Mesophyll airspace unsaturation drives C4 plant success under vapor pressure deficit stress.

A fundamental assumption in plant science posits that leaf air spaces remain vapor saturated, leading to the predominant view that stomata alone control leaf water loss. This concept has been pivotal in photosynthesis and water-use efficiency research. However, recent evidence has refuted this longstanding assumption by providing evidence of unsaturation in the leaf air space of C3 plants under relatively mild vapor pressure deficit (VPD) stress. This phenomenon represents a nonstomatal mechanism restricting water loss from the mesophyll. The potential ubiquity and physiological implications of this phenomenon, its driving mechanisms in different plant species and habitats, and its interaction with other ecological adaptations have. In this context, C4 plants spark particular interest for their importance as crops, bundle sheath cells' unique anatomical characteristics and specialized functions, and notably higher water-use efficiency relative to C3 plants. Here, we confirm reduced relative humidities in the substomatal cavity of the C4 plants maize, sorghum, and proso millet down to 80% under mild VPD stress. We demonstrate the critical role of nonstomatal control in these plants, indicating that the role of the CO2 concentration mechanism in CO2 management at a high VPD may have been overestimated. Our findings offer a mechanistic reconciliation between discrepancies in CO2 and VPD responses reported in C4 species. They also reveal that nonstomatal control is integral to maintaining an advantageous microclimate of relatively higher CO2 concentrations in the mesophyll air space of C4 plants for carbon fixation, proving vital when these plants face VPD stress.

Fast dehydration reduces bundle sheath conductance in C4 maize and sorghum.

In the face of anthropogenic warming, drought poses an escalating threat to food production. C4 plants offer promise in addressing this threat. C4 leaves operate a biochemical CO2 concentrating mechanism that exchanges metabolites between two partially isolated compartments (mesophyll and bundle sheath), which confers high-productivity potential in hot climates boosting water use efficiency. However, when C4 leaves experience dehydration, photosynthesis plummets. This paper explores the physiological mechanisms behind this decline. In a fast dehydration experiment, we measured the fluxes and isotopic composition of water and CO2 in the gas exchanged by leaves, and we interpreted results using a novel biochemical model and analysis of elasticity. Our findings show that, while CO2 supply to the mesophyll and to the bundle sheath persisted during dehydration, there was a decrease in CO2 conductance at the bundle sheath-mesophyll interface. We interpret this as causing a slowdown of intercellular metabolite exchange - an essential feature of C4 photosynthesis. This would impede the supply of reducing power to the bundle sheath, leading to phosphoglycerate accumulation and feedback inhibition of Rubisco carboxylation. The interplay between this rapid sensitivity and the effectiveness of coping strategies that C4 plants deploy may be an overlooked driver of their competitive performance.

Unsaturation in the air spaces of leaves and its implications.

Modern plant physiological theory stipulates that the resistance to water movement from plants to the atmosphere is overwhelmingly dominated by stomata. This conception necessitates a corollary assumption-that the air spaces in leaves must be nearly saturated with water vapour; that is, with a relative humidity that does not decline materially below unity. As this idea became progressively engrained in scientific discourse and textbooks over the last century, observations inconsistent with this corollary assumption were occasionally reported. Yet, evidence of unsaturation gained little traction, with acceptance of the prevailing framework motivated by three considerations: (1) leaf water potentials measured by either thermocouple psychrometry or the Scholander pressure chamber are largely consistent with the framework; (2) being able to assume near saturation of intercellular air spaces was transformational to leaf gas exchange analysis; and (3) there has been no obvious mechanism to explain a variable, liquid-phase resistance in the leaf mesophyll. Here, we review the evidence that refutes the assumption of universal, near saturation of air spaces in leaves. Refining the prevailing paradigm with respect to this assumption provides opportunities for identifying and developing mechanisms for increased plant productivity in the face of increasing evaporative demand imposed by global climate change.

C4 maize and sorghum are more sensitive to rapid dehydration than C3 wheat and sunflower.

The high productive potential, heat resilience, and greater water use efficiency of C4 over C3 plants attract considerable interest in the face of global warming and increasing population, but C4 plants are often sensitive to dehydration, questioning the feasibility of their wider adoption. To resolve the primary effect of dehydration from slower from secondary leaf responses originating within leaves to combat stress, we conducted an innovative dehydration experiment. Four crops grown in hydroponics were forced to a rapid yet controlled decrease in leaf water potential by progressively raising roots of out of the solution while measuring leaf gas exchange. We show that, under rapid dehydration, assimilation decreased more steeply in C4 maize and sorghum than in C3 wheat and sunflower. This reduction was due to a rise of nonstomatal limitation at triple the rate in maize and sorghum than in wheat and sunflower. Rapid reductions in assimilation were previously measured in numerous C4 species across both laboratory and natural conditions. Hence, we deduce that high sensitivity to rapid dehydration might stem from the disturbance of an intrinsic aspect of C4 bicellular photosynthesis. We posit that an obstruction to metabolite transport between mesophyll and bundle sheath cells could be the cause.

