In conclusion, the combined use of the δ¹³C value, Na⁺/K⁺ ratio, and canopy structural variables can help identify accessions that are well-adapted to drought and salinity, also considering the desirable plant characteristics. Tall wheatgrass stress tolerance could be used to expand forage production under a changing climate.

Functional plasticity of Capsicum annuum var. glabriusculum through multiple traits

The diversity of functional traits still has not been studied enough in model plant species, even less so in little-known species. This experiment was carried out under the extreme heat of Sonoran Desert, using shading nets and under conditions where the availability of water and nutrients was not a stress factor. We evaluated how the low, intermediate and high sunlight regimes impact survival and promote multiple alterations on phenological and ecophysiological response of cultivated Capsicum annuum var. glabriusculum plants. Extremely warm temperatures promoted a high heat sum in degrees days throughout plants development. Most plants grown in high sunlight regimes did not survive; under intermediate sunlight regimes survival was high and plants developed vegetative and reproductively; but under low sunlight regimes plants survival was high; however, they developed just vegetatively. Photosynthetic response to light suggests that plants are physiologically acclimated to low and intermediate irradiance, whereas the CO₂ assimilation curves suggest contrasting photosynthetic capacity traits. Under the intermediate sunlight regimes, plants strengthened their performance through multiple functional traits (e.g. CO₂ and water diffusion traits, photosynthetic capacity, respiration, among others). Consequently, their biomass gain was faster and proportionally higher by 76 % with an investment of 14 % in fruits development. The principal components analysis extracted the main explanatory functional traits: photosynthetic nitrogen allocation, stomatal limitation, mesophyll conductance, Rubisco maximum carboxylation velocity, among others. In conclusion, phenological response and multiple functional traits determine plants acclimation to sunlight regimes and extremely warm temperatures in short term.

A model-guided holistic review of exploiting natural variation of photosynthesis traits in crop improvement

Breeding for improved leaf photosynthesis is considered as a viable approach to increase crop yield. Whether it should be improved in combination with other traits has not been assessed critically. Based on the quantitative crop model GECROS that interconnects various traits to crop productivity, we review natural variation in relevant traits, from biochemical aspects of leaf photosynthesis to morpho-physiological crop characteristics. While large phenotypic variations (sometimes >2-fold) for leaf photosynthesis and its underlying biochemical parameters were reported, few quantitative trait loci (QTL) were identified, accounting for a small percentage of phenotypic variation. More QTL were reported for sink size (that feeds back on photosynthesis) or morpho-physiological traits (that affect canopy productivity and duration), together explaining a much greater percentage of their phenotypic variation. Traits for both photosynthetic rate and sustaining it during grain filling were strongly related to nitrogen-related traits. Much of the molecular basis of known photosynthesis QTL thus resides in genes controlling photosynthesis indirectly. Simulation using GECROS demonstrated the overwhelming importance of electron transport parameters, compared with the maximum Rubisco activity that largely determines the commonly studied light-saturated photosynthetic rate. Exploiting photosynthetic natural variation might significantly improve crop yield if nitrogen uptake, sink capacity, and other morpho-physiological traits are co-selected synergistically.

Perspective of ACC-deaminase producing bacteria in stress agriculture

The 1-aminocyclopropane-1-carboxylate deaminase (ACCD) enzyme plays an important role in stress alleviation of both biotic and abiotic stressors in plants and thereby enhances their growth under harsh environmental conditions. In-depth analysis of AcdS gene encoding for ACC deaminase reveals its presence in diverse microorganisms including bacteria and fungi. Particularly, plant growth-promoting bacteria (PGPB) containing ACCD supports plant growth by modulating the level of 'stress ethylene' and cleaving its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) into α-ketobutyrate and ammonia, enabling PGPB to utilize ACC as a carbon and nitrogen source. The reduced synthesis of ethylene in plants further relieves the ethylene inhibition of plant growth and development, and improves plant resistance to various stressors. Therefore, the dual role of microbial ACCD makes it a cost-effective and eco-friendly biocatalyst for sustainable agricultural productions. The inducible ACCD encoding gene AcdS is differentially regulated by varying environmental conditions. Successful generation of transgenic plants with microbial AcdS gene enhanced biotic and abiotic stress tolerance in plants. In the present review, we discuss the importance of ACCD-producing PGPB for their ability to reduce ethylene production and the promotion of plant growth under stress conditions. We also highlighted the development of transgenic plants by overexpressing bacterial AcdS gene to improve their performance under stress conditions.

