The role of leaf width and conductances to CO2 in determining water use efficiency in C4 grasses

Stomatal response to VPD in C4 plants with different biochemical sub-pathways

C4 NAD-malic enzyme (NAD-ME) species occurs in drier regions and exhibit different drought responses compared to C4 NADP-malic enzyme (NADP-ME) species. However, a physiological mechanism explaining the geographical discrepancies remains uncertain. This study examined gas exchange patterns that might explain different distributions observed between two subtypes of C4 photosynthesis. We measured the response of leaf gas exchange to vapour pressure deficit (VPD) and CO2 in plants from six distinct C4 clades having closely related NAD-ME and NADP-ME species using a Li-Cor 6400 gas exchange system. We found that NAD-ME species exhibited greater relative reductions in stomatal conductance with increases in VPD than NADP-ME species but observed no consistent subtype differences in C4 cycle activity as indicated by the initial slope of the A response to intercellular CO2 concentration. Based on these results, we hypothesise the greater response of gs to increasing VPD may enable NAD-ME plants to outperform NADP-ME plants in hot, dry environments where VPD is normally high.

Harnessing photosynthetic C18O16O discrimination dynamics under leaf water nonsteady state to estimate mesophyll conductance: a new, regression-based method

Mesophyll conductance (gm) is a crucial plant trait that can significantly limit photosynthesis. Measurement of photosynthetic C18O16O discrimination (Δ18O) has proved to be the only viable means of resolving gm in both C3 and C4 plants. However, the currently available methods to exploit Δ18O for gm estimation are error prone due to their inadequacy in constraining the degree of oxygen isotope exchange (θ) during mesophyll CO2 hydration. Here, we capitalized on experimental manipulation of leaf water isotopic dynamics to establish a novel, nonsteady state, regression-based approach for simultaneous determination of gm and θ from online Δ18O measurements. We demonstrated the methodological and theoretical robustness of this new Δ18O-gm estimation approach and showed through measurements on several C3 and C4 species that this approach can serve as a benchmark method against which to identify previously-unrecognized biases of the existing Δ18O-gm methods. Our results highlight the unique value of this nonsteady state-based approach for contributing to ongoing efforts toward quantitative understanding of mesophyll conductance for crop yield improvement and carbon cycle modeling.

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.

Schizachyrium scoparium (C4) better tolerates drought than Andropogon gerardii (C4) via constant CO2 supply for photosynthesis during water stress

Climate change is dramatically altering global precipitation patterns across terrestrial ecosystems, making it critically important that we understand both how and why plant species vary in their drought sensitivities. Andropogon gerardii and Schizachyrium scoparium, both C4 grasses, provide a model system for understanding the physiological mechanisms that determine how species of a single functional type can differ in drought responses, an issue remains a critical gap in our ability to model and predict the impacts of drought on grassland ecosystems. Despite its greater lability of foliar water content, previous experiments have demonstrated that S. scoparium maintains higher photosynthetic capacity during droughts. It is therefore likely that the ability of S. scoparium to withstand drought instead derives from a greater metabolic resistance to drought. Here, we tested the following hypotheses: (H1) A. gerardii is more vulnerable to drought than S. scoparium at both the population and organismal levels, (H2) A. gerardii is less stomatally flexible than S. scoparium, and (H3) A. gerardii is more metabolically limited than S. scoparium. Our results indicate that it is actually stomatal limitations of CO2 supply that limit A. gerardii photosynthesis during drought. Schizachyrium scoparium was more drought-resistant than A. gerardii based on long-term field data, organismal biomass production and physiological gas exchange measurements. While both S. scoparium and A. gerardii avoided metabolic limitation of photosynthesis, CO2 supply of A. gerardii was greatly reduced during late-stage drought stress. That two common, co-occurring C4 species possess such different responses to drought highlights the physiological variability inherent within plant functional groups and underscores the need for more studies of C4 drought tolerance.

Mulched Drip Fertigation with Growth Inhibitors Reduces Bundle-Sheath Cell Leakage and Improves Photosynthesis Capacity and Barley Production in Semi-Arid Regions

