Nutrient supply enhanced the increase in intrinsic water‐use efficiency of a temperate seminatural grassland in the last century

Under the increase in atmospheric CO₂ during the last century, variable increases in the intrinsic water‐use efficiency (W_i), i.e., the ratio between carbon assimilation rate (A) and stomatal conductance (g_s), of C₃ vegetation have been observed. Here, we ask if long‐term nutrient status and especially nitrogen supply have an effect on the CO₂ response of W_i in a temperate seminatural C₃ grassland. This analysis draws on the long‐term trends (1915–2009) in W_i, derived from carbon isotope analysis, of archived hay and herbage from the Park Grass Experiment at Rothamsted (South‐East England). Plant samples came from five fertilizer treatments, each with different annual nitrogen (N; 0, 48 or 96 kg ha^?1), phosphorus (P; 0 or 35 kg ha^?1) and potassium (K; 0 or 225 kg ha^?1) applications, with lime as required to maintain soil pH near 7. Carbon isotope discrimination (¹³Δ) increased significantly (P < 0.001) on the Control (0.9‰ per 100 ppm CO₂ increase). This trend differed significantly (P < 0.01) from those observed on the fertilized treatments (PK only: 0.4‰ per 100 ppm CO₂ increase, P < 0.001; Low N only, Low N+PK, High N+PK: no significant increase). The ¹³Δ trends on fertilized treatments did not differ significantly from each other. However, N status, assessed as N fertilizer supply plus an estimate of biologically fixed N, was negatively related (r² = 0.88; P < 0.02) to the trend for ¹³Δ against CO₂. Other indices of N status exhibited similar relationships. Accordingly, the increase in W_i at High N+PK was twice that of the Control (+28% resp. +13% relative to 1915). In addition, the CO₂ responsiveness of ¹³Δ was related to the grass content of the plant community. This may have been due to the greater CO₂ responsiveness of g_s in grasses relative to forbs. Thus, the greater CO₂ response of grass‐rich fertilized swards may be related to effects of nutrient supply on botanical composition.     

The influence of (photo)respiration on carbon isotope discrimination in plants

The contribution which (photo)respiration makes to carbon isotope discrimination (Δ¹³C) was examined by conducting simultaneous gas exchange measurements and isotopic analysis of carbon dioxide passing over leaves of Triticum aestivum and Phaseolus vulgaris, via manipulations of the carbon isotope composition (δ¹³C) of source CO₂ during growth and measurement. Dark respiration only altered net Δ¹³C (Δ_obs) at low CO₂ assimilation, and was sensitive to source CO₂δ¹³C during measurement. Photorespiration reduced Δ_obs relative to Δ¹³C predicted from p_i/p_a (Δ_i) over the full range of CO₂ assimilation, to a greater degree under elevated oxygen partial pressure (pO₂), indicating fractionation during photorespiration (f) in T. aestivum. For P. vulgaris, Δ_obs was insensitive to elevated pO₂ at higher assimilation rates, suggesting that f was minimal. A model was developed to calculate gross discrimination (Δ_ps), independent of (photo)respiration, from which estimates of f were obtained for T. aestivum (3.3‰) and P. vulgaris (0.5‰). Because photorespiratory fractionation varies interspecifically, and influences net Δ¹³C which is directly reflected in leaf δ¹³C, consideration of (photo)respiratory fractionation is necessary when interpreting δ¹³C of leaf material, especially under conditions where (photo)respiratory CO₂ losses make a large relative contribution to total plant carbon budgets.     

Intrinsic water‐use efficiency of temperate seminatural grassland has increased since 1857: an analysis of carbon isotope discrimination of herbage from the Park Grass Experiment