Assessing the CO2 concentration at the surface of photosynthetic mesophyll cells.

We present a robust estimation of the CO2 concentration at the surface of photosynthetic mesophyll cells (cw ), applicable under reasonable assumptions of assimilation distribution within the leaf. We used Capsicum annuum, Helianthus annuus and Gossypium hirsutumas model plants for our experiments. We introduce calculations to estimate cw using independent adaxial and abaxial gas exchange measurements, and accounting for the mesophyll airspace resistances. The cw was lower than adaxial and abaxial estimated intercellular CO2 concentrations (ci ). Differences between cw and the ci of each surface were usually larger than 10 µmol mol-1 . Differences between adaxial and abaxial ci ranged from a few µmol mol-1 to almost 50 µmol mol-1 , where the largest differences were found at high air saturation deficits (ASD). Differences between adaxial and abaxial ci and the ci estimated by mixing both fluxes ranged from -30 to +20 µmol mol-1 , where the largest differences were found under high ASD or high ambient CO2 concentrations. Accounting for cw improves the information that can be extracted from gas exchange experiments, allowing a more detailed description of the CO2 and water vapor gradients within the leaf.

Linking water use efficiency with water use strategy from leaves to communities.

Limitations and utility of three measures of water use characteristics were evaluated: water use efficiency (WUE), intrinsic WUE and marginal water cost of carbon gain ( ∂ E / ∂ A ) estimated, respectively, as ratios of assimilation (A) to transpiration (E), of A to stomatal conductance (gs ) and of sensitivities of E and A with variation in gs . Only the measure ∂ E / ∂ A estimates water use strategy in a way that integrates carbon gain relative to water use under varying environmental conditions across scales from leaves to communities. This insight provides updated and simplified ways of estimating ∂ E / ∂ A and adds depth to understanding ways that plants balance water expenditure against carbon gain, uniquely providing a mechanistic means of predicting water use characteristics under changing environmental scenarios.

The Chlamydomonas reinhardtii chloroplast envelope protein LCIA transports bicarbonate in planta.

LCIA (low CO2-inducible protein A) is a chloroplast envelope protein associated with the CO2-concentrating mechanism of the green alga Chlamydomonas reinhardtii. LCIA is postulated to be a HCO3- channel, but previous studies were unable to show that LCIA was actively transporting bicarbonate in planta. Therefore, LCIA activity was investigated more directly in two heterologous systems: an Escherichia coli mutant (DCAKO) lacking both native carbonic anhydrases and an Arabidopsis mutant (ßca5) missing the plastid carbonic anhydrase ßCA5. Neither DCAKO nor ßca5 can grow in ambient CO2 conditions, as they lack carbonic anhydrase-catalyzed production of the necessary HCO3- concentration for lipid and nucleic acid biosynthesis. Expression of LCIA restored growth in both systems in ambient CO2 conditions, which strongly suggests that LCIA is facilitating HCO3- uptake in each system. To our knowledge, this is the first direct evidence that LCIA moves HCO3- across membranes in bacteria and plants. Furthermore, the ßca5 plant bioassay used in this study is the first system for testing HCO3- transport activity in planta, an experimental breakthrough that will be valuable for future studies aimed at improving the photosynthetic efficiency of crop plants using components from algal CO2-concentrating mechanisms.

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

Cuticular conductance to water (gcw ) 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 gcw . This has led to the neglect of gcw as a separate parameter in most common gas exchange measurements. Here, we describe a simple technique to simultaneously estimate adaxial and abaxial values of gcw , tested in two amphistomatous plant species. What we term the 'Red-Light method' is used to estimate gcw 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 gcw . While adaxial and abaxial gcw varies significantly between leaves of the same species we found that the ratio of adaxial/abaxial gcw (γn ) is stable within a plant species. This has implications for use of generic values of gcw when analysing gas exchange data. The Red-Light method can be used to estimate total cuticular conductance (gcw-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.

Do 2 H and 18 O in leaf water reflect environmental drivers differently?

