Mesophyll conductance decreases in the wild type but not in an ABA-deficient mutant (aba1) of Nicotiana plumbaginifolia under drought conditions

Basal ABA signaling balances transpiration and photosynthesis

The balance between the CO2 entry for photosynthesis and transpiration water loss is crucial for plant growth, and ABA signaling can affect this equilibrium. To test how ABA balances plant growth and environmental adaptation, we performed molecular genetics studies in the biotech crop Nicotiana benthamiana under well-watered or drought conditions. Studies on ABA signaling in crops are complicated by the multigenic nature of the PYR/PYL/RCAR ABA receptor family and its functional redundancy, which is particularly challenging in polyploid plants. We have generated a pentuple pyl mutant in the allotetraploid Nicotiana benthamiana through CRISPR/Cas9 gene editing. The pentuple mutant is impaired in 2 NbPYL1-like and 3 NbPYL8-like receptors, affecting the regulation of transpiration and several ABA-dependent transcriptional processes. RNA-seq and metabolite analysis revealed that the synthesis of galactinol, an essential precursor for the osmoprotective raffinose family of oligosaccharides, is ABA-dependent and impaired in the mutant under osmotic stress. In contrast, our results show that, under well-watered conditions, partial inactivation of ABA signaling leads to higher CO2 entry and photosynthesis in the mutant than in WT. Photosynthesis analyses revealed an increased CO2 diffusion capacity mediated by higher stomatal and mesophyll conductances, and higher substomatal CO2 concentration in the pentuple mutant. RNA-seq analyses revealed that genes associated with cell wall loosening (e.g., expansins) and porosity were strongly downregulated by ABA in WT. In summary, a partial relief of the ABA control on transpiration mediated by ABA receptors positively affects photosynthesis when water is not limited, at the expense of reduced water use efficiency.

The interplay of short-term mesophyll and stomatal conductance responses under variable environmental conditions

Understanding the short-term responses of mesophyll conductance (gm) and stomatal conductance (gsc) to environmental changes remains a challenging yet central aspect of plant physiology. This review synthesises our current knowledge of these short-term responses, which underpin CO2 diffusion within leaves. Recent methodological advances in measuring gm using online isotopic discrimination and chlorophyll fluorescence have improved our confidence in detecting short-term gm responses, but results need to be carefully evaluated. Environmental factors like vapour pressure deficit and CO2 concentration indirectly impact gm through gsc changes, highlighting some of the complex interactions between the two parameters. Evidence suggests that short-term responses of gm are not, or at least not fully, mechanistically linked to changes in gsc, cautioning against using gsc as a reliable proxy for gm. The overarching challenge lies in unravelling the mechanistic basis of short-term gm responses, which will contribute to the development of accurate models bridging laboratory insights with broader ecological implications. Addressing these gaps in understanding is crucial for refining predictions of gm behaviour under changing environmental conditions.

Cost-benefit analysis of mesophyll conductance: diversities of anatomical, biochemical and environmental determinants

BACKGROUND

Plants invest photosynthates in construction and maintenance of their structures and functions. Such investments are considered costs. These costs are recovered by the CO2 assimilation rate (A) in the leaves, and thus A is regarded as the immediate, short-term benefit. In photosynthesizing leaves, CO2 diffusion from the air to the carboxylation site is hindered by several structural and biochemical barriers. CO2 diffusion from the intercellular air space to the chloroplast stroma is obstructed by the mesophyll resistance. The inverses is the mesophyll conductance (gm). Whether various plants realize an optimal gm, and how much investment is needed for a relevant gm, remain unsolved.

SCOPE

This review examines relationships among leaf construction costs (CC), leaf maintenance costs (MC) and gm in various plants under diverse growth conditions. Through a literature survey, we demonstrate a strong linear relationship between leaf mass per area (LMA) and leaf CC. The overall correlation of CC vs. gm across plant phylogenetic groups is weak, but significant trends are evident within specific groups and/or environments. Investment in CC is necessary for an increase in LMA and mesophyll cell surface area (Smes). This allows the leaf to accommodate more chloroplasts, thus increasing A. However, increases in LMA and/or Smes often accompany other changes, such as cell wall thickening, which diminishes gm. Such factors that make the correlations of CC and gm elusive are identified.

CONCLUSIONS

For evaluation of the contribution of gm to recover CC, leaf life span is the key factor. The estimation of MC in relation to gm, especially in terms of costs required to regulate aquaporins, could be essential for efficient control of gm over the short term. Over the long term, costs are mainly reflected in CC, while benefits also include ultimate fitness attributes in terms of integrated carbon gain over the life of a leaf, plant survival and reproductive output.

Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks

Mesophyll conductance (gmCO2 ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O3 ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O3 and drought on gmCO2 are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O3 (ambient [35.2 nmol mol-1 as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO2 inside a leaf. The combination of O3 and WD significantly decreased net photosynthetic rate (PN ) regardless of the species. The reduction of photosynthesis was associated with a decrease in gmCO2 and stomatal conductance (gsCO2 ) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (Vcmax ) and maximum electron transport rate (Jmax ) with decreased diffusive conductance parameters. The reduction of gmCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between gmCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between gsCO2 and ABA under high O3 and WD due to impaired stomatal physiological behaviour, indicating that the reduction of PN was related to gmCO2 rather than gsCO2 . The results suggest that gmCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O3 pollution.

Drought stress reduces crop carbon gain due to delayed photosynthetic induction under fluctuating light conditions

Drought stress is a major limiting factor for crop growth and yield. Water availability in the field can cyclically change between drought and rewatering conditions, depending on precipitation patterns. Concurrently, light intensity under field conditions can fluctuate, inducing dynamic photosynthesis and transpiration during the crop growth period. The present study aimed to characterize carbon gain and water use in fluctuating light under drought and rewatering conditions in two major crops, namely rice and soybean. We conducted gas exchange measurements under fluctuating light conditions with rice and soybean plants exposed to drought treatment (9-13 days) imposed by withholding water and subsequent rewatering treatment (8-9 days). Drought stress significantly reduced the maximum CO₂ assimilation rate (A) in soybean but not in rice. Under drought conditions, A increased after a step increase in light and then gradually decreased in both crops, resulting in the significant reduction of steady-state A in rice and soybean. Moreover, drought stress delayed photosynthetic induction in both crops even when it had relatively small impact on maximum A. These results suggest that the drought effects on photosynthesis should be evaluated based on induction, maximum, and steady states. The delayed photosynthetic induction under drought owing to the reduced gas diffusional conductance via stomata resulted in a substantial loss of leaf carbon gain under fluctuating light conditions. Meanwhile, rewatering, after drought, completely recovered photosynthesis under fluctuating light in both crops. Therefore, the stability of photosynthetic induction can be a promising target to improve drought tolerance during crop breeding in the future.