A 150‐year‐long record of intrinsic water‐use efficiency (W_i) was derived from community‐level carbon isotope discrimination (¹³Δ) in the herbage of the unfertilized, unlimed control treatment (plot 3) of the Park Grass Experiment at Rothamsted (England) between 1857 and 2007. ¹³Δ during spring growth (first cut harvested in June) averaged 21.0‰ (±0.5‰ SD) and has not shown a long‐term trend (P=0.5) since 1857. ¹³Δ of summer/autumn growth (second cut harvested between September and November) increased from 21.3‰ to 22.0‰ (P < 0.001) between 1875 and 2007. W_i during spring growth has therefore increased by 33% since the beginning of the experiment, and W_i of summer/autumn growth has increased by 18%. The variation in ¹³Δ was mainly related to weather conditions. Plant available soil water explained 51% and 40% of the variation in spring growth ¹³Δ and summer/autumn growth ¹³Δ, respectively. In the 1857–2007 period yields have not increased, suggesting that community‐level photosynthesis has not increased either. Therefore, the increased W_i probably resulted from a decreased stomatal conductance. Vapour pressure deficit (VPD) during spring growth (March–June) has not changed since 1915, meaning that instantaneous water‐use efficiency (W_t) in spring time has increased and transpiration has probably decreased, provided that leaf temperature followed air temperature. Conversely, VPD in the months between the first and second cut has increased by 0.07 kPa since 1915, offsetting the effect of increased W_i on W_t during summer and early autumn. Our results suggest that vegetation has adjusted physiologically to elevated CO₂ by decreasing stomatal conductance in this nutrient‐limited grassland.     

Low conductances for CO2 diffusion from stomata to the sites of carboxylation in leaves of woody species

Concurrent measurements of leaf gas exchange and on-line ¹³C discrimination were used to evaluate the CO₂ conductance to diffusion from the stomatal cavity to the sites of carboxylation within the chloroplast (internal conductance; g_i). When photon irradiance was varied it appeared that g_i and/or the discrimination accompanying carboxylation also varied. Despite this problem, g_i, was estimated for leaves of peach (Prunus persica), grapefruit (Citrus paradisi), lemon (C. limon) and macadamia (Macadamia integrifolia) at saturating photon irradiance. Estimates for leaves of C. paradisi, C. limon and M. integrifolia were considerably lower than those previously reported for well-nourished herbaceous plants and ranged from 1.1 to2.2μmol CO₂ m^?2 s^?1 Pa^?1, whilst P. persica had a mean value of 3.5 μmol CO₂ m^?2 s^?1 Pa^?1. At an ambient CO₂ partial pressure of 33Pa, estimates of chloroplastic partial pressure of CO₂ (C_c) using measurements of CO₂ assimilation rate (A) and calculated values of g_i, and of partial pressure of CO₂ in the stomatal cavity (C_st) were as low as 11.2 Pa for C. limon and as high as 17.8Pa for peach. In vivo maximum rubisco activities (V_max) were also determined from estimates of C_c. This calculation showed that for a given leaf nitrogen concentration (area basis) C. paradisi and C. limon leaves had a lower V_max than P. persica, with C. paradisi and C. limon estimated to have only 10% of leaf nitrogen present as rubisco. Therefore, low CO₂ assimilation rates despite high leaf nitrogen concentrations in leaves of the evergreen species examined were explained not only by a low C_c but also by a relatively low proportion of leaf nitrogen being used for photosynthesis. We also show that simple one-dimensional equations describing the relationship between leaf internal conductance from stomatal cavities to the sites of carboxylation and carbon isotope discrimination (Δ) can lead to errors in the estimate of g_i. Potential effects of heterogeneity in stomatal aperture on carbon isotope discrimination may be particularly important and may lead to a dependence of g_i upon CO₂ assimilation rate. It is shown that for any concurrent measurement of A and Δ, the estimate of C_c is an overestimate of the correct photosynthetic capacity-weighted value, but this error is probably less than 1.0 Pa.     

Mesophyll CO2 conductance and leakiness are not responsive to short‐ and long‐term soil water limitations in the C4 plant Sorghum bicolor

Breeding economically important C₄ crops for enhanced whole‐plant water‐use efficiency (WUE_plant) is needed for sustainable agriculture. WUE_plant is a complex trait and an efficient phenotyping method that reports on components of WUE_plant, such as intrinsic water‐use efficiency (WUE_i, the rate of leaf CO₂ assimilation relative to water loss via stomatal conductance), is needed. In C₄ plants, theoretical models suggest that leaf carbon isotope composition (δ¹³C), when the efficiency of the CO₂‐concentrating mechanism (leakiness, ?) remains constant, can be used to screen for WUE_i. The limited information about how ? responds to water limitations confines the application of δ¹³C for WUE_i screening of C₄ crops. The current research aimed to test the response of ? to short‐ or long‐term moderate water limitations, and the relationship of δ¹³C with WUE_i and WUE_plant, by addressing potential mesophyll CO₂ conductance (g_m) and biochemical limitations in the C₄ plant Sorghum bicolor. We demonstrate that g_m and ? are not responsive to short‐ or long‐term water limitations. Additionally, δ¹³C was not correlated with gas‐exchange estimates of WUE_i under short‐ and long‐term water limitations, but showed a significant negative relationship with WUE_plant. The observed association between the δ¹³C and WUE_plant suggests an intrinsic link of δ¹³C with WUE_i in this C₄ plant, and can potentially be used as a screening tool for WUE_plant in sorghum.     