We compiled hydrogen and oxygen stable isotope compositions (δ2 H and δ18 O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2 H was more closely correlated with δ2 H of xylem water or atmospheric vapour, whereas leaf water δ18 O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ2 H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ18 O. We next expressed leaf water as isotopic enrichment above xylem water (Δ2 H and Δ18 O) to remove the impact of xylem water isotopic variation. For Δ2 H, leaf water still correlated with atmospheric vapour, whereas Δ18 O showed no such correlation. This was explained by covariance between air relative humidity and the Δ18 O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2 H and 18 O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

Multienvironment QTL analysis delineates a major locus associated with homoeologous exchanges for water-use efficiency and seed yield in canola.

Canola varieties exhibit variation in drought avoidance and drought escape traits, reflecting adaptation to water-deficit environments. Our understanding of underlying genes and their interaction across environments in improving crop productivity is limited. A doubled haploid population was analysed to identify quantitative trait loci (QTL) associated with water-use efficiency (WUE) related traits. High WUE in the vegetative phase was associated with low seed yield. Based on the resequenced parental genome data, we developed sequence-capture-based markers and validated their linkage with carbon isotope discrimination (Δ13 C) in an F2 population. RNA sequencing was performed to determine the expression of candidate genes underlying Δ13 C QTL. QTL contributing to main and QTL × environment interaction effects for Δ13 C and yield were identified. One multiple-trait QTL for Δ13 C, days to flower, plant height, and seed yield was identified on chromosome A09. Interestingly, this QTL region overlapped with a homoeologous exchange (HE) event, suggesting its association with the multiple traits. Transcriptome analysis revealed 121 significantly differentially expressed genes underlying Δ13 C QTL on A09 and C09, including in HE regions. Sorting out the negative relationship between vegetative WUE and seed yield is a priority. Genetic and genomic resources and knowledge so developed could improve canola WUE and yield.

Two-Source δ18O Method to Validate the CO18O-Photosynthetic Discrimination Model: Implications for Mesophyll Conductance.

Theoretical models of photosynthetic isotopic discrimination of CO2 (13C and 18O) are commonly used to estimate mesophyll conductance (g m). This requires making simplifying assumptions and assigning parameter values so that g m can be solved for as the residual term. Uncertainties in g m estimation occur due to measurement noise and assumptions not holding, including parameter uncertainty and model parametrization. Uncertainties in the 13C model have been explored previously, but there has been little testing undertaken to determine the reliability of g m estimates from the 18O model (g m18). In this study, we exploited the action of carbonic anhydrase in equilibrating CO2 with leaf water and manipulated the observed photosynthetic discrimination (Δ18O) by changing the oxygen isotopic composition of the source gas CO2 and water vapor. We developed a two-source δ18O method, whereby two measurements of Δ18O were obtained for a leaf with its gas-exchange characteristics otherwise unchanged. Measurements were performed in broad bean (Vicia faba) and Algerian oak (Quercus canariensis) in response to light and vapor pressure deficit. Despite manipulating the Δ18O by over 100‰, in most cases we observed consistency in the calculated g m18, providing confidence in the measurements and model theory. Where there were differences in g m18 estimates between source-gas measurements, we explored uncertainty associated with two model assumptions (the isotopic composition of water at the sites of CO2-water exchange, and the humidity of the leaf internal airspace) and found evidence for both. Finally, we provide experimental guidelines to minimize the sensitivity of g m18 estimates to measurement errors. The two-source δ18O method offers a flexible tool for model parameterization and provides an opportunity to refine our understanding of leaf water and CO2 fluxes.

Hydrogen isotopic differences between C3 and C4 land plant lipids: consequences of compartmentation in C4 photosynthetic chemistry and C3 photorespiration.

The 2 H/1 H ratio of carbon-bound H in biolipids holds potential for probing plant lipid biosynthesis and metabolism. The biochemical mechanism underlying the isotopic differences between lipids from C3 and C4 plants is still poorly understood. GC-pyrolysis-IRMS (gas chromatography-pyrolysis-isotope ratio mass spectrometry) measurement of the 2 H/1 H ratio of leaf lipids from controlled and field grown plants indicates that the biochemical isotopic fractionation (ε2 Hlipid_biochem ) differed between C3 and C4 plants in a pathway-dependent manner: ε2 HC4 > ε2 HC3 for the acetogenic pathway, ε2 HC4 < ε2 HC3 for the mevalonic acid pathway and the 1-deoxy-D-xylulose 5-phosphate pathway across all species examined. It is proposed that compartmentation of photosynthetic CO2 fixation into C4 mesophyll (M) and bundle sheath (BS) cells and suppression of photorespiration in C4 M and BS cells both result in C4 M chloroplastic pyruvate - the precursor for acetogenic pathway - being more depleted in 2 H relative to pyruvate in C3 cells. In addition, compartmentation in C4 plants also results in (i) the transferable H of NADPH being enriched in 2 H in C4 M chloroplasts compared with that in C3 chloroplasts for the 1-deoxy-D-xylulose 5-phosphate pathway pathway and (ii) pyruvate relatively 2 H-enriched being used for the mevalonic acid pathway in the cytosol of BS cells in comparison with that in C3 cells.