Cuticular conductance of adaxial and abaxial leaf surfaces and its relation to minimum leaf surface conductance

Cuticular conductance to water (g_cw ) is difficult to quantify for stomatous surfaces due to the complexity of separating cuticular and stomatal transpiration, and additional complications arise for determining adaxial and abaxial g_cw . This has led to the neglect of g_cw as a separate parameter in most common gas exchange measurements. Here, we describe a simple technique to simultaneously estimate adaxial and abaxial values of g_cw , tested in two amphistomatous plant species. What we term the 'Red-Light method' is used to estimate g_cw from gas exchange measurements and a known CO₂ concentration inside the leaf during photosynthetic induction under red light. We provide an easy-to-use web application to assist with the calculation of g_cw . While adaxial and abaxial g_cw varies significantly between leaves of the same species we found that the ratio of adaxial/abaxial g_cw (γ_n ) is stable within a plant species. This has implications for use of generic values of g_cw when analysing gas exchange data. The Red-Light method can be used to estimate total cuticular conductance (g_cw-T ) accurately with the most common setup of gas exchange instruments, i.e. a chamber mixing the adaxial and abaxial gases, allowing for a wide application of this technique.

The α subunit of the heterotrimeric G protein regulates mesophyll CO2 conductance and drought tolerance in rice

Mesophyll conductance g_m determines CO₂ diffusion rates from mesophyll intercellular air spaces to the chloroplasts and is an important factor limiting photosynthesis. Increasing g_m in cultivated plants is a potential strategy to increase photosynthesis and intrinsic water use efficiency (WUE_i ). The anatomy of the leaf and metabolic factors such as aquaporins and carbonic anhydrases have been identified as important determinants of g_m . However, genes involved in the regulation and modulation of g_m remain largely unknown. In this work, we investigated the role of heterotrimeric G proteins in g_m and drought tolerance in rice d1 mutants, which harbor a null mutation in the Gα subunit gene, RGA1. d1 mutants in both cv Nipponbare and cv Taichung 65 exhibited increased g_m , fostering improvement in photosynthesis, WUE_i , and drought tolerance compared with wild-type. The increased surface area of mesophyll cells and chloroplasts exposed to intercellular airspaces and the reduced cell wall and chloroplast thickness in the d1 mutant are evident contributors to the increase in g_m . Our results indicate that manipulation of heterotrimeric G protein signaling has the potential to improve crop WUE_i and productivity under drought.

Role of Hydraulic Signal and ABA in Decrease of Leaf Stomatal and Mesophyll Conductance in Soil Drought-Stressed Tomato

Drought reduces leaf stomatal conductance (g_s) and mesophyll conductance (g_m). Both hydraulic signals and chemical signals (mainly abscisic acid, ABA) are involved in regulating g_s. However, it remains unclear what role the endogenous ABA plays in g_m under decreasing soil moisture. In this study, the responses of g_s and g_m to ABA were investigated under progressive soil drying conditions and their impacts on net photosynthesis (A_n) and intrinsic water use efficiency (WUE_i) were also analyzed. Experimental tomato plants were cultivated in pots in an environment-controlled greenhouse. Reductions of g_s and g_m induced a 68-78% decline of A_n under drought conditions. While soil water potential (Ψ_soil) was over -1.01 MPa, g_s reduced as leaf water potential (Ψ_leaf) decreased, but ABA and g_m kept unchanged, which indicating g_s was more sensitive to drought than g_m. During Ψ_soil reduction from -1.01 to -1.44 MPa, Ψ_leaf still kept decreasing, and both g_s and g_m decreased concurrently following to the sustained increases of ABA content in shoot sap. The g_m was positively correlated to g_s during a drying process. Compared to g_s or g_m, WUE_i was strongly correlated with g_m/g_s. WUE_i improved within Ψ_soil range between -0.83 and -1.15 MPa. In summary, g_s showed a higher sensitivity to drought than g_m. Under moderate and severe drought at Ψ_soil ≤ -1.01 MPa, furthermore from hydraulic signals, ABA was also involved in this co-ordination reductions of g_s and g_m and thereby regulated A_n and WUE_i.

An improved theory for calculating leaf gas exchange more precisely accounting for small fluxes

The widely used theory for gas exchange proposed by von Caemmerer and Farquhar (vCF) integrates molar fluxes, mole fraction gradients and ternary effects but does not account for cuticular fluxes, for separation of the leaf surface conditions or for ternary effects within the boundary layer. The magnitude of cuticular conductance to water (g_cw) is a key factor for determining plant survival in drought but is difficult to measure and often neglected in routine gas exchange studies. The vCF ternary effect is applied to the total flux without the recognition of different pathways that are affected by it. These simplifications lead to errors in estimations of stomatal conductance, intercellular carbon dioxide concentration (C_i) and other gas exchange parameters. The theory presented here is a more precise physical approach to the electrical resistance analogy for gas exchange, resulting in a more accurate calculation of gas exchange parameters. Additionally, we extend our theory, using physiological concepts, to create a model that allows us to calculate cuticular conductance to water.

Stomatal, mesophyll conductance, and biochemical limitations to photosynthesis during induction

The dynamics of leaf photosynthesis in fluctuating light affects carbon gain by plants. Mesophyll conductance (gm) limits CO2 assimilation rate (A) under the steady state, but the extent of this limitation under non-steady-state conditions is unknown. In the present study, we aimed to characterize the dynamics of gm and the limitations to A imposed by gas diffusional and biochemical processes under fluctuating light. The induction responses of A, stomatal conductance (gs), gm, and the maximum rate of RuBP carboxylation (Vcmax) or electron transport (J) were investigated in Arabidopsis (Arabidopsis thaliana (L.)) and tobacco (Nicotiana tabacum L.). We first characterized gm induction after a change from darkness to light. Each limitation to A imposed by gm, gs and Vcmax or J was significant during induction, indicating that gas diffusional and biochemical processes limit photosynthesis. Initially, gs imposed the greatest limitation to A, showing the slowest response under high light after long and short periods of darkness, assuming RuBP-carboxylation limitation. However, if RuBP-regeneration limitation was assumed, then J imposed the greatest limitation. gm did not vary much following short interruptions to light. The limitation to A imposed by gm was the smallest of all the limitations for most of the induction phase. This suggests that altering induction kinetics of mesophyll conductance would have little impact on A following a change in light. To enhance the carbon gain by plants under naturally dynamic light environments, attention should therefore be focused on faster stomatal opening or activation of electron transport.