The effects of nitrogen stress on the stable carbon isotope composition, productivity and water use efficiency of white spruce (Picea glauca (Moench) Voss) seedlings

A, assimilation rate
a, fractionation against ¹³C for CO₂ diffusion through air
b, net fractionation against ¹³C during CO₂ fixation
C_a, ambient CO₂ concentration
C_c, CO₂ concentration at the chloroplast
C_i, intercellular CO₂ concentration
D, vapour pressure deficit
E_n, needle transpiration rate
E_p, whole plant water use
g_w, leaf internal transfer conductance to CO₂
g_s, stomatal conductance to water vapour
L, projected leaf area
NUE, nitrogen use efficiency
PEP, phosphoenolpyruvate
Rubisco, ribulose-1,5-biphosphate carboxylase
TDR, time domain reflectometry
WUE, water use efficiency
Δ, carbon isotope discrimination
δ¹³C, carbon isotope abundance parameter
δ¹³C_a, carbon isotopic composition of atmospheric CO₂
θ, volumetric soil water content

The effect of nitrogen stress on needle δ¹³C, water-use efficiency (WUE) and biomass production in irrigated and dry land white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings, representing 10 controlled crosses, were planted in the field in individual buried sand-filled cylinders. Two nitrogen treatments were imposed, nitrogen stressed and fertilized. The ranking of δ¹³C of the crosses was maintained across all combinations of water and nitrogen treatments and there was not a significant genetic versus environmental interaction. The positive relationships between needle δ¹³C, WUE and dry matter production demonstrate that it should be possible to use δ¹³C as a surrogate for WUE, and to select for increased WUE without compromising yield, even in nitrogen deficient environments. Nitrogen stressed seedlings had the lowest needle δ¹³C in both irrigated and dry land conditions. There was a positive correlation between needle nitrogen content and δ¹³C that was likely associated with increased photosynthetic capacity. There was some indication that decreased nitrogen supply led to increased stomatal conductance and hence lower WUE. There was a negative correlation between intrinsic water use efficiency and photosynthetic nitrogen use efficiency (NUE). This suggests that white spruce seedlings have the ability to maximize NUE when water becomes limited. There was significant genetic variation in NUE that was maintained across treatments. Our results suggest that in white spruce, there is no detectable effect of anaplerotic carbon fixation and that it is more appropriate to use a value of 29‰ (‘Rubisco only’) for the net discrimination against ¹³C during CO₂ fixation. This leads to excellent correspondence between values of C_i/C_a derived from gas exchange measurements or from δ¹³C.     

Leaf hydraulic conductance and mesophyll conductance are not closely related within a single species

Stomata represent one resistor in a series of resistances for carbon and water exchange between the leaf and the atmosphere; the remaining resistors occurring within the leaf, commonly represented as mesophyll conductance to CO₂, g_m, and leaf hydraulic conductance, k_Leaf. Recent studies have proposed that g_m and k_Leaf may be coordinated across species because of shared pathways. We assessed the correlation between g_m and k_Leaf within cotton, under growth CO₂ partial pressure and irradiance treatments and also with short‐term variation in irradiance and humidity. g_m was estimated using two isotopic techniques that allowed partitioning of total g_m (Δ¹³C‐g_m) into cell wall plus plasma membrane conductance (Δ¹⁸O‐g_m) and chloroplast membrane conductance (g_cm). A weak correlation was found between Δ¹³C‐g_m and k_Leaf only when measured under growth conditions. However, Δ¹⁸O‐g_m was related to k_Leaf under both short‐term environmental variation and growth conditions. Partitioning g_m showed that g_cm was not affected by short‐term changes in irradiance or correlated with k_Leaf, but was strongly reduced at high growth CO₂ partial pressure. Thus, simultaneous measurements of g_m, k_Leaf and g_cm suggest independent regulation of carbon and water transport across the chloroplast membrane with limited coordinated regulation across the cell wall and plasma membrane.     