Leaf vein fraction influences the Péclet effect and 18 O enrichment in leaf water.

The process of evaporation results in the fractionation of water isotopes such that the lighter 16 O isotope preferentially escapes the gas phase leaving the heavier 18 O isotope to accumulate at the sites of evaporation. This applies to transpiration from a leaf with the degree of fractionation dependent on a number of environmental and physiological factors that are well understood. Nevertheless, the 18 O enrichment of bulk leaf water is often less than that predicted for the sites of evaporation. The advection of less enriched water in the transpiration stream has been suggested to limit the back diffusion of enriched evaporative site water (Péclet effect); however, evidence for this effect has been varied. In sampling across a range of species with different vein densities and saturated water contents, we demonstrate the importance of accounting for the relative 'pool' sizes of the vascular and mesophyll water for the interpretation of a Péclet effect. Further, we provide strong evidence for a Péclet signal within the xylem that if unaccounted for can lead to confounding of the estimated enrichment within the mesophyll water. This has important implications for understanding variation in the effective path length of the mesophyll and hence potentially the δ18 O of organic matter.

Stable isotopes in leaf water of terrestrial plants.

Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases. Models describing evaporative enrichment of leaf water have become increasingly complex over time, reflecting enhanced spatial and temporal resolution. We recommend that practitioners choose a model with a level of complexity suited to their application, and provide guidance. At the same time, there exists some lingering uncertainty about the biophysical processes relevant to patterns of isotopic enrichment in leaf water. An important goal for future research is to link observed variations in isotopic composition to specific anatomical and physiological features of leaves that reflect differences in hydraulic design. New measurement techniques are developing rapidly, enabling determinations of both transpired and leaf water δ(18) O and δ(2) H to be made more easily and at higher temporal resolution than previously possible. We expect these technological advances to spur new developments in our understanding of patterns of stable isotope fractionation in leaf water.

D2O solvent isotope effects suggest uniform energy barriers in ribulose-1,5-bisphosphate carboxylase/oxygenase catalysis.

d-Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the most abundant enzyme on Earth and is responsible for the fixation of atmospheric CO(2) into biomass. The reaction consists of incorporation of CO(2) and solvent H(2)O into d-ribulose 1,5-bisphosphate (RuBP) to yield 3-phospho-d-glycerate. The reaction involves several proton-dependent events: abstraction and protonation during enolization of RuBP and hydrolysis and reprotonation of the six-carbon reaction intermediate (carboxyketone). Although much is known about Rubisco structure and diversity, fundamental aspects of the reaction mechanism are poorly documented. How and when are protons exchanged among substrate, amino acid residues, and solvent water, and could alterations of proton exchange influence catalytic turnover? What is the energy profile of the reaction? To answer these questions, we measured catalytic rates and the (12)CO(2)/(13)CO(2) isotope effect in isotopic waters. We show that with increasing D(2)O content, the maximal carboxylation velocity (k(cat)(c)) decreased linearly and was 1.7 times lower in pure D(2)O. By contrast, the isotope effect on the apparent Michaelis constant for CO(2) (K(c)) was unity, suggesting that H/D exchange might have occurred with the solvent in early steps thereby slowing the overall catalysis. Calculations of kinetic commitments from observed isotope effects further indicate that (1) enolization and processing of the carboxyketone are similarly rate-limiting and (2) the tendency of the carboxyketone to go backward (decarboxylation) is likely exacerbated upon deuteration. Our results thus suggest that Rubisco catalysis is achieved by a rather equal distribution of energy barriers along the reaction.

Humidity gradients in the air spaces of leaves.