A better understanding of the factors that reduce bundle-sheath cell leakage to CO2 (Փ), enhance 13C carbon isotope discrimination, and enhance the photosynthetic capacity of barley leaves will be useful to develop a nutrient- and water-saving strategy for dry-land farming systems. Therefore, barley plants were exposed to a novel nitrification inhibitor (NI) (3,4-dimethyl-1H-pyrazol-1-yl succinic acid) (DMPSA) and a urease inhibitor (UI) (N-butyl thiophosphorictriamide (NBPT)) with mulched drip fertigation treatments, which included HF (high-drip fertigation (370 mm) under a ridge furrow system), MF (75% of HF, moderate-drip fertigation under a ridge furrow system), LF (50% of HF, low-drip fertigation under a ridge furrow system), and TP (traditional planting with no inhibitors or drip fertigation strategies). The results indicated that the nitrification inhibitor combined with mulched drip fertigation significantly reduced bundle-sheath cell leakage to CO2 (Փ) as a result of increased soil water content; this was demonstrated by the light and CO2 response curves of the photosynthesis capacity (An), the apparent quantum efficiency (α), and the 13C-photosynthate distribution. In the inhibitor-based strategy, the use of the urease and nitrification inhibitors reduced Փ by 35% and 39% compared with TP. In the NI-HF strategy, it was found that barley could retain the maximum photosynthesis capacity by increasing the leaf area index (LAI), An, rubisco content, soluble protein, dry matter per plant, and productivity. The CO2 and light response curves were considerably improved in the NI-HF and NI-MF treatments due to a higher 13C carbon isotope (Δ‱), respiration rate (Rd), and Ci/Ca, therefore obtaining the minimum Փ value. With both inhibitors, there was a significant difference between HF and LF drip fertigation. The NI-MF treatment significantly increased the grain yield, total chlorophyll content, WUE, and NUE by 52%, 47%, 57%, and 45%, respectively. Collectively, the results suggest that the new nitrification inhibitor (DMPSA) with HF or MF mulched drip fertigation could be promoted in semi-arid regions in order to mitigate bundle-sheath cell leakage to CO2 (Փ), without negatively affecting barley production and leading to the nutrient and water use efficiency of barley.

Multiple highly expressed phosphoenolpyruvate carboxylase genes have divergent enzyme kinetic properties in two C4 grasses

BACKGROUND AND AIMS

Phosphoenolpyruvate (PEP) carboxylase (PEPC) catalyses the irreversible carboxylation of PEP with bicarbonate to produce oxaloacetate. This reaction powers the carbon-concentrating mechanism (CCM) in plants that perform C4 photosynthesis. This CCM is generally driven by a single PEPC gene product that is highly expressed in the cytosol of mesophyll cells. We found two C4 grasses, Panicum miliaceum and Echinochloa colona, that each have two highly expressed PEPC genes. We characterized the kinetic properties of the two most abundant PEPCs in E. colona and P. miliaceum to better understand how the enzyme's amino acid structure influences its function.

METHODS

Coding sequences of the two most abundant PEPC proteins in E. colona and P. miliaceum were synthesized by GenScript and were inserted into bacteria expression plasmids. Point mutations resulting in substitutions at conserved amino acid residues (e.g. N-terminal serine and residue 890) were created via site-directed PCR mutagenesis. The kinetic properties of semi-purified plant PEPCs from Escherichia coli were analysed using membrane-inlet mass spectrometry and a spectrophotometric enzyme-coupled reaction.

KEY RESULTS

The two most abundant P. miliaceum PEPCs (PmPPC1 and PmPPC2) have similar sequence identities (>95 %), and as a result had similar kinetic properties. The two most abundant E. colona PEPCs (EcPPC1 and EcPPC2) had identities of ~78 % and had significantly different kinetic properties. The PmPPCs and EcPPCs had different responses to allosteric inhibitors and activators, and substitutions at the conserved N-terminal serine and residue 890 resulted in significantly altered responses to allosteric regulators.

CONCLUSIONS

The two, significantly expressed C4Ppc genes in P. miliaceum were probably the result of genomes combining from two closely related C4Panicum species. We found natural variation in PEPC's sensitivity to allosteric inhibition that seems to bypass the conserved 890 residue, suggesting alternative evolutionary pathways for increased malate tolerance and other kinetic properties.

Midday water use efficiency in sorghum is linked to faster stomatal closure rate, lower stomatal aperture and higher stomatal density

Most studies assume midday gas exchange measurements capture the leaf's daytime performance. However, stomatal conductance (gs ) and photosynthesis (An ) fluctuate diurnally due to endogenous and environmental rhythms, which can affect intrinsic water use efficiency (iWUE). Six Sorghum lines with contrasting stomatal anatomical traits were grown in environmentally controlled conditions, and leaf gas exchange was measured three times a day. Stomatal anatomy and kinetic responses to light transients were also measured. The highest An and gs and the lowest iWUE were observed at midday for most lines. Diurnally averaged iWUE correlated positively with morning and midday iWUE and negatively with the time taken for stomata to close after transition to low light intensity (kclose ). There was significant variation among sorghum lines for kclose , and smaller kclose correlated with lower gs and higher stomatal density (SD) across the lines. In turn, gs was negatively correlated with SD and regulated by the operational stomatal aperture regardless of stomatal size. Altogether, our data suggest a common physiology to improve iWUE in sorghum related to the control of water loss without impacting photosynthesis relying on higher SD, lower stomatal aperture and faster stomatal closing in response to low light intensity.

Anatomical determinants of gas exchange and hydraulics vary with leaf shape in soybean

BACKGROUND AND AIMS

Leaf shape in crops can impact light distribution and carbon capture at the whole plant and canopy level. Given similar leaf inclination, narrow leaves can allow a greater fraction of incident light to pass through to lower canopy leaves by reducing leaf area index, which can potentially increase canopy-scale photosynthesis. Soybean has natural variation in leaf shape which can be utilized to optimize canopy architecture. However, the anatomical and physiological differences underlying variation in leaf shape remain largely unexplored.