Variation, coordination, and trade-offs between needle structures and photosynthetic-related traits across five Picea species: consequences on plant growth

BACKGROUND

Picea species are distributed and planted world-wide due to their great ecological and economic values. It has been reported that Picea species vary widely in growth traits in a given environment, which reflects genetic and phenotypic differences among species. However, key physiological processes underlying tree growth and the influencing factors on them are still unknown.

RESULTS

Here, we examined needle structures, needle chemical components, physiological characteristics and growth traits across five Picea species in a common garden in Tianshui, Gansu province in China: Picea glauca, P. mariana, P. likiangensis, P. koraiensis, and P. crassifolia, among which P. glauca and P. mariana were introduced from North America, P. likiangensis was from Lijiang, Yunan province in China, P. koraiensis was from Yichun, Heilongjiang province in China, and P. crassifolia was native to the experimental site. It was found that nearly all traits varied significantly among species. Tissue-level anatomical characteristics and leaf mass per area (LMA) were affected by needle size, but the variations of them were not associated with the variations in photosynthetic and biochemical capacity among species. Variations in area-based maximum photosynthesis (P_nmax) were affected by stomatal conductance (g_s), mesophyll conductance (g_m) and biochemical parameters including maximum carboxylation rate (V_cmax), and maximum electron transport rate (J_max). The fraction of N allocated to different photosynthetic apparatus displayed contrasting values among species, which contributed to the species variations in photosynthetic nitrogen use efficiency (PNUE) and P_nmax. Additionally, all growth traits were positively correlated with P_nmax and PNUE.

CONCLUSION

Needle structures are less important than needle biochemical parameters in determining the variations in photosynthetic capacity across the five Picea species. P_nmax and PNUE are closedly associated with the fraction of N allocated to photosynthetic apparatus (P_photo) compared with leaf N content per area (N_area). The tremendous growth differences among the five Picea species were substantially related to the interspecies variation in P_nmax and PNUE.

Leaf morpho-physiological comparison between native and non-native plant species in a Mediterranean island

Islands tend to be more prone to plant invasions than mainland regions, with the Mediterranean ones not being an exception. So far, a large number of studies on comparing leaf morphological and physiological traits between native and non-native plants in Mediterranean environments have been performed, although none of them on Mediterranean islands. To fill this gap, this study focuses on 14 plant species grown in a controlled growth chamber in the absence of stress. The goal was (1) to differentiate leaf morpho-physiological traits between native and non-native plants on a Mediterranean island and (2) to deepen in the underlying causes of the differential photosynthetic traits displayed by non-native species. Results showed that in Mediterranean islands, non-native plant species show on average larger values of net CO2 assimilation, stomatal conductance (gm), photosynthetic nitrogen-use efficiency, among others, and lower leaf mass per area (LMA) and leaf thickness, compared to the native species. Among the assessed traits, this study reports for the first time larger gm, and lower mesophyll conductance limitation in non-native species, which seems to be linked to their lower LMA. These novel traits need to be added to the ‘leaf physiological trait invasive syndrome’. It was also found that on a Mediterranean island, native and non-native species are placed on opposite sides of the leaf economics spectrum, with non-native species being placed on the ‘‘fast-return’’ end. In conclusion, this study demonstrates that non-native species inhabiting a Mediterranean island possess distinct leaf morphological and physiological traits compared to co-occurring native species, at least during the favorable growth season, which increases the chances of a successful invasion.