Stomata are orifices that connect the drier atmosphere with the interconnected network of more humid air spaces that surround the cells within a leaf. Accurate values of the humidities inside the substomatal cavity, wi, and in the air, wa, are needed to estimate stomatal conductance and the CO2 concentration in the internal air spaces of leaves. Both are vital factors in the understanding of plant physiology and climate, ecological and crop systems. However, there is no easy way to measure wi directly. Out of necessity, wi has been taken as the saturation water vapour concentration at leaf temperature, wsat, and applied to the whole leaf intercellular air spaces. We explored the occurrence of unsaturation by examining gas exchange of leaves exposed to various magnitudes of wsat - wa, or Δw, using a double-sided, clamp-on chamber, and estimated degrees of unsaturation from the gradient of CO2 across the leaf that was required to sustain the rate of CO2 assimilation through the upper surface. The relative humidity in the substomatal cavities dropped to about 97% under mild Δw and as dry as around 80% when Δw was large. Measurements of the diffusion of noble gases across the leaf indicated that there were still regions of near 100% humidity distal from the stomatal pores. We suggest that as Δw increases, the saturation edge retreats into the intercellular air spaces, accompanied by the progressive closure of mesophyll aquaporins to maintain the cytosolic water potential.

An orthophosphate semiconductor with photooxidation properties under visible-light irradiation.

The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization. Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In(1-x)Ni(x)TaO(4) and (V-,Fe- or Mn-)TiO(2) (refs 7,8), non-metal-ion substitution as in TiO(2-x)N(x) and Sm(2)Ti(2)O(5)S(2) (refs 9,10) or solid-solution fabrication as in (Ga(1-x)Zn(x))(N(1-x)O(x)) and ZnS-CuInS(2)-AgInS(2) (refs 11,12). Here we report a new use of Ag(3)PO(4) semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.

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 (gcw) 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 (Ci) 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.

Revisiting carbon isotope discrimination in C3 plants shows respiration rules when photosynthesis is low.

Stable isotopes are commonly used to study the diffusion of CO2 within photosynthetic plant tissues. The standard method used to interpret the observed preference for the lighter carbon isotope in C3 photosynthesis involves the model of Farquhar et al., which relates carbon isotope discrimination to physical and biochemical processes within the leaf. However, under many conditions the model returns unreasonable results for mesophyll conductance to CO2 diffusion (gm), especially when rates of photosynthesis are low. Here, we re-derive the carbon isotope discrimination model using modified assumptions related to the isotope effect of mitochondrial respiration. In particular, we treat the carbon pool associated with respiration as separate from the pool of primary assimilates. We experimentally test the model by comparing gm values measured with different CO2 source gases varying in their isotopic composition, and show that our new model returns matching gm values that are much more reasonable than those obtained with the previous model. We use our results to discuss CO2 diffusion properties within the mesophyll.

Stomatal conductance responses to evaporative demand conferred by rice drought-yield quantitative trait locus qDTY12.1.

Rice quantitative trait locus (QTL) qDTY12.1 is a major-effect drought yield QTL that was identified from a cross of Vandana (recipient parent) and Way Rarem (donor parent) through breeding efforts to improve rice yield under upland drought stress conditions. The two main physiological effects previously observed to be related to the presence of qDTY12.1 were (i) increased lateral root growth, and (ii) increased transpiration efficiency. Since relatively more progress has thus far been made on characterising the lateral root growth response related to qDTY12.1, the present study focussed on characterising how qDTY12.1 confers higher transpiration efficiency under upland drought stress in the Vandana background. In a series of field experiments in which stomatal conductance was measured across different times of day in four qDTY12.1 near isogenic lines (NILs), the NILs and Way Rarem showed consistently higher stomatal conductance than Vandana under conditions of low vapour pressure deficit (VPD) and low photosynthetically active radiation (PAR), and consistently lower stomatal conductance than Vandana under high VPD and high PAR. Leaf δ18O was higher in the qDTY12.1 NIL than in Vandana, and although this trend was previously observed for leaf δ13C it appeared to be more consistent across measurement dates and treatments for leaf δ18O. The qDTY12.1 NILs and Way Rarem tended to show greater large vein to small vein interveinal distance and mesophyll area than Vandana, also consistent across treatments. In terms of aquaporin-related plant hydraulics, variation among NILs in terms of aquaporin inhibition of root hydraulic conductivity (Lpr) was observed, with the highest-yielding NIL showing a lack of Lpr inhibition similar to Way Rarem. The results reported here suggest that the effects of qDTY12.1 are in response not only to soil moisture, but also to atmospheric conditions. An interaction among multiple mechanisms including leaf anatomy and aquaporin function appear to confer the transpiration efficiency effect of qDTY12.1.