Stress-Related Changes in the Expression and Activity of Plant Carbonic Anhydrases

The data on stress-related changes in the expression and activity of plant carbonic anhydrases (CAs) suggest that they are generally upregulated at moderate stress severity. This indicates probable involvement of CAs in adaptation to drought, high salinity, heat, high light, C_i deficit, and excess bicarbonate. The changes in CA levels under cold stress are less studied and generally represented by the downregulation of CAs excepting βCA2. Excess Cd²⁺ and deficit of Zn²⁺ specifically reduce CA activity and reduce its synthesis. Probable roles of βCAs in stress adaptation include stomatal closure, ROS scavenging and partial compensation for decreased mesophyll CO₂ conductance. βCAs play contrasting roles in pathogen responses, interacting with phytohormone signaling networks. Their role can be either negative or positive, probably depending on the host-pathogen system, pathogen initial titer, and levels of ·NO and ROS. It is still not clear why CAs are suppressed under severe stress levels. It should be noted, that the role of βCAs in the facilitation of CO₂ diffusion and their involvement in redox signaling or ROS detoxication are potentially antagonistic, as they are inactivated by oxidation or nitrosylation. Interestingly, some chloroplastic βCAs may be relocated to the cytoplasm under stress conditions, but the physiological meaning of this effect remains to be studied.

Constant ratio of Cc to Ci under various CO2 concentrations and light intensities, and during progressive drought, in seedlings of Japanese white birch

Constant mesophyll conductance (g_m), and two-resistance g_m model (involved in resistances of cell wall and chloroplast), where g_m reaches maximum under higher CO₂ concentrations, cannot describe the phenomenon that g_m decreases with increasing intercellular CO₂ concentration (C_i) under relatively higher CO₂ concentrations. Yin et al. (2020) proposed a g_m model, according to which the ratio of chloroplastic CO₂ concentration (C_c) to C_i is constant in the two-resistance g_m model, which can describe the decreasing g_m with increasing C_i. In the present study, we investigated the relationship between C_c and C_i in leaves of Japanese white birch by using simultaneous measurements of gas exchange and chlorophyll fluorescence under various CO₂ concentrations, light intensities, and during progressive drought. Across the range of ambient CO₂ from 50 to 1000 μmol mol^-1, and light intensities of 50 to 2000 μmol m^-2 s^-1, measured under well irrigation, the ratio of C_c to C_i kept constant. During the progressive drought, overestimated C_i due to stomatal patchiness and/or cuticular transpiration was empirically corrected (threshold: stomatal conductance < 0.08 mol H₂O m^-2 s^-1) from the A/C_i response measured under adequate irrigation. The ratio of C_c to C_i during progressive drought (predawn leaf potential reached ≈ - 2 MPa) also remained constant irrespective of soil drying rate in various pot sizes. The present study suggests the involvement of some physiologically regulative mechanisms to keep C_c:C_i ratio constant, which might act on g_m in addition to the physical interaction of diffusive resistances in the cell components.

Linking water relations and hydraulics with photosynthesis

For land plants, water is the principal governor of growth. Photosynthetic performance is highly dependent on the stable and suitable water status of leaves, which is balanced by the water transport capacity, the water loss rate as well as the water capacitance of the plant. This review discusses the links between leaf water status and photosynthesis, specifically focussing on the coordination of CO₂ and water transport within leaves, and the potential role of leaf capacitance and elasticity on CO₂ and water transport.

Photosynthesis and photosynthetic efficiencies along the terrestrial plant's phylogeny: lessons for improving crop photosynthesis

Photosynthesis is the basis of all life on Earth. Surprisingly, until very recently, data on photosynthesis, photosynthetic efficiencies, and photosynthesis limitations in terrestrial land plants other than spermatophytes were very scarce. Here we provide an updated data compilation showing that maximum photosynthesis rates (expressed either on an area or dry mass basis) progressively scale along the land plant's phylogeny, from lowest values in bryophytes to largest in angiosperms. Unexpectedly, both photosynthetic water (WUE) and nitrogen (PNUE) use efficiencies also scale positively through the phylogeny, for which it has been commonly reported that these two efficiencies tend to trade-off between them when comparing different genotypes or a single species subject to different environmental conditions. After providing experimental evidence that these observed trends are mostly due to an increased mesophyll conductance to CO₂ - associated with specific anatomical changes - along the phylogeny, we discuss how these findings on a large phylogenetic scale can provide useful information to address potential photosynthetic improvements in crops in the near future.

Strengthened antioxidant capacity improves photosynthesis by regulating stomatal aperture and ribulose-1,5-bisphosphate carboxylase/oxygenase activity

ABA is important for plant growth and development; however, it also inhibits photosynthesis by regulating the stomatal aperture and ribulose-1,5-bisphosphate carboxylase/oxygenase activity. Noteworthy, this negative effect can be alleviated by antioxidants including ascorbic acid (AsA) and catalase (CAT), but the underlying mechanism remains unclear. Two rice cultivars, Zhefu802 (recurrent parent) and its near-isogenic line, fgl were selected and planted in a greenhouse with 30/24 °C (day/night) under natural sunlight conditions. Compared to fgl, Zhefu802 had significantly lower net photosynthetic rate (PN) and stomatal conductance (Cond) as well as significantly higher ABA and H₂O₂ contents. However, AsA and CAT increased PN, Cond, and stomatal aperture, which decreased H₂O₂ and malondialdehyde (MDA) levels. In this process, AsA and CAT significantly increased the ribulose-1,5-bisphosphate carboxylase activity, while they strongly decreased the ribulose-1,5-bisphosphate oxygenase activity, and finally caused an obvious decrease in the ratio of photorespiration (Pr) to PN. Additionally, AsA and CAT significantly increased the expression levels of RbcS and RbcL genes of leaves, while H₂O₂ significantly decreased them, especially the RbcS gene. In summary, the removal of H₂O₂ by AsA and CAT can improve the leaf photosynthesis by alleviating the inhibition on the stomatal conductance and ribulose-1,5-bisphosphate carboxylase capacity caused by ABA.

