Mesophyll Conductance Research Articles

A meta-analysis of mesophyll conductance to CO2 in relation to major abiotic stresses in poplar species.

Mesophyll conductance (gm) determines the diffusion of CO2 from the substomatal cavities to the site of carboxylation in the chloroplasts and represents a critical component of the diffusive limitation of photosynthesis. In this study, we evaluated the average effect sizes of different environmental constraints on gm in Populus spp., a forest tree model. We collected raw data of 815 A–Ci response curves from 26 datasets to estimate gm, using a single curve-fitting method to alleviate method-related bias. We performed a meta-analysis to assess the effects of different abiotic stresses on gm. We found a significant increase in gm from the bottom to the top of the canopy that was concomitant with the increase of maximum rate of carboxylation and light-saturated photosynthetic rate (Amax). gm was positively associated with increases in soil moisture and nutrient availability, but was insensitive to increasing soil copper concentration and did not vary with atmospheric CO2 concentration. Our results showed that gm was strongly related to Amax and to a lesser extent to stomatal conductance (gs). Moreover, a negative exponential relationship was obtained between gm and specific leaf area, which may be used to scale-up gm within the canopy.

The potential implications of a plasma membrane aquaporin in improving CO2 transport capacity, photosynthetic potential and water use efficiency under contrasting CO2 source in Solanum lycopersicum (tomato)

Mesophyll conductance has been increasingly recognized as a limiting-factor for carbon transport and fixation during photosynthetic process. Despite mesophyll conductance performs a significant role in determining photosynthetic and yield potential, its molecular mechanisms are uncertain and controversy. In the present study, the function of a plasma membrane aquaporin SlPIP1;2 in facilitating CO2 transport across the plasma membrane under contrasting CO2 concentrations was evaluated, according to the photosynthetic performance of transgenic lines of overexpression and knock-out. The relative importance of PIP-channel in determining membrane permeability and mesophyll conductance is largely depend on the ambient and intercellular CO2 concentration. Under the current ambient CO2 concentration, SlPIP1;2 had minor effect on photosynthetic performance. Overexpression of the target gene substantially enhanced CO2 membrane permeability under CO2 enrichment, according to the photosynthetic performance: overexpression SlPIP1;2 significantly improved mesophyll conductance and photosynthesis rate, which translated into improvement in crop productivity and water use efficiency. The present results suggested that SlPIP1;2 is potentially involved in facilitating CO2 transport across the plasma membrane under CO2 enrichment, which subsequently affect crop productivity and water use efficiency.

Changes in Leaf-Level Nitrogen Partitioning and Mesophyll Conductance Deliver Increased Photosynthesis for Lolium perenne Leaves Engineered to Accumulate Lipid Carbon Sinks.

Diacylglycerol acyl-transferase (DGAT) and cysteine oleosin (CO) expression confers a novel carbon sink (of encapsulated lipid droplets) in leaves of Lolium perenne and has been shown to increase photosynthesis and biomass. However, the physiological mechanism by which DGAT + CO increases photosynthesis remains unresolved. To evaluate the relationship between sink strength and photosynthesis, we examined fatty acids (FA), water-soluble carbohydrates (WSC), gas exchange parameters and leaf nitrogen for multiple DGAT + CO lines varying in transgene accumulation. To identify the physiological traits which deliver increased photosynthesis, we assessed two important determinants of photosynthetic efficiency, CO2 conductance from atmosphere to chloroplast, and nitrogen partitioning between different photosynthetic and non-photosynthetic pools. We found that DGAT + CO accumulation increased FA at the expense of WSC in leaves of L. perenne and for those lines with a significant reduction in WSC, we also observed an increase in photosynthesis and photosynthetic nitrogen use efficiency. DGAT + CO L. perenne displayed no change in rubisco content or Vcmax but did exhibit a significant increase in specific leaf area (SLA), stomatal and mesophyll conductance, and leaf nitrogen allocated to photosynthetic electron transport. Collectively, we showed that increased carbon demand via DGAT+CO lipid sink accumulation can induce leaf-level changes in L. perenne which deliver increased rates of photosynthesis and growth. Carbon sinks engineered within photosynthetic cells provide a promising new strategy for increasing photosynthesis and crop productivity.