METHODS

In this study, we selected 28 diverse soybean lines with leaf length to width ratios (leaf ratio) ranging between 1.1 and 3.2. We made leaf cross-sectional, gas exchange, vein density and hydraulic measurements and studied their interrelationships among these lines.

KEY RESULTS

Our study shows that narrow leaves tend to be thicker, with an ~30 µm increase in leaf thickness for every unit increase in leaf ratio. Interestingly, thicker leaves had a greater proportion of spongy mesophyll while the proportions of palisade and paraveinal mesophyll decreased. In addition, narrow and thicker leaves had greater photosynthesis and stomatal conductance per unit area along with greater leaf hydraulic conductance.

CONCLUSIONS

Our results suggest that selecting for narrow leaves can improve photosynthetic performance and potentially provide a yield advantage in soybean.

Anatomical drivers of stomatal conductance in sorghum lines with different leaf widths grown under different temperatures

Sustaining crop productivity and resilience in water-limited environments and under rising temperatures are matters of concern worldwide. We investigated the leaf anatomical traits that underpin our recently identified link between leaf width (LW) and intrinsic water use efficiency (iWUE), as traits of interest in plant breeding. Ten sorghum lines with varying LW were grown under three temperatures to expand the range of variation of both LW and gas exchange rates. Leaf gas exchange, surface morphology and cross-sectional anatomy were measured and analysed using structural equations modelling. Narrower leaves had lower stomatal conductance (g_s ) and higher iWUE across growth temperatures. They also had smaller intercellular airspaces, stomatal size, percentage of open stomatal aperture relative to maximum, hydraulic pathway, mesophyll thickness, and leaf mass per area. Structural modelling revealed a developmental association among leaf anatomical traits that underpinned g_s variation in sorghum. Growing temperature and LW both impacted leaf gas exchange rates, but only LW directly impacted leaf anatomy. Wider leaves may be more productive under well-watered conditions, but consume more water for growth and development, which is detrimental under water stress.

CO2 mesophyll conductance regulated by light: a review

MAIN CONCLUSION

Light quality and intensity regulate plant mesophyll conductance, which has played an essential role in photosynthesis by controlling leaf structural and biochemical properties. Mesophyll conductance (g_m), a crucial physiological factor influencing the photosynthetic rate of leaves, is used to describe the resistance of CO₂ from the sub-stomatal cavity into the chloroplast up to the carboxylation site. Leaf structural and biochemical components, as well as external environmental factors such as light, temperature, and water, all impact g_m. As an essential factor of plant photosynthesis, light affects plant growth and development and plays a vital role in regulating g_m as well as determining photosynthesis and yield. This review aimed to summarize the mechanisms of g_m response to light. Both structural and biochemical perspectives were combined to reveal the effects of light quality and intensity on the g_m, providing a guide for selecting the optimal conditions for intensifying photosynthesis in plants.

Cultivation model and deficit irrigation strategy for reducing leakage of bundle sheath cells to CO2, improve 13C carbon isotope, photosynthesis and soybean yield in semi-arid areas

A better understanding of the photosynthesis and soil water storage regulation of soybean production will be helpful to develop a water conservation strategy under a rain-fed farming system. Reducing the leakage of CO2 bundle sheath cells and improving the photosynthesis capacity and gas exchange characteristics of soybean leaves will contribute to increase yield under the dryland agricultural system and provide a scientific basis. Therefore, during 2019 and 2020, soybean exposed to different cultivation modes to analyze the response curves of photosynthesis and CO2 under different deficit irrigation strategies. In this study, we used two cultivation models: RB: ridge covered with biodegradable film and furrow area not covered; CF: conventional flat land planting under four deficit irrigation modes (R: rainwater irrigation; IB: branch stage irrigation (220 mm); IP: Irrigation during podding (220 mm); IBP: branch stage irrigation (110 mm), podding stage irrigation (110 mm). Compared with CF-IBP treatment, RB-IBP had significant effects on rainwater collection, SWS, and soybean yield. Photo-response curve analysis showed that RB-IBP treatment a significant increase in Pn, Gs, Ci, Tr, leaf WUE, and chlorophyll ab content. Under different irrigation strategies, maximum net photosynthetic rate (Pnmax), light saturation point (LSP), and apparent quantum efficiency under RB-IBP treatment (α), Pn under respiration rate and CO2 response curve were significantly higher than that under CF cultivation mode. Compared with RB culture mode under different irrigation strategies, CF cultivation mode significantly increases Δ13C and CO2 sheath cell leakage (Փ); it also led to a significant decline in the ratio of Ci/Ca concentration. This study shows that RB-IBP treatment is the best water-saving strategy because it means reducing the leakage of CO2 from the bundle sheath, thus significantly increasing soil water storage, photosynthetic capacity, and soybean yield.

