Root hydraulic conductivity of Larrea tridentata and Helianthus annuus under elevated CO2

While investigations into shoot responses to elevated atmospheric CO₂ are extensive, few studies have focused on how an elevated atmospheric CO₂ environment might impact root functions such as water uptake and transport. Knowledge of functional root responses may be particularly important in ecosystems where water is limiting if predictions about global climate change are true. In this study we investigated the effect of elevated CO₂ on the root hydraulic conductivity (L_p) of a C₃ perennial, Larrea tridentata, and a C₃ annual, Helianthus annuus. The plants were grown in a glasshouse under ambient (360 μmol mol^–1) and elevated (700 μmol mol^–1) CO₂. The L_p through intact root systems was measured using a hydrostatic pressure-induced flow system. Leaf gas exchange was also determined for both species and leaf water potential (ψ_leaf) was determined in L. tridentata. The L_p of L. tridentata roots was unchanged by an elevated CO₂ growth environment. Stomatal conductance (g_s) and transpiration (E) decreased and photosynthetic rate (A_net) and Ψ_leaf increased in L. tridentata. There were no changes in biomass, leaf area, stem diameter or root : shoot (R : S) ratio for L. tridentata. In H. annuus, elevated CO₂ induced a nearly two-fold decrease in root L_p. There was no effect of growth under elevated CO₂ on A_net, g_s, E, above- and below-ground dry mass, R : S ratio, leaf area, root length or stem diameter in this species. The results demonstrate that rising atmospheric CO₂ can impact water uptake and transport in roots in a species-specific manner. Possible mechanisms for the observed decrease in root L_p in H. annuus under elevated CO₂ are currently under investigation and may relate to either axial or radial components of root L_p.     

Stomatal conductance and photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine

Recent work has shown that stomatal conductance (g_s) and assimilation (A) are responsive to changes in the hydraulic conductance of the soil to leaf pathway (K_L), but no study has quantitatively described this relationship under controlled conditions where steady‐state flow is promoted. Under steady‐state conditions, the relationship between g_s, water potential (Ψ) and K_L can be assumed to follow the Ohm's law analogy for fluid flow. When boundary layer conductance is large relative to g_s, the Ohm's law analogy leads to g_s = K_L (Ψ_soil?Ψ_leaf)/D, where D is the vapour pressure deficit. Consequently, if stomata regulate Ψ_leaf and limit A, a reduction in K_L will cause g_s and A to decline. We evaluated the regulation of Ψ_leaf and A in response to changes in K_L in well‐watered ponderosa pine seedlings (Pinus ponderosa). To vary K_L, we systematically reduced stem hydraulic conductivity (k) using an air injection technique to induce cavitation while simultaneously measuring Ψ_leaf and canopy gas exchange in the laboratory under constant light and D. Short‐statured seedlings (< 1 m tall) and hour‐long equilibration times promoted steady‐state flow conditions. We found that Ψ_leaf remained constant near ? 1·5 MPa except at the extreme 99% reduction of k when Ψ_leaf fell to ? 2·1 MPa. Transpiration, g_s, A and K_L all declined with decreasing k (P < 0·001). As a result of the near homeostasis in bulk Ψ_leaf, g_s and A were directly proportional to K_L (R² > 0·90), indicating that changes in K_L may affect plant carbon gain.     

Aquaporin regulation in roots controls plant hydraulic conductance,stomatal conductance,and leaf water potential in Pinus radiata under water stress

Stomatal regulation is crucial for forest species performance and survival on drought‐prone sites. We investigated the regulation of root and shoot hydraulics in three Pinus radiata clones exposed to drought stress and its coordination with stomatal conductance (g_s) and leaf water potential (Ψ_leaf). All clones experienced a substantial decrease in root‐specific root hydraulic conductance (K_root‐r) in response to the water stress, but leaf‐specific shoot hydraulic conductance (K_shoot‐l) did not change in any of the clones. The reduction in K_root‐r caused a decrease in leaf‐specific whole‐plant hydraulic conductance (K_plant‐l). Among clones, the larger the decrease in K_plant‐l, the more stomata closed in response to drought. Rewatering resulted in a quick recovery of K_root‐r and g_s. Our results demonstrated that the reduction in K_plant‐l, attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leaf as water stress started. We concluded that higher K_plant‐l is associated with water stress resistance by sustaining a less negative Ψ_leaf and delaying stomatal closure.     