The Response Mechanism and Threshold of Spring Wheat to Rapid Drought

In order to deeply understand the effect mechanism of rapid drought stress on the physiological and biochemical properties of crop leaves and determine drought thresholds, the potted spring wheat under two water treatments, adequate water supply and continuous drought stress, was researched. In the early stage of drought, the parameters of leaves decreased in the order of stomatal conductance (gs), intercellular CO2 concentration (Ci), maximum electron transfer rate (Jmax), mesophyll conductance (gm), photosynthetic rate (Pn,) leaf water content (LWC), triose phosphate utilization rate (TPU), transpiration rate (Tr), and maximum carboxylation rate (Vcmax). Photosynthesis was dominated by stomatal limitation and also limited by carboxylation and mesophyll limitation. The carboxylation limitation was mainly caused by the reduction of electron transport capacity. In the late stage of drought, stomatal limitation first decreased, and then mesophyll limitation decreased. During extreme drought, carboxylation limitation also decreased. With the decrease of relative soil moisture (RSM), except for Ci, which first decreased and then increased, other physicochemical parameters of leaves all showed an S-shaped, decreasing trend. Mild and severe drought thresholds were determined to be 56.6% and 43.6% of the RSM, respectively, according to the curve’s inflection point, corresponding to 16.6% and about 52.2% of the average initial decrease amplitude among all parameters. This will provide a reference for monitoring as well as an early warning of rapid drought in spring wheat.

Shifts in biochemical and physiological responses by the inoculation of arbuscular mycorrhizal fungi in Triticum aestivum growing under drought conditions

BACKGROUND

A decrease in rainfall is one of the main constraints on wheat production, although the association of wheat with arbuscular mycorrhizal fungi (AMF) may be an alternative for crop production under drought conditions. In the present study, we used three wheat cultivars (Purple, Ilustre and Maxi Baer) inoculated with two AMF strains, one obtained from the hyperarid Atacama Desert (northern Chile; Fm) and the other obtained from southern Chile (Cc). Plants were maintained under two irrigation conditions (normal irrigation and drought) and the physiological behaviour and enzymatic and nonenzymatic antioxidant activities in the shoots were determined. In addition, the phenolic compounds were identified by high-performance liquid chromatography-dioide array detection-electrospray ionization-tandem mass spectrometry and quantified.

RESULTS

AMF colonization produced higher levels of efficiency of photosystem II and photosynthetic pigments. High values of catalase in Purple-Cc, ascorbate peroxidase in Purple-Cc, glutathione reductase in Maxi-Cc and superoxide dismutase in Purple-Cc, all under stress, were registered. Of the inoculated cultivars, Purple-Cc showed the highest flavonoid levels, whereas hydroxycinnamic acids were higher in Maxi-Fm without drought, with apigenin and luteolin being the most abundant. High levels of phenols were present in the Ilustre-Fm plants without drought. Under normal irrigation, high levels of antioxidant activity were registered in the AMF treatments, whereas, under stress conditions, in general, high values were observed under the Fm inoculation.

CONCLUSION

Our results showed that the greatest antioxidant activity and phenolic content occurred in wheat plants inoculated with AMF, indicating their influence on coping with water stress, which is of importance in vast areas where global climate change is resulting in diminished rainfall. © 2021 Society of Chemical Industry.

Chiral NMR analysis reveals the environmental dependence of areolal scalemization in Piptothrix areolare