Artefactual responses of mesophyll conductance to CO2 and irradiance estimated with the variable J and online isotope discrimination methods

Studies with the variable J method have reported that mesophyll conductance (g_m) rapidly decreases with increasing intercellular CO₂ partial pressures (C_i) or decreasing irradiance. Similar responses have been suggested with the online isotope discrimination method, although with less consistency. Here we show that even when the true g_m is constant, the variable J method can produce an artefactual dependence of g_m on C_i or irradiance similar to those reported in previous studies for any of the following factors: day respiration and chloroplastic CO₂ photocompensation point are estimated with Laisk method; C_i or electron transport rate is positively biased; net photosynthetic rate is negatively biased; insufficient NADPH is assumed while insufficient ATP limits RuBP regeneration. The isotopic method produces similar artefacts if fractionation of carboxylation or C_i is positively biased or Δ¹³ negatively biased. A non‐zero chloroplastic resistance to CO₂ movement results in a qualitatively different dependence of g_m on C_i or irradiance and this dependence is only sensitive at low C_i. We thus cannot rule out the possibility that previously reported dependence of g_m on C_i or irradiance is a methodological artefact. Recommendations are made to take advantage of sensitivities of the variable J and isotopic methods for estimating g_m.     

The use of low [CO2] to estimate diffusional and non-diffusional limitations of photosynthetic capacity of salt-stressed olive saplings

In this study it has been shown that increased diffusional resistances caused by salt stress may be fully overcome by exposing attached leaves to very low [CO₂] (～ 50 µmol mol^?1), and, thus a non‐destructive‐in vivo method to correctly estimate photosynthetic capacity in stressed plants is reported. Diffusional (i.e. stomatal conductance, g_s, and mesophyll conductance to CO₂, g_m) and biochemical limitations to photosynthesis (A) were measured in two 1‐year‐old Greek olive cultivars (Chalkidikis and Kerkiras) subjected to salt stress by adding 200 mm NaCl to the irrigation water. Two sets of A–C_i curves were measured. A first set of standard A–C_i curves (i.e. without pre‐conditioning plants at low [CO₂]), were generated for salt‐stressed plants. A second set of A–C_i curves were measured, on both control and salt‐stressed plants, after pre‐conditioning leaves at [CO₂] of ～ 50 µmol mol^?1 for about 1.5 h to force stomatal opening. This forced stomata to be wide open, and g_s increased to similar values in control and salt‐stressed plants of both cultivars. After g_s had approached the maximum value, the A–C_i response was again measured. The analysis of the photosynthetic capacity of the salt‐stressed plants based on the standard A–C_i curves, showed low values of the J_max (maximum rate of electron transport) to V_cmax (RuBP‐saturated rate of Rubisco) ratio (1.06), that would implicate a reduced rate of RuBP regeneration, and, thus, a metabolic impairment. However, the analysis of the A–C_i curves made on pre‐conditioned leaves, showed that the estimates of the photosynthetic capacity parameters were much higher than in the standard A–C_i responses. Moreover, these values were similar in magnitude to the average values reported by Wullschleger (Journal of Experimental Botany 44, 907–920, 1993) in a survey of 109 C₃ species. These findings clearly indicates that: (1) salt stress did affect g_s and g_m but not the biochemical capacity to assimilate CO₂ and therefore, in these conditions, the sum of the diffusional resistances set the limit to photosynthesis rates; (2) there was a linear relationship (r² = 0.68) between g_m and g_s, and, thus, changes of g_m can be as fast as those of g_s; (3) the estimates of photosynthetic capacity based on A–C_i curves made without removing diffusional limitations are artificially low and lead to incorrect interpretations of the actual limitations of photosynthesis; and (4) the analysis of the photosynthetic properties in terms of stomatal and non‐stomatal limitations should be replaced by the analysis of diffusional and non‐diffusional limitations of photosynthesis. Finally, the C₃ photosynthesis model parameterization using in vitro‐measured and in vivo‐measured kinetics parameters was compared. Applying the in vivo‐measured Rubisco kinetics parameters resulted in a better parameterization of the photosynthesis model.     

