Photosynthetic characteristics of dwarf and fringe Rhizophora mangle L. in a Belizean mangrove

Twin Cays (Belize) is a highly oligotrophic mangrove archipelago dominated by Rhizophora mangle L. Ocean‐fringing trees are 3–7 m tall with a leaf area index (LAI) of 2.3, whereas in the interior, dwarf zone, trees are 1.5 m or less, and the LAI is 0.7. P‐fertilization of dwarf trees dramatically increases growth. As a partial explanation of these characteristics, it was hypothesized that differences in stature and growth rates would reflect differences in leaf photosynthetic capacity, as determined by the photochemical and biochemical characteristics at the chloroplast level. Gas exchange and chlorophyll fluorescence were used to compare photosynthesis of dwarf, fringe and fertilized trees. Regardless of zonation or treatment, net CO₂ exchange (A) and photosynthetic electron transport were light saturated at less than 500 µmol photons m^?2 s^?1, and low‐light quantum efficiencies were typical for healthy C₃ plants. On the other hand, light‐saturated A was linearly related to stomatal conductance (g_s), with seasonal, zonal and treatment differences in photosynthesis corresponding linearly to differences in the mean g_s. Overall, photosynthetic capacity appeared to be co‐regulated with stomatal conductance, minimizing the variability of C_i at ambient CO₂ (and hence, C_i/C_a). Based on the results of in vitro assays, regulation of photosynthesis in R. mangle appeared to be accomplished, at least in part, by regulation of Rubisco activity.     

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.     

Disentangling the contributions of ontogeny and water stress to photosynthetic limitations in almond trees

Very few studies have attempted to disentangle the respective role of ontogeny and water stress on leaf photosynthetic attributes. The relative significance of both effects on photosynthetic attributes has been investigated in leaves of field‐grown almond trees [Prunus dulcis (Mill.) D. A. Webb] during four growth cycles. Leaf ontogeny resulted in enhanced leaf dry weight per unit area (W_a), greater leaf dry‐to‐fresh weight ratio and lower N content per unit of leaf dry weight (N_w). Concomitantly, area‐based maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max), mesophyll conductance to CO₂ diffusion (gm)′ and light‐saturated net photosynthesis (A_max) declined in both well‐watered and water‐stressed almond leaves. Although g_m and stomatal conductance (g_s) seemed to be co‐ordinated, a much stronger coordination in response to ontogeny and prolonged water stress was observed between g_m and the leaf photosynthetic capacity. Under unrestricted water supply, the leaf age‐related decline of A_max was equally driven by diffusional and biochemical limitations. Under restricted soil water availability, A_max was mainly limited by g_s and, to a lesser extent, by photosynthetic capacity and g_m. When both ontogeny and water stress effects were combined, diffusional limitations was the main determinant of photosynthesis limitation, while stomatal and biochemical limitations contributed similarly.     

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.     

Effect of jasmonic acid on the stomatal and nonstomatal limitation of leaf photosynthesis in barley leaves

The effect of long-term (7 days) and shortterm (up to 2 h) treatment of barley plants with jasmonic acid (JA) on the components contributing to stomatal and nonstomatal limitation of photosynthesis was studied. Net CO₂ assimilation rate (A) responses to intercellular CO₂ concentration (C _i ), i.e., A/C _i curves, were used to assess the photosynthetic ability. Long-term treatment of barley plants with JA led to a noticeable decrease in both the initial slope of the A/C _i curves and the maximum A at saturating C _i . The proportion of stomatal and nonstomatal factors in limitation of photosynthesis depended on the applied JA concentration. Short-term treatment with JA affected neither the stomatal conductivity for CO₂ nor the rate of photosynthetic CO₂ assimilation. We suggest that JA may affect photosynthesis indirectly, either as a stress-modulating substance, or through the alterations in gene expression.     