The occurrence of racemic and enantiomerically enriched (scalemic) mixtures of secondary metabolites in their natural sources is a rare phenomenon. The unprecedent case of enantiomeric variations from levorotatory to dextrorotatory, and back to levorotatory, passing through an almost racemic mixture, was recently documented for areolal, the major epoxythymol of Piptothrix areolare. In an attempt to shed some light to understand the reasons for such an unusual behavior, herein, we evaluated this phenomenon by correlating the areolal enantiomeric purity with several environmental variables, including temperature, humidity, rain precipitation, wind speed, and radiation during over 1 year of the plant life cycle. The specific rotation and enantiomeric excess determined by ¹ H-NMR-BINOL measurements provided the scalemic variations of areolal samples isolated from the roots collected from the same location along a 427-day period. The ¹ H-NMR-BINOL methodology provided better sensitivity to enantiomeric variations than specific rotation measurements. Statistical data, including matrix correlation analysis, exploratory analysis by heatmap plotting, and the principal component analysis (PCA), suggested direct correlation of the scalemic variation with humidity, rain precipitation, and radiation variables with the best PCA explanation (78.4%) and noncritical or poor correlations in PCA explained in 60.2% and 48.4%, respectively. When variations in the optical activity parameter of any metabolite are observed, the search for scalemic mixtures along their host plant life cycle should be undertaken. Herein, this phenomenon could be associated with interactions with soil microorganisms and with evolutionary aspects of Piptothrix areolare which belongs to Asteraceae, one of the most successfully adaptable plant families.

Influence of Burning-Induced Electrical Signals on Photosynthesis in Pea Can Be Modified by Soil Water Shortage

Local damage to plants can induce fast systemic physiological changes through generation and propagation of electrical signals. It is known that electrical signals influence numerous physiological processes including photosynthesis; an increased plant tolerance to actions of stressors is a result of these changes. It is probable that parameters of electrical signals and fast physiological changes induced by these signals can be modified by the long-term actions of stressors; however, this question has been little investigated. Our work was devoted to the investigation of the parameters of burning-induced electrical signals and their influence on photosynthesis under soil water shortage in pea seedlings. We showed that soil water shortage decreased the amplitudes of the burning-induced depolarization signals (variation potential) and the magnitudes of photosynthetic inactivation (decreasing photosynthetic CO₂ assimilation and linear electron flow and increasing non-photochemical quenching of the chlorophyll fluorescence and cyclic electron flow around photosystem I) caused by these signals. Moreover, burning-induced hyperpolarization signals (maybe, system potentials) and increased photosynthetic CO₂ assimilation could be observed under strong water shortage. It was shown that the electrical signal-induced increase of the leaf stomatal conductance was a potential mechanism for the burning-induced activation of photosynthetic CO₂ assimilation under strong water shortage; this mechanism was not crucial for photosynthetic response under control conditions or weak water shortage. Thus, our results show that soil water shortage can strongly modify damage-induced electrical signals and fast physiological responses induced by these signals.

Photosynthetic Induction Under Fluctuating Light Is Affected by Leaf Nitrogen Content in Tomato

The response of photosynthetic CO₂ assimilation to changes of illumination affects plant growth and crop productivity under natural fluctuating light conditions. However, the effects of nitrogen (N) supply on photosynthetic physiology after transition from low to high light are seldom studied. To elucidate this, we measured gas exchange and chlorophyll fluorescence under fluctuating light in tomato (Solanum lycopersicum) seedlings grown with different N conditions. After transition from low to high light, the induction speeds of net CO₂ assimilation (A _N ), stomatal conductance (g _s ), and mesophyll conductance (g _m ) delayed with the decline in leaf N content. The time to reach 90% of maximum A _N , g _s and g _m was negatively correlated with leaf N content. This delayed photosynthetic induction in plants grown under low N concentration was mainly caused by the slow induction response of g _m rather than that of g _s . Furthermore, the photosynthetic induction upon transfer from low to high light was hardly limited by photosynthetic electron flow. These results indicate that decreased leaf N content declines carbon gain under fluctuating light in tomato. Increasing the induction kinetics of g _m has the potential to enhance the carbon gain of field crops grown in infertile soil.

Limits to photosynthesis: seasonal shifts in supply and demand for CO2 in Scots pine

Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (A_net ) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (g_m ) and its effect on the estimation of carboxylation capacity ( VCmax ). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over A_net . The seasonality of A_net was strongly dependent on VCmax and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that VCmax limited A_net in the spring, whereas daily temperatures and the gradual reduction of light availability limited A_net in the autumn, despite relatively high VCmax . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between g_m and A_net , but not between g_m and stomatal conductance.