Elevated [CO2] negatively impacts C4 photosynthesis under heat and water stress without penalizing biomass

Elevated [CO2] (eCO2) and water stress reduce leaf stomatal conductance (gs), which may affect leaf thermoregulation during heat waves (heat stress). Two sorghum lines, with different leaf width were grown in a glasshouse at a mean day temperature of 30 °C, under different [CO2] and watering levels, and subjected to heat stress (43 °C) for 6 d at the start of the reproductive stage. We measured leaf photosynthetic and stomatal responses to light transients before harvesting the plants. Photosynthesis at growth conditions (Agrowth) and biomass accumulation were enhanced by eCO2 under control conditions. Heat stress increased gs, especially in wider leaves, and reduced the time constant of stomatal opening (kopen) at ambient [CO2] but not eCO2. However, heat stress reduced photosynthesis under water stress and eCO2 due to increased leaf temperature and reduced evaporative cooling. eCO2 prevented the reduction of biomass under both water and heat stress, possibly due to improved plant and soil water status as a result of reduced gs. Our results suggest that the response of the C4 crop sorghum to future climate conditions depends on the trade-off between low gs needed for high water use efficiency and drought tolerance, and the high gs needed for improved thermoregulation and heat tolerance under an eCO2 future.

Mesophyll conductance response to short-term changes in pCO2 is related to leaf anatomy and biochemistry in diverse C4 grasses

Mesophyll CO₂ conductance (g_m ) in C₃ species responds to short-term (minutes) changes in environment potentially due to changes in leaf anatomical and biochemical properties and measurement artefacts. Compared with C₃ species, there is less information on g_m responses to short-term changes in environmental conditions such as partial pressure of CO₂ (pCO₂ ) across diverse C₄ species and the potential determinants of these responses. Using 16 C₄ grasses we investigated the response of g_m to short-term changes in pCO₂ and its relationship with leaf anatomy and biochemistry. In general, g_m increased as pCO₂ decreased (statistically significant increase in 12 species), with percentage increases in g_m ranging from +13% to +250%. Greater increase in g_m at low pCO₂ was observed in species exhibiting relatively thinner mesophyll cell walls along with greater mesophyll surface area exposed to intercellular air spaces, leaf N, photosynthetic capacity and activities of phosphoenolpyruvate carboxylase and Rubisco. Species with greater CO₂ responses of g_m were also able to maintain their leaf water-use efficiencies (TE_i ) under low CO₂ . Our study advances understanding of CO₂ response of g_m in diverse C₄ species, identifies the key leaf traits related to this response and has implications for improving C₄ photosynthetic models and TE_i through modification of g_m .

Genetic basis of sorghum leaf width and its potential as a surrogate for transpiration efficiency

Leaf width was correlated with plant-level transpiration efficiency and associated with 19 QTL in sorghum, suggesting it could be a surrogate for transpiration efficiency in large breeding program. Enhancing plant transpiration efficiency (TE) by reducing transpiration without compromising photosynthesis and yield is a desirable selection target in crop improvement programs. While narrow individual leaf width has been correlated with greater intrinsic water use efficiency in C₄ species, the extent to which this translates to greater plant TE has not been investigated. The aims of this study were to evaluate the correlation of leaf width with TE at the whole-plant scale and investigate the genetic control of leaf width in sorghum. Two lysimetry experiments using 16 genotypes varying for stomatal conductance and three field trials using a large sorghum diversity panel (n = 701 lines) were conducted. Negative associations of leaf width with plant TE were found in the lysimetry experiments, suggesting narrow leaves may result in reduced plant transpiration without trade-offs in biomass accumulation. A wide range in width of the largest leaf was found in the sorghum diversity panel with consistent ranking among sorghum races, suggesting that environmental adaptation may have a role in modifying leaf width. Nineteen QTL were identified by genome-wide association studies on leaf width adjusted for flowering time. The QTL identified showed high levels of correspondence with those in maize and rice, suggesting similarities in the genetic control of leaf width across cereals. Three a priori candidate genes for leaf width, previously found to regulate dorsoventrality, were identified based on a 1-cM threshold. This study provides useful physiological and genetic insights for potential manipulation of leaf width to improve plant adaptation to diverse environments.

Plasticity in stomatal behaviour across a gradient of water supply is consistent among field-grown maize inbred lines with varying stomatal patterning

Stomata regulate leaf CO₂ assimilation (A) and water loss. The Ball-Berry and Medlyn models predict stomatal conductance (g_s ) with a slope parameter (m or g₁ ) that reflects the sensitivity of g_s to A, atmospheric CO₂  and humidity, and is inversely related to water use efficiency (WUE). This study addressed knowledge gaps about what the values of m and g₁ are in C₄ crops under field conditions, as well as how they vary among genotypes and with drought stress. Four inbred maize genotypes were unexpectedly consistent in how m and g₁ decreased as water supply decreased. This was despite genotypic variation in stomatal patterning, A and g_s . m and g₁ were strongly correlated with soil water content, moderately correlated with predawn leaf water potential (Ψ_pd ), but not correlated with midday leaf water potential (Ψ_md ). This implied that m and g₁ respond to long-term water supply more than short-term drought stress. The conserved nature of m and g₁ across anatomically diverse genotypes and water supplies suggests there is flexibility in structure-function relationships underpinning WUE. This evidence can guide the simulation of maize g_s across a range of water supply in the primary maize growing region and inform efforts to improve WUE.