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     

Diffusional conductance to CO2 is the key limitation to photosynthesis in salt‐stressed leaves of rice (Oryza sativa)

Salinity significantly limits leaf photosynthesis but the factors causing the limitation in salt‐stressed leaves remain unclear. In the present work, photosynthetic and biochemical traits were investigated in four rice genotypes under two NaCl concentration (0 and 150 mM) to assess the stomatal, mesophyll and biochemical contributions to reduced photosynthetic rate (A) in salt‐stressed leaves. Our results indicated that salinity led to a decrease in A, leaf osmotic potential, electron transport rate and CO₂ concentrations in the chloroplasts (C_c) of rice leaves. Decreased A in salt‐stressed leaves was mainly attributable to low C_c, which was determined by stomatal and mesophyll conductance. The increased stomatal limitation was mainly related to the low leaf osmotic potential caused by soil salinity. However, the increased mesophyll limitation in salt‐stressed leaves was related to both osmotic stress and ion stress. These findings highlight the importance of considering mesophyll conductance when developing salinity‐tolerant rice cultivars.     

Photosynthesis Inhibition During Gas Exchange Oscillations in ABA-Treated Helianthus annuus: Relative Role of Stomatal Patchiness and Leaf Carboxylation Capacity

Environmental factors that induce spatial heterogeneity of stomatal conductance, g _s, called stomatal patchiness, also reduce the photochemical capacity of CO₂ fixation, yet current methods cannot distinguish between the relative effect of stomatal patchiness and biochemical limitations on photosynthetic capacity. We evaluate effects of stomatal patchiness and the biochemical capacity of CO₂ fixation on the sensitivity of net photosynthetic rate (P _N) to stomatal conductance (g _s), θ (θ = δP _N/g _s). A qualitative model shows that stomatal patchiness increases the sensitivity θ while reduced biochemical capacity of CO₂ fixation lowers θ. We used this feature to distinguish between stomatal patchiness and mesophyll impairments in the photochemistry of CO₂ fixation. We compared gas exchange of sunflower (Helianthus annuus L.) plants grown in a growth chamber and fed abscisic acid, ABA (10⁻⁵ M), for 10 d with control plants (-ABA). P _N and g _s oscillated more frequently in ABA-treated than in control plants when the leaves were placed into the leaf chamber and exposed to a dry atmosphere. When compared with the initial CO₂ response measured at the beginning of the treatment (day zero), both ABA and control leaves showed reduced P _N at particular sub-stomatal CO₂ concentration (c _i) during the oscillations. A lower reduction of P _N at particular g _s indicated overestimation of c _i due to stomatal patchiness and/or omitted cuticular conductance, g _c. The initial period of damp oscillation was characterised by inhibition of chloroplast processes while stomatal patchiness prevailed at the steady state of gas exchange. The sensitivity θ remained at the original pre-treatment values at high g _s in both ABA and control plants. At low g _s, θ decreased in ABA-treated plants indicating an ABA-induced impairment of chloroplast processes. In control plants, g _c neglected in the calculation of g _s was the likely reason for apparent depression of photosynthesis at low g _s. This revised version was published online in August 2006 with corrections to the Cover Date.     

Do all leaf photosynthesis parameters of rice acclimate to elevated CO2, elevated temperature,and their combination,in FACE environments?

Leaf photosynthesis of crops acclimates to elevated CO₂ and temperature, but studies quantifying responses of leaf photosynthetic parameters to combined CO₂ and temperature increases under field conditions are scarce. We measured leaf photosynthesis of rice cultivars Changyou 5 and Nanjing 9108 grown in two free‐air CO₂ enrichment (FACE) systems, respectively, installed in paddy fields. Each FACE system had four combinations of two levels of CO₂ (ambient and enriched) and two levels of canopy temperature (no warming and warmed by 1.0–2.0°C). Parameters of the C₃ photosynthesis model of Farquhar, von Caemmerer and Berry (the FvCB model), and of a stomatal conductance (g_s) model were estimated for the four conditions. Most photosynthetic parameters acclimated to elevated CO₂, elevated temperature, and their combination. The combination of elevated CO₂ and temperature changed the functional relationships between biochemical parameters and leaf nitrogen content for Changyou 5. The g_s model significantly underestimated g_s under the combination of elevated CO₂ and temperature by 19% for Changyou 5 and by 10% for Nanjing 9108 if no acclimation was assumed. However, our further analysis applying the coupled g_s–FvCB model to an independent, previously published FACE experiment showed that including such an acclimation response of g_s hardly improved prediction of leaf photosynthesis under the four combinations of CO₂ and temperature. Therefore, the typical procedure that crop models using the FvCB and g_s models are parameterized from plants grown under current ambient conditions may not result in critical errors in projecting productivity of paddy rice under future global change.     