CO2 diffusion in tobacco: a link between mesophyll conductance and leaf anatomy.

The partial pressure of CO2 at the sites of carboxylation within chloroplasts depends on the conductance to CO2 diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (gm). We investigated how gm varies with leaf age and through a tobacco (Nicotiana tabacum) canopy by combining gas exchange and carbon isotope measurements using tunable diode laser spectroscopy. We combined these measurements with the anatomical characterization of leaves. CO2 assimilation rate, A, and gm decreased as leaves aged and moved lower in the canopy and were linearly correlated. This was accompanied by large anatomical changes including an increase in leaf thickness. Chloroplast surface area exposed to the intercellular airspace per unit leaf area (Sc) also decreased lower in the canopy. Older leaves had thicker mesophyll cell walls and gm was inversely proportional to cell wall thickness. We conclude that reduced gm of older leaves lower in the canopy was associated with a reduction in Sc and a thickening of mesophyll cell walls.

Anatomical variation of mesophyll conductance due to salt stress in Populus cathayana females and males growing under different inorganic nitrogen sources.

Synergistic regulation in leaf architecture and photosynthesis is essential for salt tolerance. However, how plant sex and inorganic nitrogen sources alter salt stress-dependent photosynthesis remains unknown. Leaf anatomical characteristics and photosynthesis of Populus cathayana Rehder females and males were investigated under salt stress conditions combined with nitrate NO3- and ammonium NH4+ supplies to clarify the underlying mechanisms. In salt-stressed females, we observed an increased mesophyll spongy cell density, a reduced chloroplast density, a decreased surface area of chloroplasts adjacent to the intercellular air space (Sc/S) and an increased mesophyll cell area per transverse section width (S/W), consequently causing mesophyll conductance (gm) and photosynthesis inhibition, especially under NH4+ supply. Conversely, males with a greater mesophyll palisade tissue thickness and chloroplast density, but a lower spongy cell density had lower S/W and higher Sc/S, and higher gm and photosynthesis. NH4+-fed females had a lower CO2 conductance through cell wall and stromal conductance perpendicular to the cell wall, but a higher chloroplast conductance from the cell wall (gcyt1) than females supplied with NO3-, whereas males had a higher chloroplast conductance and lower CO2 conductance through cell wall when supplied with NO3- instead of NH4+ under salt stress. These findings indicate sex-specific strategies in coping with salt stress related to leaf anatomy and gm under both types of nitrogen supplies, which may contribute to sex-specific CO2 capture and niche segregation.

Yield and photosynthetic rate of wheat under continuously high temperature

In tropical regions high temperature occurs throughout wheat growth cycle and is a major factor influencing growth and yield in this area. The aim of this work is to describe the responses to continuously-high temperatures on growth, photosynthesis and yield of wheat, under controlled conditions. Nias and Dewata (Indonesian wheat varieties) and Axe and Gladius (Australian varieties) were tested in growth room experiment with temperature and photoperiod chosen to simulate conditions on Lombok Island, at lowland (32/23°C) and highland (28/20°C) sites. The third temperature (25/15°C) was selected to represent temperature in a more temperate wheat-producing area. High temperature reduced yield and dry matter accumulation which was associated with a reduction in photosynthetic rate and stomata conductance and an increase in respiration rate. The reduction in photosynthetic rate at high temperature was not only due to lower stomatal conductance but also non-stomatal effects as mesophyll conductance and quantum yield were lower. Genetic variability in response to heat stress was evident with the Indonesian varieties being more tolerant to high temperatures than Australian varieties. Nias and Dewata produced higher yield and biomass and maintaining higher rates of photosynthesis. Maintaining high photosynthetic rate and high stomata conductance, are important characters in adapting wheat into tropical environment such as Lombok Island.