Coordination of stomata and vein patterns with leaf width underpins water-use efficiency in a C4 crop

Despite its importance for crop water use and productivity, especially in drought-affected environments, the underlying mechanisms of variation in intrinsic water-use efficiency (iWUE = net photosynthesis/stomatal conductance for water vapour, g_sw ) are not well understood, especially in C₄ plants. Recently, we discovered that leaf width (LW) correlated negatively with iWUE and positively with g_sw across several C₄ grasses. Here, we confirmed these relationships within 48 field-grown genotypes differing in LW in Sorghum bicolor, a C₄ crop adapted to dry and hot conditions. We measured leaf gas exchange and modelled leaf energy balance three times a day, alongside anatomical traits as potential predictors of iWUE. LW correlated negatively with iWUE and stomatal density, but positively with g_sw , interveinal distance of longitudinal veins, and the percentage of stomatal aperture relative to maximum. Energy balance modelling showed that wider leaves needed to open their stomata more to generate a more negative leaf-to-air temperature difference, especially at midday when air temperatures exceeded 40°C. These results highlight the important role that LW plays in shaping iWUE through coordination of vein and stomatal traits and by affecting stomatal aperture. Therefore, LW could be used as a predictor of higher iWUE among sorghum genotypes.

Rapid stomatal closure contributes to higher water use efficiency in major C4 compared to C3 Poaceae crops

Understanding water use characteristics of C3 and C4 crops is important for food security under climate change. Here, we aimed to clarify how stomatal dynamics and water use efficiency (WUE) differ in fluctuating environments in major C3 and C4 crops. Under high and low nitrogen conditions, we evaluated stomatal morphology and kinetics of stomatal conductance (gs) at leaf and whole-plant levels in controlled fluctuating light environments in four C3 and five C4 Poaceae species. We developed a dynamic photosynthesis model, which incorporates C3 and C4 photosynthesis models that consider stomatal dynamics, to evaluate the contribution of rapid stomatal opening and closing to photosynthesis and WUE. C4 crops showed more rapid stomatal opening and closure than C3 crops, which could be explained by smaller stomatal size and higher stomatal density in plants grown at high nitrogen conditions. Our model analysis indicated that accelerating the speed of stomatal closure in C3 crops to the level of C4 crops could enhance WUE up to 16% by reducing unnecessary water loss during low light periods, whereas accelerating stomatal opening only minimally enhanced photosynthesis. The present results suggest that accelerating the speed of stomatal closure in major C3 crops to the level of major C4 crops is a potential breeding target for the realization of water-saving agriculture.

High intrinsic water use efficiency is underpinned by high stomatal aperture and guard cell potassium flux in C3 and C4 grasses grown at glacial CO2 and low light

We compared how stomatal morphology and physiology control intrinsic leaf water use efficiency (iWUE) in two C3 and six C4 grasses grown at ambient (400 µmol mol-1) or glacial CO2 (180 µmol mol-1) and high (1000 µmol m-2 s-1) or low light intensity (200 µmol m-2 s-1). C4 grasses tended to have higher iWUE and CO2 assimilation rates, and lower stomatal conductance (gs), operational stomatal aperture (aop), and guard cell K+ influx rate relative to C3 grasses, while stomatal size (SS) and stomatal density (SD) did not vary according to the photosynthetic type. Overall, iWUE and gs depended most on aop and density of open stomata. In turn, aop correlated with K+ influx, stomatal opening speed on transition to high light, and SS. Species with higher SD had smaller and faster-opening stomata. Although C4 grasses operated with lower gs and aop at ambient CO2, they showed a greater potential to open stomata relative to maximal stomatal conductance (gmax), indicating heightened stomatal sensitivity and control. We uncovered promising links between aop, gs, iWUE, and K+ influx among C4 grasses, and differential K+ influx responses of C4 guard cells to low light, revealing molecular targets for improving iWUE in C4 crops.

Limitation of C4 photosynthesis by low carbonic anhydrase activity increases with temperature but does not influence mesophyll CO2 conductance

The CO2-concentrating mechanism (CCM) in C4 plants is initiated by the uptake of bicarbonate (HCO3-) via phosphoenolpyruvate carboxylase (PEPC). Generation of HCO3- for PEPC is determined by the interaction between mesophyll CO2 conductance and the hydration of CO2 to HCO3- by carbonic anhydrase (CA). Genetic reduction of CA was previously shown not to limit C4 photosynthesis under ambient atmospheric partial pressures of CO2 (pCO2). However, CA activity varies widely across C4 species and it is unknown if there are specific environmental conditions (e.g. high temperature) where CA may limit HCO3- production for C4 photosynthesis. Additionally, CA activity has been suggested to influence mesophyll conductance, but this has not been experimentally tested. We hypothesize that CA activity can limit PEPC at high temperatures, particularly at low pCO2, but does not directly influence gm. Here we tested the influence of genetically reduced CA activity on photosynthesis and gm in the C4 plant Zea mays under a range of pCO2 and temperatures. Reduced CA activity limited HCO3- production for C4 photosynthesis at low pCO2 as temperatures increased, but did not influence mesophyll conductance. Therefore, high leaf CA activity may enhance C4 photosynthesis under high temperature when stomatal conductance restricts the availability of atmospheric CO2.