Comparisons of photosynthetic and anatomical traits between wild and domesticated cotton

Mesophyll conductance (gm) is a crucial leaf trait contributing to the photosynthetic rate (AN). Plant domestication typically leads to an enhancement of AN that is often associated with profound anatomical modifications, but it is unclear which of these structural alterations influence gm. We analyzed the implication of domestication on leaf anatomy and its effect on gm in 26 wild and 31 domesticated cotton genotypes (Gossypium sp.) grown under field conditions. We found that domesticated genotypes had higher AN but similar gm to wild genotypes. Consistent with this, domestication did not translate into significant differences in the fraction of mesophyll occupied by intercellular air spaces (fias) or mesophyll and chloroplast surface area exposed to intercellular air space (Sm/S and Sc/S, respectively). However, leaves of domesticated genotypes were significantly thicker, with larger but fewer mesophyll cells with thinner cell walls. Moreover, domesticated genotypes had higher cell wall conductance (gcw) but smaller cytoplasmic conductance (gcyt) than wild genotypes. It appears that domestication in cotton has not generally led to significant improvement in gm, in part because their thinner mesophyll cell walls (increasing gcw) compensate for their lower gcyt, itself due to larger distance between plasmalemma and chloroplast envelopes.

Mesophyll conductance exerts a significant limitation on photosynthesis during light induction

Past studies have established mesophyll diffusion conductance to CO₂ (g_m ) as a variable and significant limitation to plant photosynthesis under steady-state conditions. However, the role of g_m in influencing photosynthesis (A) during the transient period of light induction is largely unknown. We combined gas exchange measurements with laser-enabled carbon isotope discrimination measurements to assess g_m during photosynthetic induction, using Arabidopsis as the measurement species. Our measurements revealed three key findings: (1) we found that the rate at which g_m approached steady state during induction was not necessarily faster than the induction rate of the carboxylation process, contradictory to what has been suggested in previous studies; (2) g_m displayed a strong and consistent coordination with A under both induction and steady-state settings, hinting that the mechanism driving g_m -A coupling does not require physiological stability as a prerequisite; and (3) photosynthetic limitation analysis of our data revealed that when integrated over the entire induction period, the relative limitation of A imposed by g_m can be as high as > 35%. The present study provides the first demonstration of the important role of g_m in limiting CO₂ assimilation during photosynthetic induction, thereby pointing to a need for more research attention to be devoted to g_m in future induction studies.

Dynamic changes in cell wall composition of mature sunflower leaves under distinct water regimes affect photosynthesis

In previous work, we identified that exposure to limited water availability induced changes in cell wall composition of mature Helianthus annuus L. leaves that affected mesophyll conductance to CO2 diffusion (gm). However, it is unclear on which timescale these changes in cell wall composition occurred. Here, we subjected H. annuus to control (i.e. water availability), different levels of short-term water deficit stress (ST), long-term water deficit stress (LT), and long-term water deficit stress followed by gradual recoveries addressed at different timescales (LT-Rec) to evaluate the dynamics of modifications in the main composition of cell wall (cellulose, hemicelluloses, pectins and lignins) affecting photosynthesis. During gradual ST treatments, pectins enhancement was associated with gm decline. However, during LT-Rec, pectins content decreased significantly after only 5 h, while hemicelluloses and lignins amounts changed after 24 h, all being uncoupled from gm. Surprisingly, lignins increased by around 200% compared with control and were related to stomatal conductance to gas diffusion (gs) during LT-Rec. Although we suspect that the accuracy of the protocols to determine cell wall composition should be re-evaluated, we demonstrate for the first time that a highly dynamic cell wall composition turnover differently affects photosynthesis in plants subjected to distinct water regimes.