Temperature responses of photosynthesis and stomatal conductance in rice and wheat plants

The mechanisms for the different temperature (T) responses of leaf photosynthetic rate (A) and stomatal conductance (gs) have not been thoroughly elucidated, and the uncertainties and limitations facing the optimal-stomata models have not been systematically investigated. In the present study, the temperature responses of A, gs, and mesophyll conductance (gm) were studied in the selected rice and wheat genotypes, and the C3 photosynthesis model and three widely used optimal-stomata models were introduced to investigate these two questions. The photosynthetic limiting process varied significantly between rice and wheat plants. The photosynthetic limitation and gm had strong impacts on the temperature response of λ, the key parameter for the optimal-stomata models. λ was increased with T when photosynthesis was limited by the RuBP carboxylation, while it was decreased with T when it was limited by the RuBP regeneration. Because of the negative correlation between λ and gs, the differing λ-T relationships may explain the different gs-T relationships between the selected genotypes. This is the first study using the optimal-stomata models to interpret the differential gs-T relationships and highlights the importance of estimating gm and the photosynthetic limitations before applying the optimal-stomata models.

Resurrection plants optimize photosynthesis despite very thick cell walls by means of chloroplast distribution

Resurrection plants are vascular species able to sustain extreme desiccation in their vegetative tissues. Despite its potential interest, the role of leaf anatomy in CO2 diffusion and photosynthesis under non-stressed conditions has not been explored in these species. Net CO2 assimilation (An) and its underlying diffusive, biochemical and anatomical determinants were assessed in 10 resurrection species from diverse locations, including ferns and homoiochlorophyllous and poikilochlorophyllous angiosperms. Data obtained were compared to previously published results in desiccation-sensitive ferns and angiosperms. An in resurrection plants was mostly driven by mesophyll conductance to CO2 (gm) and limited by CO2 diffusion. Resurrection species had a larger cell wall thickness (Tcw) than desiccation sensitive plants, a feature associated with limited CO2 diffusion in the mesophyll, but also greater chloroplast exposure to intercellular spaces (Sc), which usually leads to higher gm. This combination enabled a higher An per Tcw compared to desiccation sensitive species. Resurrection species possess unusual anatomical features that possibly confer stress tolerance (thick cell walls) without compromising the photosynthetic capacity (high chloroplast exposure). This mechanism is particularly successful in resurrection ferns, which display higher photosynthesis than their desiccation sensitive counterparts.

Stomatal, mesophyll and biochemical limitations to soil drought and rewatering in relation to intrinsic water-use efficiency in Manchurian ash and Mongolian oak

The balance between stomatal and mesophyll conductance has been reported to directly influence intrinsic water-use efficiency (WUEi), but it is unclear whether variations in stomatal and mesophyll limitations (lsc and lm, respectively) affect WUEi during soil drought and rewatering. Limitations (lsc, lm and biochemical limitation, lb) and WUEi were measured in Manchurian ash (Fraxinus mandshurica Rupr.) and Mongolian oak (Quercus mongolica Fish. ex Ledeb) saplings exposed to two levels of water stress (moderate and severe) and rewatering in this study. The primary limiting factors for photosynthesis among lsc, lm, and lb and their correlations with WUEi were assessed. Both lsc and lm increased with water stress, while lb decreased, and during recovery, lsc and lm decreased, while lb increased. Photosynthesis limitations directly influenced WUEi, with WUEi being larger after rewatering than during water stress, increasing as biochemical capacity weakened at the expense of diffusional limitations (lsc and lm). Stomatal closure was more limiting to net photosynthesis in Mongolian oak than in Manchurian ash during the drought and recovery treatments; consistently higher WUEi was measured in the former species, possibly due to a more developed root system and larger leaf hydraulic conductance and stem vessel diameter in the former species.

Increased photosynthesis from a deep-shade to high-light regime occurs by enhanced CO2 diffusion into the leaf of Selaginella martensii

The current understanding of photosynthesis across land plant phylogeny strongly indicates that ancient vascular plants are mainly limited by strong constitutive CO2 diffusional constraints, particularly low stomatal and mesophyll conductance. Considering that the lycophyte Selaginella martensii can demonstrate long-term light acclimation, this study addresses the regulation extent of CO2 assimilation in this species cultivated under contrasting light regimes of deep shade, medium shade and high light. Comparative analyses of photosynthetic traits, CO2 conductance and leaf morpho-anatomy revealed acclimation plasticity similar to that of seed plants, though occurring in the context of an inherently low photosynthetic capacity typical of lycophytes. Specific modulations of the stomatal density and aperture, chloroplast surface exposed to mesophyll airspaces and cell wall thickness sustained a marked improvement in CO2 diffusion from deep shade to high light. However, the maximum carboxylation rate was comparatively less effectively upregulated, leading to a greater incidence of biochemical limitations of photosynthesis. Because of a low carboxylation capacity under any light regime, a lycophyte prevents potential photodamage to the chloroplast by not only exploiting the thermal dissipation of excess absorbed energy but also diverting a large fraction of photosynthetic electrons to sinks alternative to carboxylation.