Genome-Wide Association Studies of Soybean Yield-Related Hyperspectral Reflectance Bands Using Machine Learning-Mediated Data Integration Methods

In conjunction with big data analysis methods, plant omics technologies have provided scientists with cost-effective and promising tools for discovering genetic architectures of complex agronomic traits using large breeding populations. In recent years, there has been significant progress in plant phenomics and genomics approaches for generating reliable large datasets. However, selecting an appropriate data integration and analysis method to improve the efficiency of phenome-phenome and phenome-genome association studies is still a bottleneck. This study proposes a hyperspectral wide association study (HypWAS) approach as a phenome-phenome association analysis through a hierarchical data integration strategy to estimate the prediction power of hyperspectral reflectance bands in predicting soybean seed yield. Using HypWAS, five important hyperspectral reflectance bands in visible, red-edge, and near-infrared regions were identified significantly associated with seed yield. The phenome-genome association analysis of each tested hyperspectral reflectance band was performed using two conventional genome-wide association studies (GWAS) methods and a machine learning mediated GWAS based on the support vector regression (SVR) method. Using SVR-mediated GWAS, more relevant QTL with the physiological background of the tested hyperspectral reflectance bands were detected, supported by the functional annotation of candidate gene analyses. The results of this study have indicated the advantages of using hierarchical data integration strategy and advanced mathematical methods coupled with phenome-phenome and phenome-genome association analyses for a better understanding of the biology and genetic backgrounds of hyperspectral reflectance bands affecting soybean yield formation. The identified yield-related hyperspectral reflectance bands using HypWAS can be used as indirect selection criteria for selecting superior genotypes with improved yield genetic gains in large breeding populations.

High light and temperature reduce photosynthetic efficiency through different mechanisms in the C4 model Setaria viridis

C₄ plants frequently experience high light and high temperature conditions in the field, which reduce growth and yield. However, the mechanisms underlying these stress responses in C₄ plants have been under-explored, especially the coordination between mesophyll (M) and bundle sheath (BS) cells. We investigated how the C₄ model plant Setaria viridis responded to a four-hour high light or high temperature treatment at photosynthetic, transcriptomic, and ultrastructural levels. Although we observed a comparable reduction of photosynthetic efficiency in high light or high temperature treated leaves, detailed analysis of multi-level responses revealed important differences in key pathways and M/BS specificity responding to high light and high temperature. We provide a systematic analysis of high light and high temperature responses in S. viridis, reveal different acclimation strategies to these two stresses in C₄ plants, discover unique light/temperature responses in C₄ plants in comparison to C₃ plants, and identify potential targets to improve abiotic stress tolerance in C₄ crops.

Role of bundle sheath conductance in sustaining photosynthesis competence in sugarcane plants under nitrogen deficiency

The role of bundle sheath conductance (g_bs) in sustaining sugarcane photosynthesis under nitrogen deficiency was investigated. Sugarcane was grown under different levels of nitrogen supply and g_bs was estimated using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence at 21% or 2% [O₂] and varying air [CO₂] and light intensity. Maximum rates of PEPC carboxylation, Rubisco carboxylation, and ATP production increased with an increase in leaf nitrogen concentration (LNC) from 1 to 3 g m^-2. Low nitrogen supply reduced Rubisco and PEPC abundancies, the quantum efficiency of CO₂ assimilation and g_bs. Because of reduced g_bs, low photosynthetic rates were not associated with increased leakiness under nitrogen deficiency. In fact, low nitrogen supply increased bundle sheath cell wall thickness, probably accounting for low g_bs and increased estimates of [CO₂] at Rubisco sites. Effects of nitrogen on expression of ShPIP2;1 and ShPIP1;2 aquaporins did not explain changes in g_bs. Our data revealed that reduced Rubisco carboxylation was the main factor causing low sugarcane photosynthesis at low nitrogen supply, in contrast to the previous report on the importance of an impaired CO₂ concentration mechanism under N deficiency. Our findings suggest higher investment of nitrogen into Rubisco protein would favour photosynthesis and plant performance under low nitrogen availability.