Amelioration of plant responses to drought under elevated CO2 by rejuvenating photosynthesis and nitrogen use efficiency: implications for future climate-resilient crops

The contemporary global agriculture is beset with serious threats from diverse eco-environmental conditions causing decreases in crop yields by ~ 15%. These yield losses might increase further due to climate change scenarios leading to increased food prices triggering social unrest and famines. Urbanization and industrialization are often associated with rapid increases in greenhouse gases (GHGs) especially atmospheric CO₂ concentration [(CO₂)]. Increase in atmospheric [CO₂] significantly improved crop photosynthesis and productivity initially which vary with plant species, genotype, [CO₂] exposure time and biotic as well as abiotic stress factors. Numerous attempts have been made using different plant species to unravel the physiological, cellular and molecular effects of elevated [CO₂] as well as drought. This review focuses on plant responses to elevated [CO₂] and drought individually as well as in combination with special reference to physiology of photosynthesis including its acclimation. Furthermore, the functional role of nitrogen use efficiency (NUE) and its relation to photosynthetic acclimation and crop productivity under elevated [CO₂] and drought are reviewed. In addition, we also discussed different strategies to ameliorate the limitations of ribulose-1,5-bisphosphate (RuBP) carboxylation and RuBP regeneration. Further, improved stomatal and mesophyll conductance and NUE for enhanced crop productivity under fast changing global climate conditions through biotechnological approaches are also discussed here. We conclude that multiple gene editing approaches for key events in photosynthetic processes would serve as the best strategy to generate resilient crop plants with improved productivity under fast changing climate.

Adjustments in photosynthesis and leaf water relations are related to changes in cell wall composition in Hordeum vulgare and Triticum aestivum subjected to water deficit stress

In the current climate change scenario, understanding crops' physiological performance under water shortage is crucial to overcome drought periods. Although the implication of leaf water relations maintaining leaf turgor and stomatal functioning under water deprivation has been suggested, the relationships between photosynthesis and osmotic and elastic adjustments remain misunderstood. Similarly, only few studies in dicotyledonous analysed how changes in cell wall composition affected photosynthesis and leaf water relations under drought. To induce modifications in photosynthesis, leaf water relations and cell wall composition, Hordeum vulgare and Triticum aestivum were subjected to different water regimes: control (CL, full irrigation), moderate and severe water deficit stress (Mod WS and Sev WS, respectively). Water shortage decreased photosynthesis mainly due to stomatal conductance (g_s) declines, being accompanied by reduced osmotic potential at full turgor (π_o) and increased bulk modulus of elasticity (ε). Whereas both species enhanced pectins when intensifying water deprivation, species-dependent adjustments occurred for cellulose and hemicelluloses. From these results, we showed that π_o and ε influenced photosynthesis, particularly, g_s. Furthermore, the (Cellulose+Hemicelluloses)/Pectins ratio determined ε and mesophyll conductance (g_m) in grasses, presenting the lowest pectins content within angiosperms. Thus, we highlight the relevance of cell wall composition regulating grasses physiology during drought acclimation.

Overproduction of ABA in rootstocks alleviates salinity stress in tomato shoots

To determine whether root-supplied ABA alleviates saline stress, tomato (Solanum lycopersicum L. cv. Sugar Drop) was grafted onto two independent lines (NCED OE) overexpressing the SlNCED1 gene (9-cis-epoxycarotenoid dioxygenase) and wild type rootstocks. After 200 days of saline irrigation (EC = 3.5 dS m^-1 ), plants with NCED OE rootstocks had 30% higher fruit yield, but decreased root biomass and lateral root development. Although NCED OE rootstocks upregulated ABA-signalling (AREB, ATHB12), ethylene-related (ACCs, ERFs), aquaporin (PIPs) and stress-related (TAS14, KIN, LEA) genes, downregulation of PYL ABA receptors and signalling components (WRKYs), ethylene synthesis (ACOs) and auxin-responsive factors occurred. Elevated SlNCED1 expression enhanced ABA levels in reproductive tissue while ABA catabolites accumulated in leaf and xylem sap suggesting homeostatic mechanisms. NCED OE also reduced xylem cytokinin transport to the shoot and stimulated foliar 2-isopentenyl adenine (iP) accumulation and phloem transport. Moreover, increased xylem GA₃ levels in growing fruit trusses were associated with enhanced reproductive growth. Improved photosynthesis without changes in stomatal conductance was consistent with reduced stress sensitivity and hormone-mediated alteration of leaf growth and mesophyll structure. Combined with increases in leaf nutrients and flavonoids, systemic changes in hormone balance could explain enhanced vigour, reproductive growth and yield under saline stress.