High Stomatal Conductance in the Tomato Flacca Mutant Allows for Faster Photosynthetic Induction

Due to their slow movement and closure upon shade, partially closed stomata can be a substantial limitation to photosynthesis in variable light intensities. The abscisic acid deficient flacca mutant in tomato (Solanum lycopersicum) displays very high stomatal conductance (g_s ). We aimed to determine to what extent this substantially increased g_s affects the rate of photosynthetic induction. Steady-state and dynamic photosynthesis characteristics were measured in flacca and wildtype leaves, by the use of simultaneous gas exchange and chlorophyll fluorometry. The steady-state response of photosynthesis to CO₂, maximum quantum efficiency of photosystem II photochemistry (F_v/F_m ), as well as mesophyll conductance to CO₂ diffusion were not significantly different between genotypes, suggesting similar photosynthetic biochemistry, photoprotective capacity, and internal CO₂ permeability. When leaves adapted to shade (50 µmol m^-2 s^-1) at 400 µbar CO₂ partial pressure and high humidity (7 mbar leaf-to-air vapour pressure deficit, VPD) were exposed to high irradiance (1500 µmol m^-2 s^-1), photosynthetic induction was faster in flacca compared to wildtype leaves, and this was attributable to high initial g_s in flacca (~0.6 mol m^-2 s^-1): in flacca, the times to reach 50 (t₅₀ ) and 90% (t₉₀ ) of full photosynthetic induction were 91 and 46% of wildtype values, respectively. Low humidity (15 mbar VPD) reduced g_s and slowed down photosynthetic induction in the wildtype, while no change was observed in flacca; under low humidity, t₅₀ was 63% and t₉₀ was 36% of wildtype levels in flacca. Photosynthetic induction in low CO₂ partial pressure (200 µbar) increased g_s in the wildtype (but not in flacca), and revealed no differences in the rate of photosynthetic induction between genotypes. Effects of higher g_s in flacca were also visible in transients of photosystem II operating efficiency and non-photochemical quenching. Our results show that at ambient CO₂ partial pressure, wildtype g_s is a substantial limitation to the rate of photosynthetic induction, which flacca overcomes by keeping its stomata open at all times, and it does so at the cost of reduced water use efficiency.

Dynamic changes in ABA content in water-stressed Populus nigra: effects on carbon fixation and soluble carbohydrates

BACKGROUND AND AIMS

Hydraulic and chemical signals operate in tandem to regulate systemic plant responses to drought. Transport of abscisic acid (ABA) through the xylem and phloem from the root to shoot has been suggested to serve as the main signal of water deficit. There is evidence that ABA and its ABA-glycosyl-ester (ABA-GE) are also formed in leaves and stems through the chloroplastic 2-C-methylerythritol-5-phosphate (MEP) pathway. This study aimed to evaluate how hormonal and hydraulic signals contribute to optimize stomatal (gs), mesophyll (gm) and leaf hydraulic (Kleaf) conductance under well-watered and water-stressed conditions in Populus nigra (black poplar) plants. In addition, we assessed possible relationships between ABA and soluble carbohydrates within the leaf and stem.

METHODS

Plants were subjected to three water treatments: well-watered (WW), moderate stress (WS1) and severe stress (WS2). This experimental set-up enabled a time-course analysis of the response to water deficit at the physiological [leaf gas exchange, plant water relations, (Kleaf)], biochemical (ABA and its metabolite/catabolite quantification in xylem sap, leaves, wood, bark and roots) and molecular (gene expression of ABA biosynthesis) levels.

KEY RESULTS

Our results showed strong coordination between gs, gm and Kleaf under water stress, which reduced transpiration and increased intrinsic water use efficiency (WUEint). Analysis of gene expression of 9-cis-epoxycarotenoid dioxygenase (NCED) and ABA content in different tissues showed a general up-regulation of the biosynthesis of this hormone and its finely-tuned catabolism in response to water stress. Significant linear relationships were found between soluble carbohydrates and ABA contents in both leaves and stems, suggesting a putative function for this hormone in carbohydrate mobilization under severe water stress.

CONCLUSIONS

This study demonstrates the tight regulation of the photosynthetic machinery by levels of ABA in different plants organs on a daily basis in both well-watered and water stress conditions to optimize WUEint and coordinate whole plant acclimation responses to drought.

Elevated CO2-induced changes in mesophyll conductance and anatomical traits in wild type and carbohydrate-metabolism mutants of Arabidopsis

Decreases in photosynthetic rate, stomatal conductance (gs), and mesophyll conductance (gm) are often observed under elevated CO2 conditions. However, which anatomical and/or physiological factors contribute to the decrease in gm is not fully understood. Arabidopsis thaliana wild-type and carbon-metabolism mutants (gwd1, pgm1, and cfbp1) with different accumulation patterns of non-structural carbohydrates were grown at ambient (400 ppm) and elevated (800 ppm) CO2. Anatomical and physiological traits of leaves were measured to investigate factors causing the changes in gm and in the mesophyll resistance (expressed as the reciprocal of mesophyll conductance per unit chloroplast surface area facing to intercellular space, Sc/gm). When grown at elevated CO2, all the lines showed increases in cell wall mass, cell wall thickness, and starch content, but not in leaf thickness. gm measured at 800 ppm CO2 was significantly lower than at 400 ppm CO2 in all the lines. Changes in Sc/gm were associated with thicker cell walls rather than with excess starch content. The results indicate that the changes in gm and Sc/gm that occur in response to elevated CO2 are independent of non-structural carbohydrates, and the cell wall represents a greater limitation factor for gm than starch.

Size-dependent variation in leaf functional traits and nitrogen allocation trade-offs in Robinia pseudoacacia and Cornus controversa

Tree species vary in how they invest resources to different functions throughout their life histories, and investigating the detailed patterns of ontogenetic changes in key functional traits will aid in predicting forest dynamics and ecosystem processes. In this context, we investigated size-dependent changes in key leaf functional traits and nitrogen (N) allocation trade-offs in black locust (Robinia pseudoacacia L., an N-fixing pioneer species) and giant dogwood (Cornus controversa Hemsl., a mid-successional species), which have different life-history strategies, especially in their light use. We found that the leaf mass per area and leaf carbon concentrations increased linearly with tree size (diameter at breast height, DBH), whereas leaf N concentrations decreased nonlinearly, with U- and hump-shaped patterns in black locust and giant dogwood, respectively. We also discovered large differences in N allocation between the two species. The fraction of leaf N invested in cell walls was much higher in black locust than in giant dogwood, while the opposite was true for the light harvesting N fraction. Furthermore, these fractions were related to DBH to varying degrees: the cell wall N fraction increased with DBH for both species, whereas the light harvesting N fraction of giant dogwood decreased nonlinearly and that of black locust remained constant. Instead, black locust reduced the fraction of leaf N invested in other N pools, resulting in a smaller fraction compared to that of giant dogwood. On the other hand, both species had similar fraction of leaf N invested in ribulose-1,5-bisphosphate carboxylase/oxygenase across tree size. This study indicated that both species increased leaf mechanical toughness through characteristic changes in N allocation trade-offs over the lifetimes of the trees.