Gas exchange and photosynthetic acclimation over subambient to elevated CO2 in a C3–C4 grassland

Atmospheric CO₂ (C_a) has risen dramatically since preglacial times and is projected to double in the next century. As part of a 4‐year study, we examined leaf gas exchange and photosynthetic acclimation in C₃ and C₄ plants using unique chambers that maintained a continuous C_a gradient from 200 to 550 µmol mol^?1 in a natural grassland. Our goals were to characterize linear, nonlinear and threshold responses to increasing C_a from past to future C_a levels. Photosynthesis (A), stomatal conductance (g_s), leaf water‐use efficiency (A/g_s) and leaf N content were measured in three common species: Bothriochloa ischaemum, a C₄ perennial grass, Bromus japonicus, a C₃ annual grass, and Solanum dimidiatum, a C₃ perennial forb. Assimilation responses to internal CO₂ concentrations (A/C_i curves) and photosynthetically active radiation (A/PAR curves) were also assessed, and acclimation parameters estimated from these data. Photosynthesis increased linearly with C_a in all species (P < 0.05). S. dimidiatum and B. ischaemum had greater carboxylation rates for Rubisco and PEP carboxylase, respectively, at subambient than superambient C_a (P < 0.05). To our knowledge, this is the first published evidence of A up‐regulation at subambient C_a in the field. No species showed down‐regulation at superambient C_a. Stomatal conductance generally showed curvilinear decreases with C_a in the perennial species (P < 0.05), with steeper declines over subambient C_a than superambient, suggesting that plant water relations have already changed significantly with past C_a increases. Resource‐use efficiency (A/g_s and A/leaf N) in all species increased linearly with C_a. As both C₃ and C₄ plants had significant responses in A, g_s, A/g_s and A/leaf N to C_a enrichment, future C_a increases in this grassland may not favour C₃ species as much as originally thought. Non‐linear responses and acclimation to low C_a should be incorporated into mechanistic models to better predict the effects of past and present rising C_a on grassland ecosystems.     

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].     

Different photosynthetic responses of wild and cultivated plants to high irradiance

In addition to other factors, high altitude (HA) environment is characterized by high photosynthetic photon flux density (PPFD). Photosynthetic characteristics of wild and cultivated plants were studied at different irradiances at Losar, India (altitude 4 200 m). Wild plants were tolerant to high PPFDs. Slopes of curve between net photosynthetic rate (P _N) and intercellular CO₂ concentration (C _i) or stomatal conductance (g _s) increased with increase in irradiance suggesting insensitivity or tolerance of these plants to higher PPFD. Cultivated plants, however, were sensitive to higher PPFD, their slopes of curves between P _N and C _i or g _s decreased with increased PPFD. Tolerance or insensitivity to higher PPFD was an important parameter affecting plant performance at HA.     

Pre‐dawn stomatal opening does not substantially enhance early‐morning photosynthesis in Helianthus annuus

Most C₃ plant species have partially open stomata during the night especially in the 3–5 h before dawn. This pre‐dawn stomatal opening has been hypothesized to enhance early‐morning photosynthesis (A) by reducing diffusion limitations to CO₂ at dawn. We tested this hypothesis in cultivated Helianthus annuus using whole‐shoot gas exchange, leaf level gas exchange and modelling approaches. One hour pre‐dawn low‐humidity treatments were used to reduce pre‐dawn stomatal conductance (g). At the whole‐shoot level, a difference of pre‐dawn g (0.40 versus 0.17 mol m^?2 s^?1) did not significantly affect A during the first hour after dawn. Shorter term effects were investigated with leaf level gas exchange measurements and a difference of pre‐dawn g (0.10 versus 0.04 mol m^?2 s^?1) affected g and A for only 5 min after dawn. The potential effects of a wider range of stomatal apertures were explored with an empirical model of the relationship between A and intercellular CO₂ concentration during the half‐hour after dawn. Modelling results demonstrated that even extremely low pre‐dawn stomatal conductance values have only a minimal effect on early‐morning A for a few minutes after dawn. Thus, we found no evidence that pre‐dawn stomatal opening enhances A.     