Leaf nitrogen have a better relationship with photosynthesis performance across wheat species under elevated CO2 and drought

Elevated CO₂ concentrations and persistent drought are predicted to inhibit the photosynthetic performance and yield of crops. Such effects vary considerably between species groups. Modern cultivated (Triticum aestivum L., AABBDD) and wild (Triticum dicoccoides Korn L., AABB and Triticum monococcum L., AA) species of wheat were subjected to elevated CO₂ concentration (ambient concentration +200 μmol mol^-1) and drought (well watered: 75-85% of the field water capacity; drought: 50-60% of the field water capacity) at open-top chamber experimental facilities. Elevated CO₂ concentration decreased the limitation of stomatal morphology traits on stomatal conductance regulation. This could compensate the disadvantage of plants who has low stomatal density and large single stomatal area as well as low leaf water use efficiency such as modern wheat Z9023 in drought acclimation in the future CO₂ rising world. Moreover, elevated CO₂ concentration largely increased the dependence of light harvesting and electron transportation performance per photosynthesis system II reaction center, maximum rubisco carboxylation rate, and maximum Ribulose-1,5-bisphosphate regeneration rate on leaf nitrogen concentration across the selected wheat species and water regimes. Modern cultivated cultivars Z9023 and CH58 have higher photosynthetic performance per unit of leaf nitrogen than wild species under elevated CO₂ concentrations. The increasing CO₂ may present opportunities to breeders and possibly allow them to select for cultivars with better photosynthetic nitrogen use efficiency response to future CO₂ rising climate.

Phenotypic diploidization in plant functional traits uncovered by synthetic neopolyploids in Dianthus broteri

Whole-genome duplication and post-polyploidization genome downsizing play key roles in the evolution of land plants; however, the impact of genomic diploidization on functional traits still remains poorly understood. Using Dianthus broteri as a model, we compared the ecophysiological behaviour of colchicine-induced neotetraploids (4xNeo) to diploids (2x) and naturally occurring tetraploids (4xNat). Leaf gas-exchange and chlorophyll fluorescence analyses were performed in order to asses to what extent post-polyploidization evolutionary processes have affected 4xNat. Genomic diploidization and phenotypic novelty were evident. Distinct patterns of variation revealed that post-polyploidization processes altered the phenotypic shifts directly mediated by genome doubling. The photosynthetic phenotype was affected in several ways but the main effect was phenotypic diploidization (i.e. 2x and 4xNat were closer to each other than to 4xNeo). Overall, our results show the potential benefits of considering experimentally synthetized versus naturally established polyploids when exploring the role of polyploidization in promoting functional divergence.

Reduced photosynthesis in Arabidopsis thaliana atpme17.2 and atpae11.1 mutants is associated to altered cell wall composition

The cell wall is a complex and dynamic structure that determines plants' performance by constant remodeling of its compounds. Although cellulose is its major load-bearing component, pectins are crucial to determine wall characteristics. Changes in pectin physicochemical properties, due to pectin remodeling enzymes (PRE), induce the rearrangement of cell wall compounds, thus, modifying wall architecture. In this work, we tested for the first time how cell wall dynamics affect photosynthetic properties in Arabidopsis thaliana pectin methylesterase atpme17.2 and pectin acetylesterase atpae11.1 mutants in comparison to wild-type Col-0. Our results showed maintained PRE activities comparing mutants with wild-type and no significant differences in cellulose, but cell wall non-cellulosic neutral sugars contents changed. Particularly, the amount of galacturonic acid (GalA) - which represents to some extent the pectin cell wall proportion - was reduced in the two mutants. Additionally, physiological characterization revealed that mutants presented a decreased net CO₂ assimilation (A_N ) because of reductions in both stomatal (g_s ) and mesophyll conductances (g_m ). Thus, our results suggest that atpme17.2 and atpae11.1 cell wall modifications due to genetic alterations could play a significant role in determining photosynthesis.