Effects of instantaneous and growth CO2 levels and abscisic acid on stomatal and mesophyll conductances

C₃ photosynthesis is often limited by CO₂ diffusivity or stomatal (g_s ) and mesophyll (g_m ) conductances. To characterize effects of stomatal closure induced by either high CO₂ or abscisic acid (ABA) application on g_m , we examined g_s and g_m in the wild type (Col-0) and ost1 and slac1-2 mutants of Arabidopsis thaliana grown at 390 or 780 μmol mol^-1 CO₂ . Stomata of these mutants were reported to be insensitive to both high CO₂ and ABA. When the ambient CO₂ increased instantaneously, g_m decreased in all these plants, whereas g_s in ost1 and slac1-2 was unchanged. Therefore, the decrease in g_m in response to high CO₂ occurred irrespective of the responses of g_s . g_m was mainly determined by the instantaneous CO₂ concentration during the measurement and not markedly by the CO₂ concentration during the growth. Exogenous application of ABA to Col-0 caused the decrease in the intercellular CO₂ concentration (C_i ). With the decrease in C_i , g_m did not increase but decreased, indicating that the response of g_m to CO₂ and that to ABA are differently regulated and that ABA content in the leaves plays an important role in the regulation of g_m .

Interspecific differences in how sink-source imbalance causes photosynthetic downregulation among three legume species

BACKGROUND AND AIMS

Sink-source imbalance could cause an accumulation of total non-structural carbohydrates (TNC; soluble sugar and starch) in source leaves. We aimed to clarify interspecific differences in how sink-source imbalance and TNC causes the downregulation of photosynthesis among three legume plants. The TNC in source leaves was altered by short-term manipulative treatments, and its effects on photosynthetic characteristics were evaluated.

METHODS

Soybean, French bean and azuki bean were grown under high nitrogen availability. After primary leaves were fully expanded, they were subjected to additional treatments: defoliation except for two primary leaves; transfer to low nitrogen conditions; transfer to low nitrogen conditions and defoliation; or irradiation by light-emitting diodes. Physiological and anatomical traits such as TNC content, maximum photosynthetic rate, cell wall content and δ13C values of primary leaves and whole-plant growth were examined.

KEY RESULTS

Among the three legume plants, the downregulation of maximum photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content was co-ordinated with an increase in TNC only in French bean. Rubisco did not decrease with an increase in TNC in soybean and azuki bean. The defoliation treatment caused an increase in cell wall content especially in soybean, and maximum photosynthesis decreased despite resulting in a higher Rubisco content. This indicates that a decrease in mesophyll conductance could cause photosynthetic downregulation, which was confirmed by an increase in δ13C.

CONCLUSION

The present results suggest that a downregulation of photosynthesis in response to increased levels of TNC in source leaves can result not only from decreases in Rubisco content, but also from anatomical factors, such as an increase in cell wall thickness leading to reduced chloroplast CO2 concentrations.

Mesophyll conductance to CO2 in leaves of Siebold's beech (Fagus crenata) seedlings under elevated ozone

Ozone is an air pollutant that negatively affects photosynthesis in woody plants. Previous studies suggested that ozone-induced reduction in photosynthetic rates is mainly attributable to a decrease of maximum carboxylation rate (V_cmax) and/or maximum electron transport rate (J_max) estimated from response of net photosynthetic rate (A) to intercellular CO₂ concentration (C_i) (A/C_i curve) assuming that mesophyll conductance for CO₂ diffusion (g_m) is infinite. Although it is known that C_i-based V_cmax and J_max are potentially influenced by g_m, its contribution to ozone responses in C_i-based V_cmax and J_max is still unclear. In the present study, therefore, we analysed photosynthetic processes including g_m in leaves of Siebold's beech (Fagus crenata) seedlings grown under three levels of ozone (charcoal-filtered air or ozone at 1.0- or 1.5-times ambient concentration) for two growing seasons in 2016-2017. Leaf gas exchange and chlorophyll fluorescence were simultaneously measured in July and September of the second growing season. We determined the A, stomatal conductance to water vapor and g_m, and analysed A/C_i curve and A/C_c curve (C_c: chloroplast CO₂ concentration). We also determined the Rubisco and chlorophyll contents in leaves. In September, ozone significantly decreased C_i-based V_cmax. At the same time, ozone decreased g_m, whereas there was no significant effect of ozone on C_c-based V_cmax or the contents of Rubisco and chlorophyll in leaves. These results suggest that ozone-induced reduction in C_i-based V_cmax is a result of the decrease in g_m rather than in carboxylation capacity. The decrease in g_m by elevated ozone was offset by an increase in C_i, and C_c did not differ depending on ozone treatment. Since C_c-based V_cmax was also similar, A was not changed by elevated ozone. We conclude that g_m is an important factor for reduction in C_i-based V_cmax of Siebold's beech under elevated ozone.

Is Nitrogen a Key Determinant of Water Transport and Photosynthesis in Higher Plants Upon Drought Stress?

Drought stress is a major global issue limiting agricultural productivity. Plants respond to drought stress through a series of physiological, cellular, and molecular changes for survival. The regulation of water transport and photosynthesis play crucial roles in improving plants' drought tolerance. Nitrogen (N, ammonium and nitrate) is an essential macronutrient for plants, and it can affect many aspects of plant growth and metabolic pathways, including water relations and photosynthesis. This review focuses on how drought stress affects water transport and photosynthesis, including the regulation of hydraulic conductance, aquaporin expression, and photosynthesis. It also discusses the cross talk between N, water transport, and drought stress in higher plants.

Three Methods of Estimating Mesophyll Conductance Agree Regarding its CO₂ Sensitivity in the Rubisco-Limited Ci Range

Whether the mesophyll conductance to CO₂ movement (gm) within leaves of C₃ plants changes with CO₂ concentration remains a matter of debate, particularly at low CO₂ concentrations. We tested for changes in gm over the range of sub-stomatal CO₂ concentrations (Ci) for which Rubisco activity limited photosynthesis (A) in three plant species grown under the same conditions. Mesophyll conductance was estimated by three independent methods: the oxygen sensitivity of photosynthesis, variable J fluorescence combined with gas exchange, and the curvature of the Rubisco-limited A vs. Ci curve. The latter assay used a new method of rapidly obtaining data points at approximately every 3 μmol mol-1 for Rubisco-limited A vs. Ci curves, allowing separate estimates of curvature over limited Ci ranges. In two species, soybean and sunflower, no change in gm with Ci was detected using any of the three methods of estimating gm. In common bean measured under the same conditions as the other species, all three methods indicated large decreases in gm with increasing Ci. Therefore, change in gm with Ci in the Rubsico-limited region of A vs. Ci curves depended on the species, but not on the method of estimating gm.