Modelling the discrimination of 13CO2 above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales

Fluxes and concentrations of carbon dioxide and ¹³CO₂ provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of ¹³CO₂ within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model (CANISOTOPE) to simulate concentration profiles of ¹³CO₂, its flux density across the canopy–atmosphere interface and leaf‐level photosynthetic discrimination against ¹³CO₂ is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against ¹³CO₂, by the deciduous forest canopy (Δ_canopy), was computed to be 22·4‰, but it varied between 18 and 27‰. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Δ_canopy occurred early and late in the growing season. Diurnal and seasonal variations in Δ_canopy resulted from a strong dependence of Δ_canopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy‐scale water use efficiency (CO₂ assimilation/transpiration) and Δ_canopy was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well‐mixed environments.     

Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator,the gentoo penguin (Pygoscelis papua)

Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ¹³C_C‐D and Δ¹⁵N_C‐D, respectively). We found that essential AA δ¹³C values and source AA δ¹⁵N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessential AA Δ¹³C_C‐D values reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDF_Glu‐Phe = 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ¹⁵N_C‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDF_Glu‐Phe equation with the avian‐specific TDF_Glu‐Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDF_Glu‐Phe of 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.     

Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd

Native scrub‐oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO₂] (approx. 350 μmol mol^?1) and to high [CO₂] (increased by 350 μmol mol^?1). Stomatal and photosynthetic acclimation to high [CO₂] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (g_s) was approximately 40% lower in the high‐ compared to low‐[CO₂]‐grown plants when measured at their respective growth concentrations. Reciprocal measurements of g_s in both high‐ and low‐[CO₂]‐grown plants showed that there was negative acclimation in the high‐[CO₂]‐grown plants (9–16% reduction in g_s when measured at 700 μmol mol^?1), but these were small compared to those for net CO₂ assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO₂] (c_i) which showed a reduction in stomatal sensitivity at low c_i in the high‐[CO₂]‐grown plants. Stomatal density showed no change in response to growth in high growth [CO₂]. Long‐term stomatal and photosynthetic acclimation to growth in high [CO₂] did not markedly change the 2·5‐ to 3‐fold increase in gas‐exchange‐derived water use efficiency caused by high [CO₂].     

Assessing the effects of nitrogen deposition and climate on carbon isotope discrimination and intrinsic water‐use efficiency of angiosperm and conifer trees under rising CO2 conditions

The objective of this study is to globally assess the effects of atmospheric nitrogen deposition and climate, associated with rising levels of atmospheric CO₂, on the variability of carbon isotope discrimination (Δ¹³C), and intrinsic water‐use efficiency (iWUE) of angiosperm and conifer tree species. Eighty‐nine long‐term isotope tree‐ring chronologies, representing 23 conifer and 13 angiosperm species for 53 sites worldwide, were extracted from the literature, and used to obtain long‐term time series of Δ¹³C and iWUE. Δ¹³C and iWUE were related to the increasing concentration of atmospheric CO₂ over the industrial period (1850–2000) and to the variation of simulated atmospheric nitrogen deposition and climatic variables over the period 1950–2000. We applied generalized additive models and linear mixed‐effects models to predict the effects of climatic variables and nitrogen deposition on Δ¹³C and iWUE. Results showed a declining Δ¹³C trend in the angiosperm and conifer species over the industrial period and a 16.1% increase of iWUE between 1850 and 2000, with no evidence that the increased rate was reduced at higher ambient CO₂ values. The temporal variation in Δ¹³C supported the hypothesis of an active plant mechanism that maintains a constant ratio between intercellular and ambient CO₂ concentrations. We defined linear mixed‐effects models that were effective to describe the variation of Δ¹³C and iWUE as a function of a set of environmental predictors, alternatively including annual rate (N_rate) and long‐term cumulative (N_cum) nitrogen deposition. No single climatic or atmospheric variable had a clearly predominant effect, however, Δ¹³C and iWUE showed complex dependent interactions between different covariates. A significant association of N_rate with iWUE and Δ¹³C was observed in conifers and in the angiosperms, and N_cum was the only independent term with a significant positive association with iWUE, although a multi‐factorial control was evident in conifers.     