Kinetic variation in grass phosphoenolpyruvate carboxylases provides opportunity to enhance C4 photosynthetic efficiency

The high rates of photosynthesis and the carbon-concentrating mechanism (CCM) in C₄ plants are initiated by the enzyme phosphoenolpyruvate (PEP) carboxylase (PEPC). The flow of inorganic carbon into the CCM of C₄ plants is driven by PEPC's affinity for bicarbonate (K_HCO3 ), which can be rate limiting when atmospheric CO₂ availability is restricted due to low stomatal conductance. We hypothesize that natural variation in K_HCO3 across C₄ plants is driven by specific amino acid substitutions to impact rates of C₄ photosynthesis under environments such as drought that restrict stomatal conductance. To test this hypothesis, we measured K_HCO3 from 20 C₄ grasses to compare kinetic properties with specific amino acid substitutions. There was nearly a twofold range in K_HCO3 across these C₄ grasses (24.3 ± 1.5 to 46.3 ± 2.4 μm), which significantly impacts modeled rates of C₄ photosynthesis. Additionally, molecular engineering of a low-HCO₃^- affinity PEPC identified key domains that confer variation in K_HCO3 . This study advances our understanding of PEPC kinetics and builds the foundation for engineering increased-HCO₃^- affinity and C₄ photosynthetic efficiency in important C₄ crops.

Planting models and mulching material strategies to reduce bundle sheath cell leakage and improve photosynthetic capacity and maize production in semi-arid climate

Better understanding of soil water storage and photosynthetic regulation of maize production will be useful to develop a water-saving strategy in rain-fed conditions. Therefore, maize crop was grown under the different cultivation practices for analyzed light and CO₂-response curves under various mulching strategies during 2017-2018 years. Six different treatments were used such as the following: PP, ridges and furrows zone covered with plastic film mulching; PS, ridges covered with plastic film and furrows zone with stalk mulching; PN, ridges covered with plastic film and furrows zone without mulching; TP, conventional flat planting with plastic film mulching; TS, conventional flat planting with stalk mulching; and TN, conventional flat planting without mulching. The PP treatment had considerable effects on rainwater collection, improved SWS, and maize productivity than that of TP treatment. Significantly increase of SWS was observed under the PP treatment as a result photosynthetic capacity (A_n) improved under light and CO₂-response curves, apparent quantum efficiency (α), respiration rate, total chlorophyll ab content, and ¹³C-photosynthates distribution in different organs. Under the PP and TP treatments, the maize might keep a great photosynthetic capacity at the post-flowering stage through improving A_n, LAI, soluble protein, Rubisco contents, and grain yield. The CO₂ and light-response curves were significantly enhanced at the PP treatment due to higher ¹³C carbon isotope (Δ‰) and Ci/Ca as a result lower bundle sheath to leakiness of CO₂ (ɸ) compared with the rest of all treatments. The results suggested that PP cultivation practice was the best water-saving strategy because it reduced bundle sheath leakiness to CO₂ (ɸ); as a result there's a significant improvement in soil water storage, LAI, ¹³C-photosynthates distribution, photosynthetic capacity parameters, and maize production.

Cold acclimation of mesophyll conductance, bundle-sheath conductance and leakiness in Miscanthus × giganteus

The cold acclimations of mesophyll conductance (g_m ), bundle-sheath conductance (g_bs ) and the CO₂ concentrating mechanism (CCM) of C₄ plants have not been well studied. Here, we estimated the temperature response of g_m , g_bs and leakiness (ϕ), the amount of concentrated CO₂ that escapes the bundle-sheath cells, for the chilling-tolerant C₄ plant Miscanthus × giganteus grown at 14 and 25°C. To estimate these parameters, we combined the C₄ -enzyme-limited photosynthesis model and the Δ¹³ C discrimination model. These combined models were parameterised using in vitro activities of carbonic anhydrase (CA), pyruvate, phosphate dikinase (PPDK), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and phosphoenolpyruvate carboxylase (PEPc). Cold-grown Miscanthus plants increased in vitro activities of RuBisCO and PPDK but decreased PEPc activity compared with warm-grown plants. Mesophyll conductance and g_bs responded strongly to measurement temperatures but did not differ between plants from the two growth temperatures. Furthermore, modelling showed that ϕ increased with measurement temperatures for both cold-grown and warm-grown plants, but was only marginally larger in cold-grown compared with warm-grown plants. Our results in Miscanthus support that g_m and g_bs are unresponsive to growth temperature and that the CCM is able to acclimate to cold through increased activity of PPDK and RuBisCO.

Emerging roles for carbonic anhydrase in mesophyll conductance and photosynthesis

Carbonic anhydrase (CA) is an abundant protein in most photosynthesizing organisms and higher plants. This review paper considers the physiological importance of the more abundant CA isoforms in photosynthesis, through their effects on CO₂ diffusion and other processes in photosynthetic organisms. In plants, CA has multiple isoforms in three different families (α, β and γ) and is mainly known to catalyze the CO₂↔HCO3- equilibrium. This reversible conversion has a clear role in photosynthesis, primarily through sustaining the CO₂ concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Despite showing the same major reaction mechanism, the three main CA families are evolutionarily distinct. For different CA isoforms, cellular localization and total gene expression as a function of developmental stage are predicted to determine the role of each family in relation to the net assimilation rate. Reaction-diffusion modeling and observational evidence support a role for CA activity in reducing resistance to CO₂ diffusion inside mesophyll cells by facilitating CO₂ transfer in both gas and liquid phases. In addition, physical and/or biochemical interactions between CAs and other membrane-bound compartments, for example aquaporins, are suggested to trigger a CO₂ -sensing response by stomatal movement. In response to environmental stresses, changes in the expression level of CAs and/or stimulated deactivation of CAs may correspond with lower photosynthetic capacity. We suggest that further studies should focus on the dynamics of the relationship between the activity of CAs (with different subcellular localization, abundance and gene expression) and limitations due to CO₂ diffusivity through the mesophyll and supply of CO₂ to photosynthetic reactions.