Effects of elevated CO2, drought and temperature on the water relations and gas exchange of groundnut (Arachis hypogaea) stands grown in controlled environment glasshouses

Stands of groundnut (Arachis hypogaea L. cv. Kadiri‐3) were grown in controlled environment glasshouses at mean atmospheric CO₂ concentrations of 375 or 700 μmol mol^?1 and daily mean air temperatures of 28 or 32°C on irrigated or drying soil profiles. Leaf water (Ψ_l) and solute potential (Ψ_s), relative water content (RWC), stomatal conductance (g_l) and net photosynthesis (P_n) were measured at midday for the youngest mature leaf throughout the growing season. Elevated CO₂ and temperature had no detectable effect on the water relations of irrigated plants, but higher values of RWC, Ψ_l and Ψ_s were maintained for longer under elevated CO₂ during progressive drought. Turgor potential (Ψ_p) reached zero when Ψ_l declined to ?1.6 to ?1.8 MPa in all treatments; turgor was lost sooner when droughted plants were grown under ambient CO₂. A 4°C increase in mean air temperature had no effect on Ψ_s in droughted plants, but elicited a small increase in Ψ_l; midday g_l values were lower under elevated than under ambient CO₂, and Ψ_l and g_l declined below ?1.5 MPa and 0.25 cm s^?1, respectively, as the soil dried. Despite the low g_l values recorded for droughted plants late in the season, P_n was maintained under elevated CO₂, but declined to zero 3 weeks before final harvest under ambient CO₂. Concurrent reductions in g_l and increases in water use efficiency under elevated CO₂ prolonged photosynthetic activity during drought and increased pod yields relative to plants grown under ambient CO₂. The implications of future increases in atmospheric CO₂ for the productivity of indeterminate C₃ crops grown in rainfed subsistence agricultural systems in the semi‐arid tropics are discussed.     

Hydrologic balance in an intact temperate forest ecosystem under ambient and elevated atmospheric CO2 concentration

Abstract Increasing atmospheric CO₂ concentration decreases stomatal conductance in many species, but the savings of water from reduced transpiration may permit the forest to retain greater leaf area index (L). Therefore, the net effect on water use in forest ecosystems under a higher CO₂ atmosphere is difficult to predict. The free air CO₂ enrichment (FACE) facility (n = 3) in a 14‐m tall (in 1996) Pinus taeda L. stand was designed to reduce uncertainties in predicting such responses. Continuous measurements of precipitation, throughfall precipitation, sap flux, and soil moisture were made over 3.5 years under ambient (CO₂^a) and elevated (CO₂^e) ambient + 200 µmol mol^?1). Annual stand transpiration under ambient CO₂ conditions accounted for 84–96% of latent heat flux measured with the eddy‐covariance technique above the canopy. Under CO₂^e, P. taeda transpired less per unit of leaf area only when soil drought was severe. Liquidambar styraciflua, the other major species in the forest, used progressively less water, settling at 25% reduction in sap flux density after 3.5 years under CO₂^e. Because P. taeda dominated the stand, and severe drought periods were of relatively short duration, the direct impact of CO₂^e on water savings in the stand was undetectable. Moreover, the forest used progressively more water under CO₂^e, probably because soil moisture availability progressively increased, probably owing to a reduction in soil evaporation caused by more litter buildup in the CO₂^e plots. The results suggest that, in this forest, the effect of CO₂^e on transpiration was greater indirectly through enhanced litter production than directly through reduced stomatal conductance. In forests composed of species more similar to L. styraciflua, water savings from stomatal closure may dominate the response to CO₂^e.     

Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

Stomatal sensitivity to vapour pressure difference over a subambient to elevated CO2 gradient in a C3/C4 grassland

In the present study the response of stomatal conductance (g_s) to increasing leaf‐to‐air vapour pressure difference (D) in early season C₃ (Bromus japonicus) and late season C₄ (Bothriochloa ischaemum) grasses grown in the field across a range of CO₂ (200–550 µmol mol^?1) was examined. Stomatal sensitivity to D was calculated as the slope of the response of g_s to the natural log of externally manipulated D (dg_s/dlnD). Increasing D and CO₂ significantly reduced g_s in both species. Increasing CO₂ caused a significant decrease in stomatal sensitivity to D in Br. japonicus, but not in Bo. ischaemum. The decrease in stomatal sensitivity to D at high CO₂ for Br. japonicus fit theoretical expectations of a hydraulic model of stomatal regulation, in which g_s varies to maintain constant transpiration and leaf water potential. The weaker stomatal sensitivity to D in Bo. ischaemum suggested that stomatal regulation of leaf water potential was poor in this species, or that non‐hydraulic signals influenced guard cell behaviour. Photosynthesis (A) declined with increasing D in both species, but analyses of the ratio of intercellular to atmospheric CO₂ (C_i/C_a) suggested that stomatal limitation of A occurred only in Br. japonicus. Rising CO₂ had the greatest effect on g_s and A in Br. japonicus at low D. In contrast, the strength of stomatal and photosynthetic responses to CO₂ were not affected by D in Bo. ischaemum. Carbon and water dynamics in this grassland are dominated by a seasonal transition from C₃ to C₄ photosynthesis. Interspecific variation in the response of g_s to D therefore has implications for predicting seasonal ecosystem responses to CO₂.     