Diurnal and seasonal variations in stomatal conductance of rice at elevated atmospheric CO2 under fully open‐air conditions

Understanding of leaf stomatal responses to the atmospheric CO₂ concentration, [CO₂], is essential for accurate prediction of plant water use under future climates. However, limited information is available for the diurnal and seasonal changes in stomatal conductance (g_s) under elevated [CO₂]. We examined the factors responsible for variations in g_s under elevated [CO₂] with three rice cultivars grown in an open‐field environment under flooded conditions during two growing seasons (a total of 2140 individual measurements). Conductance of all cultivars was generally higher in the morning and around noon than in the afternoon, and elevated [CO₂] decreased g_s by up to 64% over the 2 years (significantly on 26 out of 38 measurement days), with a mean g_s decrease of 23%. We plotted the g_s variations against three parameters from the Ball‐Berry model and two revised versions of the model, and all parameters explained the g_s variations well at each [CO₂] in the morning and around noon (R² > 0.68), but could not explain these variations in the afternoon (R² < 0.33). The present results provide an important basis for modelling future water use in rice production.     

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.     

Desert dogma revisited: coupling of stomatal conductance and photosynthesis in the desert shrub, Larrea tridentata

The success of the desert shrub Larrea tridentata (creosotebush) has been largely attributed to temperature acclimation and stomatal control of photosynthesis (A) under drought stress. However, there is a paucity of field data on these relationships. To address this void, we conducted a joint field and modelling study that encompassed a diverse set of environmental conditions. At a Larrea‐dominated site in southern New Mexico we manipulated soil moisture during the growing season over a 2‐year period and measured plant pre‐dawn water potential (Ψ_pd), stomatal conductance (g) and A of individual shrubs. We used these data to develop a semi‐mechanistic photosynthesis model (A–Season) that explicitly couples internal CO₂ (C_i) and g. Vapour pressure deficit (VPD) and Ψ_pd affect instantaneous g in a manner that is consistent with a biophysical model of stomatal regulation of leaf water potential. C_i is modelled as a function of g, derived from a simplification of a typical A–C_i curve. After incorporating the effects of growing temperature on stomatal behaviour, the model was able to capture the large diurnal fluctuations in A, g and C_i and the observed hysteresis in g versus C_i dynamics. Our field data and application of the A–Season model suggest that dogma attributed to Larrea's success is supported with regard to stomatal responses to VPD and Ψ_pd, but not for mechanisms of temperature acclimation and CO₂ demand.     

Quantitative trait loci controlling water use efficiency and related traits in Quercus robur L.

Genetic variation for intrinsic water use efficiency (W _i) and related traits was estimated in a full-sib family of Quercus robur L. over 3 years. The genetic linkage map available for this F1 family was used to locate quantitative trait loci (QTL) for W _i, as estimated by leaf carbon stable isotope composition (δ ¹³C) or the ratio of net CO₂ assimilation rate (A) to stomatal conductance to water vapour (g _w) and related leaf traits. Gas exchange measurements were used to standardize estimates of A and g _w and to model the sensitivity of g _w to leaf-to-air vapour pressure deficit (sg^VPD). δ ¹³C varied by more than 3‰ among the siblings, which is equivalent to 40% variation of W _i. Most of the studied traits exhibited high clonal mean repeatabilities (>50%; proportion of clonal mean variability in global variance). Repeatabilities for δ ¹³C, leaf mass per area (LMA) and leaf nitrogen content were higher than 70%. For δ ¹³C, ten QTLs were detected, one of which was detected repeatedly for all 3 years and consistently explained more than 20% of measured variance. Four genomic regions were found in which co-localizing traits linked variation in W _i to variations in leaf chlorophyll and nitrogen content, LMA and sg^VPD. A positive correlation using clonal means between δ ¹³C and A/g _w, as well as a co-localisation of QTL detected for both traits, can be seen as validation of the theoretical model linking the genetic architecture of these two traits.     

On the need to incorporate sensitivity to CO2 transfer conductance into the Farquhar–von Caemmerer–Berry leaf photosynthesis model