Linking remote sensing parameters to CO2 assimilation rates at a leaf scale

Solar-induced chlorophyll fluorescence (SIF) and photochemical reflectance index (PRI) are expected to be useful for remote sensing of photosynthetic activity at various spatial scales. This review discusses how chlorophyll fluorescence and PRI are related to the CO₂ assimilation rate at a leaf scale. Light energy absorbed by photosystem II chlorophylls is allocated to photochemistry, fluorescence, and heat dissipation evaluated as non-photochemical quenching (NPQ). PRI is correlated with NPQ because it reflects the composition of xanthophylls, which are involved in heat dissipation. Assuming that NPQ is uniquely related to the photochemical efficiency (quantum yield of photochemistry), photochemical efficiencies can be assessed from either chlorophyll fluorescence or PRI. However, this assumption may not be held under some conditions such as low temperatures and photoinhibitory environments. Even in such cases, photosynthesis may be estimated more accurately if both chlorophyll fluorescence and PRI are determined simultaneously. To convert from photochemical efficiency to CO₂ assimilation, environmental responses in stomatal conductance also need to be considered. Models linking chlorophyll fluorescence and PRI with CO₂ assimilation rates will contribute to understanding and future prediction of the global carbon cycle.

Cell wall thickness and composition are involved in photosynthetic limitation

The key role of cell walls in setting mesophyll conductance to CO2 (gm) and, consequently, photosynthesis is reviewed. First, the theoretical properties of cell walls that can affect gm are presented. Then, we focus on cell wall thickness (Tcw) reviewing empirical evidence showing that Tcw varies strongly among species and phylogenetic groups in a way that correlates with gm and photosynthesis; that is, the thicker the mesophyll cell walls, the lower the gm and photosynthesis. Potential interplays of gm, Tcw, dehydration tolerance, and hydraulic properties of leaves are also discussed. Dynamic variations of Tcw in response to the environment and their implications in the regulation of photosynthesis are discussed, and recent evidence suggesting an influence of cell wall composition on gm is presented. We then propose a hypothetical mechanism for the influence of cell walls on photosynthesis, combining the effects of thickness and composition, particularly pectins. Finally, we discuss the prospects for using biotechnology for enhancing photosynthesis by altering cell wall-related genes.

Photosynthetic activity increases with leaf size and intercellular spaces in an allomorphic lianescent aroid Rhodospatha oblongata

This study aimed to investigate leaf anatomy, as well as photosynthetic gas exchange, that underlie the improvement in light foraging capacity, which appears to occur in aroid vines seeking light exposure. Three levels of plant height (soil level, 3 m and 6 m) were categorised for the aroid vine Rhodospatha oblongata Poepp. to represent the transition from ground to canopy. Compared with shaded leaves, leaves exposed to high light conditions were thicker, presenting a larger, spongy parenchyma characterised by a larger transversal area of intercellular spaces. In addition to the increase in maximum CO2 assimilation (Amax) and thicker and larger leaf lamina, we found an increased light saturation point, light compensation point and water use efficiency at 500 µmol PPFD. Nitrogen content per leaf dry mass remained constant across habitats, but Amax/N was 1.5-times greater in the canopy position than in the leaves at soil level, suggesting that CO2 gain did not rely on an N-related biochemical apparatus. The lower δ13C discrimination observed at high canopy leaves corroborated the higher photosynthesis. Altogether, these results suggest that the large and exposed aroid leaves maintained carbon gain coupled with light gain through investing in a more efficient proportion of intercellular spaces and photosynthetic cell surface, which likely allowed a less pronounced CO2 gradient in substomatal-intercellular space.