Diurnal depression of leaf hydraulic conductance in a tropical tree species

Diurnal patterns of hydraulic conductance of the leaf lamina (K_leaf) were monitored in a field‐grown tropical tree species in an attempt to ascertain whether the dynamics of stomatal conductance (g_s) and CO₂ uptake (A_leaf) were associated with short‐term changes in K_leaf. On days of high evaporative demand mid‐day depression of K_leaf to between 40 and 50% of pre‐dawn values was followed by a rapid recovery after 1500 h. Leaf water potential during the recovery stage was less than ?1 MPa implying a refilling mechanism, or that loss of K_leaf was not linked to cavitation. Laboratory measurement of the response of K_leaf to Ψ_leaf confirmed that leaves in the field were operating at water potentials within the depressed region of the leaf ‘vulnerability curve’. Diurnal courses of K_leaf and Ψ_leaf predicted from measured transpiration, xylem water potential and the K_leaf vulnerability function, yielded good agreement with observed trends in both leaf parameters. Close correlation between depression of K_leaf, g_s and A_leaf suggests that xylem dysfunction in the leaf may lead to mid‐day depression of gas exchange in this species.     

Elevated CO2 effects on mesophyll conductance and its consequences for interpreting photosynthetic physiology

Mesophyll conductance (g_m) generally correlates with photosynthetic capacity, although the causal relationship between the two is unclear. The response of g_m to various CO₂ regimes was measured to determine its relationship to environmental changes that affect photosynthesis. The overall effect of CO₂ growth environment on g_m was species and experiment dependent. The data did not statistically differ from the previously shown A–g_m relationship and was unaffected by CO₂ treatment. The consequences of the CO₂ effect on g_m for interpreting photosynthesis in individual cases were investigated. Substantial effects of assumed versus calculated g_m on leaf properties estimated from gas‐exchange measurements were found. This differential error resulted in an underestimation in ratio of maximum carboxylation to electron transport, especially in plants with high photosynthetic capacity. Including g_m in the calculations also improved the agreement between maximum carboxylation rates and in vitro Rubisco measurements. It is concluded that g_m is finite and varies with photosynthetic capacity. Including g_m when calculating photosynthesis parameters from gas‐exchange data will avoid systematic errors.     

The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO2 and water vapour in the air

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO₂ and water vapour concentrations, responses of stomatal conductance (g^w_s) to boundary layer thickness have two components, one resulting from changes in intercellular CO₂ concentration (χ^c_i) and another from changes in leaf surface water vapour saturation deficit (D^w_s). The experiments and simulations also show that the boundary layer conductance (g^w_b) can significantly alter the apparent response of g^w_s to ambient air CO₂ mole fraction (χ^c_a) and water vapour mole fraction (χ^w_a). Because of the feedback loop involved the responses of g^w_s for χ^c_a and χ^w_a each include responses to both χ^c_i and D^w_s. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χ^c_i and D^w_s—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g^w_s for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g^w_s to χ^c_i and D^w_s, the apparent responses of g^w_s to D^w_a and χ^c_a depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.     

Effects of elevated O3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

Naturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open-top chambers and subjected in situ to three ozone (O₃) regimes, two concentrations of CO₂, and a combination of O₃ and CO₂ treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current-year and 1-year-old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O₃ concentrations led to a significant decline in the CO₂ compensation point (Г^*), maximum RuP₂-saturated rate of carboxylation (V_cmax), maximum rate of electron transport (J_max), maximum stomatal conductance (g_smax), and sensitivity of stomatal conductance to changes in leaf-to-air vapour pressure difference (?g_s/?D_v) in both shoot-age classes. However, the effect of elevated O₃ concentrations on the respiration rate in light (R_d) was dependent on shoot age. Elevated CO₂(700 μmol mol^?1) significantly decreased J_max and g_smax but increased R_d in 1-year-old shoots and the ?g_s/?D_v in both shoot-age classes. The interactive effects of O₃ and CO₂ on some key parameters (e.g. V_cmax and J_max) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO₂. As a consequence, the injury induced by O₃ was reduced through decreased ozone uptake in 1-year-old shoots, but not in the current-year shoots. Compared to ambient O₃ concentration, reduced O₃ concentrations (charcoal-filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O₃ concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current-year and 1-year-old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.     