Virtually all current estimates of the maximum carboxylation rate (V_cmax) of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) and the maximum electron transport rate (J_max) for C₃ species implicitly assume an infinite CO₂ transfer conductance (g_i). And yet, most measurements in perennial plant species or in ageing or stressed leaves show that g_i imposes a significant limitation on photosynthesis. Herein, we demonstrate that many current parameterizations of the photosynthesis model of Farquhar, von Caemmerer & Berry (Planta 149, 78–90, 1980 ) based on the leaf intercellular CO₂ concentration (C_i) are incorrect for leaves where g_i limits photosynthesis. We show how conventional A–C_i curve (net CO₂ assimilation rate of a leaf –A_n– as a function of C_i) fitting methods which rely on a rectangular hyperbola model under the assumption of infinite g_i can significantly underestimate V_cmax for such leaves. Alternative parameterizations of the conventional method based on a single, apparent Michaelis–Menten constant for CO₂ evaluated at C_i[K_m(CO₂)_i] used for all C₃ plants are also not acceptable since the relationship between V_cmax and g_i is not conserved among species. We present an alternative A–C_i curve fitting method that accounts for g_i through a non‐rectangular hyperbola version of the model of Farquhar et al. (1980 ). Simulated and real examples are used to demonstrate how this new approach eliminates the errors of the conventional A–C_i curve fitting method and provides V_cmax estimates that are virtually insensitive to g_i. Finally, we show how the new A–C_i curve fitting method can be used to estimate the value of the kinetic constants of Rubisco in vivo is presented     

Modelling assimilation and intercellular CO2 from measured conductance: a synthesis of approaches

A spectrum of models that estimate assimilation rate A from intercellular carbon dioxide concentration (C_i) and measured stomatal conductance to CO₂ (g_c) were investigated using leaf‐level gas exchange measurements. The gas exchange measurements were performed in a uniform loblolly pine stand (Pinus taeda L.) using the Free Air CO₂ Enrichment (FACE) facility under ambient and elevated atmospheric CO₂ for 3 years. These measurements were also used to test a newly proposed framework that combines basic properties of the A–C_i curve with a Fickian diffusion transport model to predict the relationship between C_i/C_a and g_c, where C_a is atmospheric carbon dioxide concentration. The widely used Ball–Berry model and five other models as well as the biochemical model proposed by Farquhar et al. (1980) were also reformulated to express variations in C_i/C_a as a function of their corresponding driving mechanisms. To assess the predictive capabilities of these approaches, their respective parameters were estimated from independent measurements of long‐term stable carbon isotope determinations (δ¹³C), meteorological variables, and ensemble A–C_i curves. All eight approaches reproduced the measured A reasonably well, in an ensemble sense, from measured water vapour conductance and modeled C_i/C_a. However, the scatter in the instantaneous A estimates was sufficiently large for both ambient and elevated C_a to suggest that other transient processes were not explicitly resolved by all eight parameterizations. An important finding from our analysis is that added physiological complexity in modeling C_i/C_a (when g_c is known) need not always translate to increased accuracy in predicting A. Finally, the broader utility of these approaches to estimate assimilation and net ecosystem exchange is discussed in relation to elevated atmospheric CO₂.     

Leaf gas exchange and carbon isotope composition responses to drought in a drought-avoiding (Pinus pinaster) and a drought-tolerant (Quercus petraea) species under present and elevated atmospheric CO2 concentrations

The responses of predawn leaf water potential (φ_wp), leaf conductance to water vapour diffusion (g), CO₂ assimilation rate (A) and carbon isotope competition (δ¹³C) to a soil drying cycle were assessed in Pinus pinaster, a drought-avoiding species with high stomatal sensitivity to drought, and Quercus petraea, a drought-tolerant species with lower stomatal sensitivity to drought, under present (350 μmol^?1) and elevated (700 μmol^?1) atmospheric CO₂ concentrations ([CO₂]). In P. pinaster, decreasing A in response to drought was associated with increasing plant intrinsic water use efficiency (A/g) and with decreasing calculated intercellular [CO₂] (C₁), suggesting a stomatal limitation of A. In contrast, in Q. petraea, A/g declined and C¹ increased during the drying cycle, which suggests a non-stomatal origin for the decrease in A. In P. pinaster, a negative relationship was observed between the gas exchange-derived values of C_i/C_a and δ¹³C, which conforms to the classical two-step carbon isotope discrimination model. In Q. petraea, the relationship between C₁/C_a and δ¹³C was positive. Possible causes of this discrepancy are discussed. Lower g values were observed under elevated [CO₂] than under present [CO₂] in Q. petraea, whereas g was unaffected in P. pinaster. A stimulation of A by elevated [CO₂] was found in P. pinaster but not in Q. petraea. In both species, A/g was markedly higher under elevated than under present [CO₂]. Whether the differences in the g response to elevated [CO₂] found here can be generalized to other drought-avoiding and non-avoiding species remains to be assessed.