Leaf hydraulic vulnerability triggers the decline in stomatal and mesophyll conductance during drought in rice

Understanding the physiological responses of crops to drought is important for ensuring sustained crop productivity under climate change, which is expected to exacerbate the frequency and intensity of periods of drought. Drought responses involve multiple traits, and the correlations between these traits are poorly understood. Using a variety of techniques, we estimated the changes in gas exchange, leaf hydraulic conductance, and leaf turgor in rice (Oryza sativa) in response to both short- and long-term soil drought. We performed a photosynthetic limitation analysis to quantify the contributions of each limiting factor to the resultant overall decrease in photosynthesis during drought. Biomass, leaf area, and leaf width significantly decreased during the 2-week drought treatment, but leaf mass per area and leaf vein density increased. Light-saturated photosynthetic rate declined dramatically during soil drought, mainly due to the decrease in stomatal conductance (gs) and mesophyll conductance (gm). Stomatal modeling suggested that the decline in leaf hydraulic conductance explained most of the decrease in stomatal closure during the drought treatment, and may also trigger the drought-related decrease of stomatal conductance and mesophyll conductance. The results of this study provide insight into the regulation of carbon assimilation under drought conditions.

Singlet oxygen triggers chloroplast rupture and cell death in the zeaxanthin epoxidase defective mutant aba1 of Arabidopsis thaliana under high light stress

The two Arabidopsis thaliana mutants, aba1 and max4, were previously identified as sharing a number of co-regulated genes with both the flu mutant and Arabidopsis cell suspension cultures exposed to high light (HL). On this basis, we investigated whether aba1 and max4 were generating high amounts of singlet oxygen (¹O₂) and activating ¹O₂-mediated cell death. Thylakoids of aba1 produced twice as much ¹O₂ as thylakoids of max4 and wild type (WT) plants when illuminated with strong red light. ¹O₂ was measured using the spin probe 2,2,6,6-tetramethyl-4-piperidone hydrochloride. 77-K chlorophyll fluorescence emission spectra of thylakoids revealed lower aggregation of the light harvesting complex II in aba1. This was rationalized as a loss of connectivity between photosystem II (PSII) units and as the main cause for the high yield of ¹O₂ generation in aba1. Up-regulation of the ¹O₂ responsive gene AAA-ATPase was only observed with statistical significant in aba1 under HL. Two early jasmonate (JA)-responsive genes, JAZ1 and JAZ5, encoding for two repressor proteins involved in the negative feedback regulation of JA signalling, were not up-regulated to the WT plant levels. Chloroplast aggregation followed by chloroplast rupture and eventual cell death was observed by confocal imaging of the fluorescence emission of leaf cells of transgenic aba1 plants expressing the chimeric fusion protein SSU-GFP. Cell death was not associated with direct ¹O₂ cytotoxicity in aba1, but rather with a delayed stress response. In contrast, max4 did not show evidence of ¹O₂-mediated cell death. In conclusion, aba1 may serve as an alternative model to other ¹O₂-overproducing mutants of Arabidopsis for investigating ¹O₂-mediated cell death.

Persistent negative temperature response of mesophyll conductance in red raspberry (Rubus idaeus L.) leaves under both high and low vapour pressure deficits: a role for abscisic acid?

The temperature dependence of mesophyll conductance (g_m ) was measured in well-watered red raspberry (Rubus idaeus L.) plants acclimated to leaf-to-air vapour pressure deficit (VPDL) daytime differentials of contrasting amplitude, keeping a fixed diurnal leaf temperature (T_leaf ) rise from 20 to 35 °C. Contrary to the great majority of g_m temperature responses published to date, we found a pronounced reduction of g_m with increasing T_leaf irrespective of leaf chamber O₂ level and diurnal VPDL regime. Leaf hydraulic conductance was greatly enhanced during the warmer afternoon periods under both low (0.75 to 1.5 kPa) and high (0.75 to 3.5 kPa) diurnal VPDL regimes, unlike stomatal conductance (g_s ), which decreased in the afternoon. Consequently, the leaf water status remained largely isohydric throughout the day, and therefore cannot be evoked to explain the diurnal decrease of g_m . However, the concerted diurnal reductions of g_m and g_s were well correlated with increases in leaf abscisic acid (ABA) content, thus suggesting that ABA can induce a significant depression of g_m under favourable leaf water status. Our results challenge the view that the temperature dependence of g_m can be explained solely from dynamic leaf anatomical adjustments and/or from the known thermodynamic properties of aqueous solutions and lipid membranes.​.

Carbon dioxide and water transport through plant aquaporins

Aquaporins are channel proteins that function to increase the permeability of biological membranes. In plants, aquaporins are encoded by multigene families that have undergone substantial diversification in land plants. The plasma membrane intrinsic proteins (PIPs) subfamily of aquaporins is of particular interest given their potential to improve plant water relations and photosynthesis. Flowering plants have between 7 and 28 PIP genes. Their expression varies with tissue and cell type, through development and in response to a variety of factors, contributing to the dynamic and tissue specific control of permeability. There are a growing number of PIPs shown to act as water channels, but those altering membrane permeability to CO₂ are more limited. The structural basis for selective substrate specificities has not yet been resolved, although a few key amino acid positions have been identified. Several regions important for dimerization, gating and trafficking are also known. PIP aquaporins assemble as tetramers and their properties depend on the monomeric composition. PIPs control water flux into and out of veins and stomatal guard cells and also increase membrane permeability to CO₂ in mesophyll and stomatal guard cells. The latter increases the effectiveness of Rubisco and can potentially influence transpiration efficiency.

Determination of drought tolerance related traits in Helianthus argophyllus, Helianthus annuus, and their hybrids

Drought is a major constraint for sunflower (Helianthus annuus) production worldwide. Drought tolerance traits have been identified in the related wild species Helianthus argophyllus. This study was initiated to develop sunflower drought-tolerant genotypes by crossing cultivated sunflower with this species and analyze drought tolerance traits in the H. annuus and H. argophyllus populations, H. annuus intraspecific hybrids, and H. annuus × H. argophyllus interspecific hybrids along with the commercial hybrid Hysun-33 under three stress regimes: exogenous application of ABA, both by foliar spray and irrigation, and 5% PEG-induced osmotic stress. H. argophyllus populations had a significantly lower leaf area and higher water-use efficiency and leaf cuticular wax content under all treatments, and maintained a higher net photosynthetic rate and stomatal conductance under osmotic stress. Small leaf area and high cuticular waxes content of the wild species were, however, not inherited in interspecific hybrids which suggested for selection in F₂ for these traits. Therefore, transgressive plants were selected in the F₂ population to establish F₃ plant progenies with silver-leafed canopy of H. argophyllus which showed higher achene yield under stress condition. These results are discussed with a view to using H. argophyllus to improve drought tolerance in cultivated sunflower.