Stomatal responses to changes in vapor pressure deficit reflect tissue‐specific differences in hydraulic conductance

The vapor pressure deficit (D) of the atmosphere can negatively affect plant growth as plants reduce stomatal conductance to water vapor (g_wv) in response to increasing D, limiting the ability of plants to assimilate carbon. The sensitivity of g_wv to changes in D varies among species and has been correlated with the hydraulic conductance of leaves (K_leaf), but the hydraulic conductance of other tissues has also been implicated in plant responses to changing D. Among the 19 grass species, we found that K_leaf was correlated with the hydraulic conductance of large longitudinal veins (K_lv, r² = 0.81), but was not related to K_root (r² = 0.01). Stomatal sensitivity to D was correlated with K_leaf relative to total leaf area (r² = 0.50), and did not differ between C₃ and C₄ species. Transpiration (E) increased in response to D, but 8 of the 19 plants showed a decline in E at high D, indicative of an ‘apparent feedforward’ response. For these individuals, E began to decline at lower values of D in plants with low K_root (r² = 0.72). These results show the significance of both leaf and root hydraulic conductance as drivers of plant responses to evaporative demand.     

On the relationship between leaf anatomy and CO2 diffusion through the mesophyll of hypostomatous leaves

Internal conductances to CO₂ transfer from the stomatal cavity to sites of carboxylation (g_i) in hypostomatous sun-and shade-grown leaves of citrus, peach and Macadamia trees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m^?2) for all leaves, including sclerophyllous Macadamia, whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high g_i, the chloro-plast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a), the lowest tissue density (T_d) and the highest chloro-plast surface area (S_c) exposed to ias. There were negative correlations between g_i and leaf thickness or D/a, but positive correlations between g_i and S_c or S_c/T_d. We developed a one-dimensional diffusion model which partitioned g_i into a gaseous diffusion conductance through the ias (g_ias) plus a liquid-phase conductance through mesophyll cell walls (g_cw). The model accounted for a significant amount of variation (r₂=0.80) in measured g_i by incorporating both components. The g_ias component was related to the one-dimensional path-length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The g_cw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between g_i and leaf thickness or D/a related to g_ias whereas positive correlations between g_i and S_c or S_c/T_d, related to g_cw. The g_cw was consistently lower than g_ias, and thus represented a greater constraint on CO₂ diffusion in the mesophylls of these hypostomatous species.     

CO2 transfer conductance, leaf structure and carbon isotope composition of Polygonum cuspidatum leaves from low and high altitudes

Anatomy and some physiological characteristics of the leaves in Polygonum cuspidatum Sieb. et Zucc., a dioecious clonal herb, were compared between two populations, one from a lowland in Shizuoka City (10 m above sea level), and another from a highland on Mt. Fuji (2500 m above sea level). Leaf mass per area (LMA) of the highland plants was about twice that of the lowland plants. The greater leaf thickness, thicker mesophyll cell walls and higher mesophyll cell density in the highland leaves contributed to the larger LMA. Although mesophyll area exposed to intercellular airspaces was greater in the highland leaves than in the lowland leaves by 30%, the surface area of chloroplasts facing intercellular airspaces was similar between these leaves. CO₂ transfer conductance inside the leaf (g_i) of the highland leaves (0·75 μmol m^?2 s^?1 Pa^?1) is the lowest recorded for herbaceous plants and was only 40% of that in the lowland leaves. On the other hand, the difference in stomatal conductance was small. δ¹³C values in the leaf dry matter were greater in the highland leaves by 4‰. These data and the estimation of CO₂ partial pressures in the intercellular air spaces and in the chloroplast suggested that the greater dry matter δ¹³C in the highland leaves, indicative of lower long‐term ratio of the chloroplast stroma to the ambient CO₂ partial pressures, would be mainly attributed to their lower g_i.     

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.     