Linking canopy-scale mesophyll conductance and phloem sugar δ13 C using empirical and modelling approaches.

Summary Interpreting phloem carbohydrate or xylem tissue carbon isotopic composition as measures of water‐use efficiency or past tree productivity requires in‐depth knowledge of the factors altering the isotopic composition within the pathway from ambient air to phloem contents and tree ring. One of least understood of these factors is mesophyll conductance (g m).We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative g m descriptions and a simple transport of sugars from foliage down the trunk. We parameterised the model for a boreal Scots pine stand and compared simulated g m responses with weather variations. We further compared the simulated δ13C of new photosynthates among the different g m descriptions and against measured phloem sugar δ13C.Simulated g m estimates of the seven descriptions varied according to weather conditions, resulting in varying estimates of phloem δ13C during cold/moist and warm/dry periods. The model succeeded in predicting a drought response and a postdrought release in phloem sugar δ13C indicating suitability of the model for inverse prediction of leaf processes from phloem isotopic composition.We suggest short‐interval phloem sampling during and after extreme weather conditions to distinguish between mesophyll conductance drivers for future model development.

Photosynthetic performance responses in different phyiological development stages of some Brassica juncea genotypes in field condition

In this study Brassica juncea genotypes were planted as fall sowing at the experimental fields located at Yenimahalle-Ankara location based on randomized block design. In order to investigate photosynthetic performances of three B. juncea genotypes; net photosynthetic rate = PN [μmol(CO2) m-2s-1], transpiration = E (mmol m-2s-1), stomatal conductance = gs (mmol m-2s-1), intercellular/ambient CO2 air= Ci [μmol (CO2) mol-1(hava)], mesophyll conductance= gm [mmol(CO2)/m2s1] and photosynthetic water use efficiency = PWUE [μmol(CO2)/mol(H2O)] parameters (model LCi Photosynthesis System, ADC Bioscientific Ltd., Hertfordshire, UK) were measured and phenological aspects were recorded at different physiological stages as booting, anthesis and grain filling during experiment. Photosynthetic rate and stomatal conductance were associated with seed yield in B. juncea genotypes and the selection of genotypes with high gas exchange may provide development of mustard (B. juncea) genotypes with high yield.

Whole-tree mesophyll conductance reconciles isotopic and gas-exchange estimates of water-use efficiency.

Photosynthetic water-use efficiency (WUE) describes the link between terrestrial carbon (C) and water cycles. Estimates of intrinsic WUE (iWUE) from gas exchange and C isotopic composition (δ13 C) differ due to an internal conductance in the leaf mesophyll (gm ) that is variable and seldom computed. We present the first direct estimates of whole-tree gm , together with iWUE from whole-tree gas exchange and δ13 C of the phloem (δ13 Cph ). We measured gas exchange, online 13 C-discrimination, and δ13 Cph monthly throughout spring, summer, and autumn in Eucalyptus tereticornis grown in large whole-tree chambers. Six trees were grown at ambient temperatures and six at a 3°C warmer air temperature; a late-summer drought was also imposed. Drought reduced whole-tree gm . Warming had few direct effects, but amplified drought-induced reductions in whole-tree gm . Whole-tree gm was similar to leaf gm for these same trees. iWUE estimates from δ13 Cph agreed with iWUE from gas exchange, but only after incorporating gm . δ13 Cph was also correlated with whole-tree 13 C-discrimination, but offset by -2.5±0.7‰, presumably due to post-photosynthetic fractionations. We conclude that δ13 Cph is a good proxy for whole-tree iWUE, with the caveats that post-photosynthetic fractionations and intrinsic variability of gm should be incorporated to provide reliable estimates of this trait in response to abiotic stress.