The evolution of the role of ABA in the regulation of water-use efficiency: From biochemical mechanisms to stomatal conductance

Abscisic acid is found in a wide variety of organisms. In the plant kingdom, ABA's role in mediating responses to abiotic stress has been conserved and enhanced throughout evolution. The emergence of plants to terrestrial environments required the development of mechanisms to cope with ongoing and severe abiotic stress such as drought and rapid changes in humidity and temperature. The common understanding is that terrestrial plants evolved strategies ranging from desiccation-tolerance mechanisms (mosses) to drought tolerance (CAM plants), to better exploit different ecological niches. In between these divergent water regulation strategies, ABA plays a significant role in managing plants' adaptation to new environments by optimizing water-use efficiency (WUE) under particular environmental conditions. ABA plays some very different roles in the regulation of WUE. ABA's role in the regulation of guard cells and transpiration has yielded a wide variety of WUE-regulation mechanisms, ranging from no sensitivity (ferns) to low sensitivity (anisohydric behavior) to hypersensitivity to ABA (isohydric behavior and putatively CAM plants). ABA also plays a role in the regulation of non-stomatal, biochemical mechanisms of WUE regulation. In angiosperms, this includes the control of osmotic adjustment and morphological changes, including changes in leaf size, stomatal density, stomatal size and root development. Under severe stress, ABA also appears to initiate leaf senescence via transcriptional regulation, to directly inhibit photosynthesis.

Partial Root-Zone Drying of Olive (Olea europaea var. 'Chetoui') Induces Reduced Yield under Field Conditions

The productivity of olive trees in arid and semi-arid environments is closely linked to irrigation. It is necessary to improve the efficiency of irrigation techniques to optimise the amount of olive fruit produced in relation to the volume of water used. Partial root-zone drying (PRD) is a water saving irrigation technique that theoretically allows the production of a root-to-shoot signal that modifies the physiology of the above-ground parts of the plant; specifically reducing stomatal conductance (gs) and improving water use efficiency (WUE). Partial root-zone drying has been successfully applied under field conditions to woody and non-woody crops; yet the few previous trials with olive trees have produced contrasting results. Thirty year-old olive trees (Olea europaea 'var. Chetoui') in a Tunisian grove were exposed to four treatments from May to October for three-years: 'control' plants received 100% of the potential evapotranspirative demand (ETc) applied to the whole root-zone; 'PRD100' were supplied with an identical volume of water to the control plants alternated between halves of the root-zone every ten-days; 'PRD50' were given 50% of ETc to half of the root-system, and; 'rain-fed' plants received no supplementary irrigation. Allowing part of the root-zone to dry resulted in reduced vegetative growth and lower yield: PRD100 decreased yield by ~47% during productive years. During the less productive years of the alternate bearing cycle, irrigation had no effect on yield; this suggests that withholding of water during 'off-years' may enhance the effectiveness of irrigation over a two-year cycle. The amount and quality of oil within the olive fruit was unaffected by the irrigation treatment. Photosynthesis declined in the PRD50 and rain-fed trees due to greater diffusive limitations and reduced biochemical uptake of CO2. Stomatal conductance and the foliar concentration of abscisic acid (ABA) were not altered by PRD100 irrigation, which may indicate the absence of a hormonal root-to-shoot signal. Rain-fed and PRD50 treatments induced increased stem water potential and increased foliar concentrations of ABA, proline and soluble sugars. The stomata of the olive trees were relatively insensitive to super-ambient increases in [CO2] and higher [ABA]. These characteristics of 'hydro-passive' stomatal behaviour indicate that the 'Chetoui' variety of olive tree used in this study lacks the physiological responses required for the successful exploitation of PRD techniques to increase yield and water productivity. Alternative irrigation techniques such as partial deficit irrigation may be more suitable for 'Chetoui' olive production.

Methods of mesophyll conductance estimation: its impact on key biochemical parameters and photosynthetic limitations in phosphorus-stressed soybean across CO2

Despite the development of various methods, the rapid estimation of mesophyll conductance (gm ) for a large number of samples is still a daunting challenge. Although the accurate estimation of gm is critical to partition photosynthetic limitations by stomatal (Ls ) and mesophyll (Lm ) conductance and by photo-biochemical (Lb ) processes, the impact of various gm estimation methods on this is ambiguous. As phosphorus (P) starvation and elevated CO2 (eCO2 ) strongly affect photosynthetic processes, their combined effect on the proportional changes in these limitations are not well understood. To investigate this, while also evaluating distinct recent methods of gm estimation sharing few common theories and assumptions, soybean was grown under a range of P nutrition at ambient and eCO2 . Methods significantly affected gm and carboxylation efficiency (VCmax ) but not other photosynthetic parameters. In all the methods, all photosynthetic parameters responded similarly to treatments. However, the percentage difference between VCmax assuming finite and infinite gm was highly inconsistent among methods. The primary mechanism responsible for P limitation to soybean photosynthesis was not CO2 diffusion limitations but Lb comprised of reduced chlorophyll, photochemistry and biochemical processes. The eCO2 decreased Lb but increased Lm without affecting Ls across leaf P concentration. Although each method explored advances of our understanding about gm variability, they all require assumptions of varying degrees, which lead to the discrepancy in the gm values. Among the methods, the oxygen sensitivity-based gm estimation appeared to be suitable for the quick assessment of a large number of samples or genotypes. Digital tools are provided for the easy estimation of gm for some methods.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

Why small fluxes matter: the case and approaches for improving measurements of photosynthesis and (photo)respiration

Since its inception, the Farquhar et al. (1980) model of photosynthesis has been a mainstay for relating biochemistry to environmental conditions from chloroplast to global levels in terrestrial plants. Many variables could be assigned from basic enzyme kinetics, but the model also required measurements of maximum rates of photosynthetic electron transport (J max ), carbon assimilation (Vcmax ), conductance of CO2 into (g s ) and through (g m ) the leaf, and the rate of respiration during the day (R d ). This review focuses on improving the accuracy of these measurements, especially fluxes from photorespiratory CO2, CO2 in the transpiration stream, and through the leaf epidermis and cuticle. These fluxes, though small, affect the accuracy of all methods of estimating mesophyll conductance and several other photosynthetic parameters because they all require knowledge of CO2 concentrations in the intercellular spaces. This review highlights modified methods that may help to reduce some of the uncertainties. The approaches are increasingly important when leaves are stressed or when fluxes are inferred at scales larger than the leaf.