Recent developments in mesophyll conductance in C3, C4, and crassulacean acid metabolism plants

The conductance of carbon dioxide (CO₂ ) from the substomatal cavities to the initial sites of CO₂ fixation (g_m ) can significantly reduce the availability of CO₂ for photosynthesis. There have been many recent reviews on: (i) the importance of g_m for accurately modelling net rates of CO₂ assimilation, (ii) on how leaf biochemical and anatomical factors influence g_m , (iii) the technical limitation of estimating g_m , which cannot be directly measured, and (iv) how g_m responds to long- and short-term changes in growth and measurement environmental conditions. Therefore, this review will highlight these previous publications but will attempt not to repeat what has already been published. We will instead initially focus on the recent developments on the two-resistance model of g_m that describe the potential of photorespiratory and respiratory CO₂ released within the mitochondria to diffuse directly into both the chloroplast and the cytosol. Subsequently, we summarize recent developments in the three-dimensional (3-D) reaction-diffusion models and 3-D image analysis that are providing new insights into how the complex structure and organization of the leaf influences g_m . Finally, because most of the reviews and literature on g_m have traditionally focused on C₃ plants we review in the final sections some of the recent developments, current understanding and measurement techniques of g_m in C₄ and crassulacean acid metabolism (CAM) plants. These plants have both specialized leaf anatomy and either a spatially or temporally separated CO₂ concentrating mechanisms (C₄ and CAM, respectively) that influence how we interpret and estimate g_m compared with a C₃ plants.

Mesophyll conductance in land surface models: effects on photosynthesis and transpiration

The CO₂ transfer conductance within plant leaves (mesophyll conductance, g_m ) is currently not considered explicitly in most land surface models (LSMs), but instead treated implicitly as an intrinsic property of the photosynthetic machinery. Here, we review approaches to overcome this model deficiency by explicitly accounting for g_m , which comprises the re-adjustment of photosynthetic parameters and a model describing the variation of g_m in dependence of environmental conditions. An explicit representation of g_m causes changes in the response of photosynthesis to environmental factors, foremost leaf temperature, and ambient CO₂ concentration, which are most pronounced when g_m is small. These changes in leaf-level photosynthesis translate into a stronger climate and CO₂ response of gross primary productivity (GPP) and transpiration at the global scale. The results from two independent studies show consistent latitudinal patterns of these effects with biggest differences in GPP in the boreal zone (up to ~15%). Transpiration and evapotranspiration show spatially similar, but attenuated, changes compared with GPP. These changes are indirect effects of g_m caused by the assumed strong coupling between stomatal conductance and photosynthesis in current LSMs. Key uncertainties in these simulations are the variation of g_m with light and the robustness of its temperature response across plant types and growth conditions. Future research activities focusing on the response of g_m to environmental factors and its relation to other plant traits have the potential to improve the representation of photosynthesis in LSMs and to better understand its present and future role in the Earth system.

Increased adaxial stomatal density is associated with greater mesophyll surface area exposed to intercellular air spaces and mesophyll conductance in diverse C4 grasses

Mesophyll conductance (g_m ) is the diffusion of CO₂ from intercellular air spaces (IAS) to the first site of carboxylation in the mesophyll cells. In C₃ species, g_m is influenced by diverse leaf structural and anatomical traits; however, little is known about traits affecting g_m in C₄ species. To address this knowledge gap, we used online oxygen isotope discrimination measurements to estimate g_m and microscopy techniques to measure leaf structural and anatomical traits potentially related to g_m in 18 C₄ grasses. In this study, g_m scaled positively with photosynthesis and intrinsic water-use efficiency (TE_i ), but not with stomatal conductance. Also, g_m was not determined by a single trait but was positively correlated with adaxial stomatal densities (SD_ada ), stomatal ratio (SR), mesophyll surface area exposed to IAS (S_mes ) and leaf thickness. However, g_m was not related to abaxial stomatal densities (SD_aba ) and mesophyll cell wall thickness (T_CW ). Our study suggests that greater SD_ada and SR increased g_m by increasing S_mes and creating additional parallel pathways for CO₂ diffusion inside mesophyll cells. Thus, SD_ada , SR and S_mes are important determinants of C₄ -g_m and could be the target traits selected or modified for achieving greater g_m and TE_i in C₄ species.