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.

Elevated [CO2] benefits coffee growth and photosynthetic performance regardless of light availability

Despite being evolved in shaded environments, most coffee (Coffea arabica L.) is cultivated worldwide under sparse shade or at full sunlight. Coffee is ranked as greatly responsive to climate change (CC), and shading has been considered an important management strategy for mitigating the harmful CC outcomes on the crop. However, there is no information on the effects of enhanced [CO2] (eCa) on coffee performance in response to light availability. Here, we examined how carbon assimilation and use are affected by eCa in combination with contrasting light levels. For that, greenhouse-grown plants were submitted to varying light levels (16 or 7.5 mol photons m−2 day−1) and [CO2] (ca. 380 or 740 μmol mol−1 air) over six months. We demonstrated that both high light and eCa improved growth and photosynthetic performance, independently. Despite marginal alterations in biomass partitioning, some allometric changes, such as higher root biomass-to-total leaf area and lower leaf area ratio under the combination of eCa and high light were found. Stimulation of photosynthetic rates by eCa occurred with no direct effect on stomatal and mesophyll conductances, and no signs of photosynthetic down-regulation were found irrespective of treatments. Particularly at high light, eCa led to decreases in both photorespiration rates and oxidative pressure. Overall, our novel findings suggest that eCa could tandemly act with shading to mitigate the harmful CC effects on coffee sustainability.

Anatomically induced changes in rice leaf mesophyll conductance explain the variation in photosynthetic nitrogen use efficiency under contrasting nitrogen supply

BackgroundThe ratio of CO2 mesophyll conductance (gm) to Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) content has been suggested to positively affect photosynthetic nitrogen use efficiency (PNUE). The anatomical basis of gm has been quantified, but information on the relationship between cell-level anatomies and PNUE is less advanced. Here, hydroponic experiments were conducted in rice plants supplied with ammonium (NH4+) and nitrate (NO3−) under three N levels (low, 0.71 mM; intermediate, 2.86 mM; high, 7.14 mM) to investigate the gas exchange parameters, leaf anatomical structure and PNUE.ResultsThe results showed a lower PNUE in plants supplied with high nitrogen and NH4+, which was positively correlated with the gm/Rubisco ratio. A one-dimensional within-leaf model revealed that the resistance to CO2 diffusion in the liquid phase (rliq) dominated the overall mesophyll resistance (rm), in which CO2 transfer resistance in the cell wall, cytoplasm and stroma were significantly affected by nitrogen supply. The chloroplast surface area exposed to intercellular space (Sc) per Rubisco rather than the gm/Sc ratio was positively correlated with PNUE and was thus considered a key component influencing PNUE.ConclusionIn conclusion, our study emphasized that Sc was the most important anatomical trait in coordinating gm and PNUE with contrasting N supply.

The complementary role of root and leaf PIP1 and PIP2 aquaporins drives the anisohydric behavior in Helinathus annuus L.

This study aimed at determining the role of aquaporins (AQPs) in the anisohydric behavior of sunflower. Deactivation of AQPs by mercury revealed that they could play vital role in the stomatal regulation and this role was not time-dependent under the control conditions. Sunflower seedlings were exposed to drought or to osmotic or ionic factor of NaCl and KCl. Under control and optimal conditions, sunflower leaves behaved in an anisohydric manner (maintaining high stomatal conductance (gs) at 13:00 compared to 9:00). At 13:00: 1) the upregulation of PIP1;1 could improve the mesophyll conductance to CO2, leading to enhancement of the photosynthetic rate (A) and consequently increasing the gs and 2) PIP2s were downregulated in the leaves, perhaps to decrease the hydraulic conductance and control excess water loss via transpiration but at the same time these genes were upregulated in the roots to improve root to leaf hydraulic conductance and maintain the positive water balance of the whole plant. Under drought, the decrease in gs and A at 13:00 could imply a shift towards near-isohydric behavior. The downregulation of PIP1s and further downregulation of PIP2s could indicate their involvement in this shift by decreasing CO2 permeability and avoiding the possible loss of water from the plant roots to soil, respectively. The anisohydric behavior of leaves treated with KCl, but not NaCl, (25 or 150 mM) could be due to the positive impact of K+ on stomata and the regulation of PIP1s and PIP2s. The anisohydric behavior could be achieved by a synchronized regulation of leaf and root AQPs.