Abscisic Acid Induces Rapid Reductions in Mesophyll Conductance to Carbon Dioxide

The rate of photosynthesis (A) of plants exposed to water deficit is a function of stomatal (gs) and mesophyll (gm) conductance determining the availability of CO2 at the site of carboxylation within the chloroplast. Mesophyll conductance often represents the greatest impediment to photosynthetic uptake of CO2, and a crucial determinant of the photosynthetic effects of drought. Abscisic acid (ABA) plays a fundamental role in signalling and co-ordination of plant responses to drought; however, the effect of ABA on gm is not well-defined. Rose, cherry, olive and poplar were exposed to exogenous ABA and their leaf gas exchange parameters recorded over a four hour period. Application with ABA induced reductions in values of A, gs and gm in all four species. Reduced gm occurred within one hour of ABA treatment in three of the four analysed species; indicating that the effect of ABA on gm occurs on a shorter timescale than previously considered. These declines in gm values associated with ABA were not the result of physical changes in leaf properties due to altered turgor affecting movement of CO2, or caused by a reduction in the sub-stomatal concentration of CO2 (Ci). Increased [ABA] likely induces biochemical changes in the properties of the interface between the sub-stomatal air-space and mesophyll layer through the actions of cooporins to regulate the transport of CO2. The results of this study provide further evidence that gm is highly responsive to fluctuations in the external environment, and stress signals such as ABA induce co-ordinated modifications of both gs and gm in the regulation of photosynthesis.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

Impact of cuticle on calculations of the CO2 concentration inside leaves

Water vapor over-estimates the CO 2 entering leaves during photosynthesis because the cuticle and epidermis transmit more water vapor than CO 2 . Direct measurements of internal CO 2 concentrations may be preferred. The CO2 concentration inside leaves (c i) is typically calculated from the relationship between water vapor diffusing out while CO2 diffuses in. Diffusion through the cuticle/epidermis is usually not considered. This study was undertaken to determine how much the calculations would be affected by including cuticle properties. Previous studies indicate that measurable amounts of CO2 and water vapor move through the cuticle, although much less CO2 than water vapor. The present experiments were conducted with sunflower (Helianthus annuus L) leaves in a gas exchange apparatus designed to directly measure c i, while simultaneously calculating c i. Results showed that, in normal air, calculated c i were always higher than directly measured ones, especially when abscisic acid was fed to the leaves to close the stomata and cause gas exchange to be dominated by the cuticle. The effect was attributed mostly to the reliance on the gas phase for the calculations without taking cuticle properties into account. Because cuticle properties are usually unknown and vary with the turgor of the leaf, which can stretch the waxes, it is difficult to include cuticle properties in the calculation. It was concluded that direct measurement of c i may be preferable to the calculations.

Manipulation of the hypocotyl sink activity by reciprocal grafting of two Raphanus sativus varieties: its effects on morphological and physiological traits of source leaves and whole-plant growth

To reveal whether hypocotyl sink activities are regulated by the aboveground parts, and whether physiology and morphology of source leaves are affected by the hypocotyl sink activities, we conducted grafting experiments using two Raphanus sativus varieties with different hypocotyl sink activities. Comet (C) and Leafy (L) varieties with high and low hypocotyl sink activities were reciprocally grafted and resultant plants were called by their scion and stock such as CC, LC, CL and LL. Growth, leaf mass per area (LMA), total non-structural carbohydrates (TNCs) and photosynthetic characteristics were compared among them. Comet hypocotyls in CC and LC grew well regardless of the scions, whereas Leafy hypocotyls in CL and LL did not. Relative growth rate was highest in LL and lowest in CC. Photosynthetic capacity was correlated with Rubisco (ribulose 1·5-bisphosphate carboxylase/oxygenase) content but unaffected by TNC. High C/N ratio and accumulation of TNC led to high LMA and structural LMA. These results showed that the hypocotyl sink activity was autonomously regulated by hypocotyl and that the down-regulation of photosynthesis was not induced by TNC. We conclude that the change in the sink activity alters whole-plant growth through the changes in both biomass allocation and leaf morphological characteristics in R. sativus.

Aquaporins in Plants

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms. In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations, transport selectivity, and regulation properties. Plant aquaporins are localized in the plasma membrane, endoplasmic reticulum, vacuoles, plastids and, in some species, in membrane compartments interacting with symbiotic organisms. Plant aquaporins can transport various physiological substrates in addition to water. Of particular relevance for plants is the transport of dissolved gases such as carbon dioxide and ammonia or metalloids such as boron and silicon. Structure-function studies are developed to address the molecular and cellular mechanisms of plant aquaporin gating and subcellular trafficking. Phosphorylation plays a central role in these two processes. These mechanisms allow aquaporin regulation in response to signaling intermediates such as cytosolic pH and calcium, and reactive oxygen species. Combined genetic and physiological approaches are now integrating this knowledge, showing that aquaporins play key roles in hydraulic regulation in roots and leaves, during drought but also in response to stimuli as diverse as flooding, nutrient availability, temperature, or light. A general hydraulic control of plant tissue expansion by aquaporins is emerging, and their role in key developmental processes (seed germination, emergence of lateral roots) has been established. Plants with genetically altered aquaporin functions are now tested for their ability to improve plant tolerance to stresses. In conclusion, research on aquaporins delineates ever expanding fields in plant integrative biology thereby establishing their crucial role in plants.

Rapid response of leaf photosynthesis in two fern species Pteridium aquilinum and Thelypteris dentata to changes in CO2 measured by tunable diode laser absorption spectroscopy

We investigated stomatal conductance (g(s)) and mesophyll conductance (g(m)) in response to atmospheric CO2 concentration [CO2] in two primitive land plants, the fern species Pteridium aquilinum and Thelypteris dentata, using the concurrent measurement of leaf gas exchange and carbon isotope discrimination. [CO2] was initially decreased from 400 to 200 μmol mol(-1), and then increased from 200 to 700 μmol mol(-1), and finally decreased from 700 to 400 μmol mol(-1). Analysis by tunable diode laser absorption spectroscopy (TDLAS) revealed a rapid and continuous response in g m within a few minutes. In most cases, both ferns showed rapid and significant responses of g m to changes in [CO2]. The largest changes (quote % decrease) were obtained when [CO2] was decreased from 400 to 200 μmol mol(-1). This is in contrast to angiosperms where an increase in g(m) is commonly observed at low [CO2]. Similarly, fern species observed little or no response of g(s) to changes in [CO2] whereas, a concomitant decline of g(m) and g(s) with [CO2] is often reported in angiosperms. Together, these results suggest that regulation of g(m) to [CO2] may differ between angiosperms and ferns.

Superimposed behaviour of gm under ABA-induced stomata closing and low CO2

This article comments on: Mesophyll conductance decreases in the wild type but not in an ABA‐deficient mutant (aba1) of Nicotiana plumbaginifolia under drought conditions