The effects on Arbutus unedo L. of long-term exposure to elevated CO2

Arbutus unedo is a sclerophyllous evergreen, characteristic of Mediterranean coastal scrub vegetation. In Italy, trees of A. unedo have been found close to natural CO₂ vents where the mean atmospheric carbon dioxide concentration is about 2200 μmol mol^?1. Comparisons were made between trees growing in elevated and ambient CO₂ concentrations to test for evidence of adaptation to long-term exposure to elevated CO₂. Leaves formed at elevated CO₂ have a lower stomatal density and stomatal index and higher specific leaf area than those formed at ambient CO₂, but there was no change in carbon to nitrogen ratios of the leaf tissue. Stomatal conductance was lower at elevated CO₂ during rapid growth in the spring. In mid-summer, under drought stress, stomatal closure of all leaves occurred and in the autumn, when stress was relieved, the conductance of leaves at both elevated and ambient CO₂ increased. In the spring, the stomatal conductance of the new flush of leaves at ambient CO₂ was higher than the leaves at elevated CO₂, increasing instantaneous water use efficiency at elevated CO₂. Chlorophyll fluorescence measurements suggested that elevated CO₂ provided some protection against photoinhibition in mid-summer. Analysis of A/C_i curves showed that there was no evidence of either upward or downward regulation of photosynthesis at elevated CO₂. It is therefore anticipated that A. unedo will have higher growth rates as the ambient CO₂ concentrations increase.     

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].     

Stomatal acclimation over a subambient to elevated CO2 gradient in a C3/C4 grassland

An investigation to determine whether stomatal acclimation to [CO₂] occurred in C₃/C₄ grassland plants grown across a range of [CO₂] (200–550 µmol mol^?1) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance (g_s) to a range of intercellular CO₂ (a g_s–C_i curve) at each growth [CO₂] in the third and fourth growing seasons of the treatment. The g_s–C_i response curves for Solanum dimidiatum (C₃ perennial forb) differed significantly across [CO₂] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO₂] in this species was also found as maximum g_s (g_smax; g_s measured at the lowest C_i) increased with decreasing growth [CO₂] only below 400 µmol mol^?1. The substantial increase in g_s at subambient [CO₂] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (V_cmax; r² = 0·27) and was not associated with CO₂ saturated photosynthesis (A_max). The response of g_s to C_i did not vary with growth [CO₂] in Bromus japonicus (C₃ annual grass) or Bothriochloa ischaemum (C₄ perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO₂] in both C₃ species, was not correlated with g_s. Larger stomatal size at subambient [CO₂], however, may be associated with stomatal acclimation in S. dimidiatum. Incorporating stomatal acclimation into modelling studies could improve the ability to predict changes in ecosystem water fluxes and water availability with rising CO₂ and to understand their magnitudes relative to the past.     

Altitudinal variation in photosynthetic capacity, diffusional conductance and δ13C of butterfly bush (Buddleja davidii) plants growing at high elevations

In this study, we have examined several physiological, biochemical and morphological features of Buddleja davidii plants growing at 1300 m above sea level (a.s.l.) and 3400 m a.s.l., respectively, to identify coordinated changes in leaf properties in response to reduced CO₂ partial pressure (P_a). Our results confirmed previous findings that foliar δ¹³C, photosynthetic capacity and foliar N concentration on a leaf area basis increased, whereas stomatal conductance (g_s) decreased with elevation. The net CO₂ assimilation rate (A_max), maximum rate of electron transport (J_max) and respiration increased significantly with elevation, although no differences were found in carboxylation efficiency of Rubisco (V_cmax). Consequently, also the J_max to V_cmax ratio was significantly increased by elevation, indicating that the functional balance between Ribulose‐1,5‐biphosphate (RuBP) consumption and RuBP regeneration changes as elevation increases. Our results also indicated a homeostatic response of CO₂ transfer conductance inside the leaf (mesophyll conductance, g_m) to increasing elevation. In fact, with elevation, g_m also increased compensating for the strong decrease in g_s and, thus, in the P_i (intercellular partial pressure of CO₂) to P_a ratio, leading to similar chloroplast partial pressure of CO₂ (P_c) to P_a ratio at different elevations. Because there were no differences in V_cmax, also A measured at similar PPFD and leaf temperature did not differ statistically with elevation. As a consequence, a clear relationship was found between A and g_m, and between A and the sum of g_s and g_m. These data suggest that the higher dry mass δ¹³C of leaves at the higher elevation, indicative of lower long‐term P_c/P_a ratio, cannot be attributed to changes either in diffusional resistances or in carboxylation efficiency. We speculate that because temperature significantly decreases as the elevation increases, it dramatically affects CO₂ diffusion and hence P_c/P_a and, consequently, is the primary factor influencing ¹³C discrimination at high elevation.     

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