The Light Dependence of Mesophyll Conductance and Relative Limitations on Photosynthesis in Evergreen Sclerophyllous Rhododendron Species.

Mesophyll conductance (gm) limits CO2 diffusion from sub-stomatal internal cavities to the sites of RuBP carboxylation. However, the response of gm to light intensity remains controversial. Furthermore, little is known about the light response of relative mesophyll conductance limitation (lm) and its effect on photosynthesis. In this study, we measured chlorophyll fluorescence and gas exchange in nine evergreen sclerophyllous Rhododendron species. gm was maintained stable across light intensities from 300 to 1500 μmol photons m−2 s−1 in all these species, indicating that gm did not respond to the change in illumination in them. With an increase in light intensity, lm gradually increased, making gm the major limiting factor for area-based photosynthesis (AN) under saturating light. A strong negative relationship between lm and AN was found at 300 μmol photons m−2 s−1 but disappeared at 1500 μmol photons m−2 s−1, suggesting an important role for lm in determining AN at sub-saturating light. Furthermore, the light-dependent increase in lm led to a decrease in chloroplast CO2 concentration (Cc), inducing the gradual increase of photorespiration. A higher lm under saturating light made AN more limited by RuBP carboxylation. These results indicate that the light response of lm plays significant roles in determining Cc, photorespiration, and the rate-limiting step of AN.

Mesophyll conductance: walls, membranes and spatial complexity.

A significant resistance to CO2 diffusion is imposed by mesophyll tissue inside leaves. Mesophyll resistance, rm (or its reciprocal, mesophyll conductance, gm ), reduces the rate at which Rubisco can fix CO2 , increasing the water and nitrogen costs of carbon acquisition. gm varies in proportion to the surface area of chloroplasts exposed to intercellular airspace per unit leaf area. It also depends on the thickness and effective porosity of the cell wall and the CO2 permeabilities of membranes. As no measurements exist for the effective porosity of mesophyll cell walls, and CO2 permeability values are too low to account for observed rates of CO2 assimilation, conclusions from modelling must be treated with caution. There is great variation in the mesophyll resistance per unit chloroplast area for a given cell wall thickness, which may reflect differences in effective porosity. While apparent gm can vary with CO2 and irradiance, the underlying conductance at the cellular level may remain unchanged. Dynamic changes in apparent gm arise for spatial reasons and because chloroplasts differ in their photosynthetic composition and operate in different light environments. Measurements of the temperature sensitivity of membrane CO2 permeability are urgently needed to explain variation in temperature responses of gm .

Accounting for mesophyll conductance substantially improves 13 C-based estimates of intrinsic water-use efficiency.

Carbon isotope discrimination (Δ) has been used widely to infer intrinsic water-use efficiency (iWUE) of C3 plants, a key parameter linking carbon and water fluxes. Despite the essential role of mesophyll conductance (gm ) in photosynthesis and Δ, its effect on Δ-based predictions of iWUE has generally been neglected. Here, we derive a mathematical expression of iWUE as a function of Δ that includes gm (iWUEmes ) and exploits the gm -stomatal conductance (gsc ) relationship across drought-stress levels and plant functional groups (deciduous or semideciduous woody, evergreen woody and herbaceous species) in a global database. iWUEmes was further validated with an independent dataset of online-Δ and CO2 and H2 O gas exchange measurements with seven species. Drought stress reduced gsc and gm by nearly one-half across all plant functional groups, but had no significant effect on the gsc :gm ratio, with a well supported value of 0.79±0.07 (95% CI, n=198). gm was negatively correlated to iWUE. Incorporating the gsc :gm ratio greatly improved estimates of iWUE, compared with calculations that assumed infinite gm . The inclusion of the gsc :gm ratio, fixed at 0.79 when gm was unknown, proved desirable to eliminate significant errors in estimating iWUE from Δ across various C3 vegetation types.