Gas exchange and resource-use efficiency of Leymus cinereus (Poaceae): diurnal and seasonal responses to naturally declining soil moisture

We examined factors that limit diurnal and seasonal photosynthesis in Leymus cinereus, a robust tussock grass from shrub-steppes of western North America. Data from plants in a natural stand and in experimental field plots indicate that this bunchgrass has 1) a high photosynthetic capacity, 2) high leaf nitrogen content and high nitrogen-use efficiency, 3) a steep leaf-to-air diffusion gradient for carbon dioxide, which enhances intrinsic water-use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO₂ assimilation (A) and stomatal conductance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderately stressed plants reduced intercellular carbon dioxide concentration (c_i) by ≈40 μl liter^-1. The maximum rate of A achieved during the day for severely stressed plants (predawn water potential = -4 MPa) was one-third and daily carbon gain per unit leaf area was about one-fourth that of well-watered plants. For plants in the natural stand, CO₂-saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas there was little effect on A at CO₂ ambient levels or on carboxylation efficiency until predawn water potentials reached -1.8 MPa. Nitrogen-use efficiency declined with diminishing soil moisture, but there was no seasonal change in stomatal limitation or instantaneous water-use efficiency as estimated from A vs. c_i curves at optimal leaf temperature and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diumally, but over the growing season, stomatal limitation estimated from A vs. c_i curves is relatively constant because maximum stomatal conductance is closely tuned to the CO₂ assimilatory capacity of the mesophyll.     

Chloride as a macronutrient increases water‐use efficiency by anatomically driven reduced stomatal conductance and increased mesophyll diffusion to CO2

Chloride (Cl^?) has been recently described as a beneficial macronutrient, playing specific roles in promoting plant growth and water‐use efficiency (WUE). However, it is still unclear how Cl^? could be beneficial, especially in comparison with nitrate (NO₃^?), an essential source of nitrogen that shares with Cl^? similar physical and osmotic properties, as well as common transport mechanisms. In tobacco plants, macronutrient levels of Cl^? specifically reduce stomatal conductance (g_s) without a concomitant reduction in the net photosynthesis rate (A_N). As stomata‐mediated water loss through transpiration is inherent in the need of C₃ plants to capture CO₂, simultaneous increase in photosynthesis and WUE is of great relevance to achieve a sustainable increase in C₃ crop productivity. Our results showed that Cl^?‐mediated stimulation of larger leaf cells leads to a reduction in stomatal density, which in turn reduces g_s and water consumption. Conversely, Cl^? improves mesophyll diffusion conductance to CO₂ (g_m) and photosynthetic performance due to a higher surface area of chloroplasts exposed to the intercellular airspace of mesophyll cells, possibly as a consequence of the stimulation of chloroplast biogenesis. A key finding of this study is the simultaneous improvement of A_N and WUE due to macronutrient Cl^? nutrition. This work identifies relevant and specific functions in which Cl^? participates as a beneficial macronutrient for higher plants, uncovering a sustainable approach to improve crop yield.     

Water availability affects seasonal CO2‐induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO₂ (eCO₂) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (g_s) and increased soil water content (V_SWC) and second, through increased leaf internal CO₂ (C_i) and decreased stomatal limitations (S_lim). It is unclear if such findings from temperate grassland studies similarly pertain to warmer ecosystems with periodic water deficits. We tested these mechanisms in three important C₃ herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO₂‐induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%–50% with a 150 μmol mol^?1 increase in atmospheric CO₂ across seasons. This eCO₂‐induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily V_SWC. The highest photosynthetic enhancement by eCO₂ (>30%) was observed during the most water‐limited period, for example, with V_SWC <0.07 in this sandy surface soil. Under eCO₂ there was neither a significant decrease in g_s in the three herbaceous species, nor increases in V_SWC, indicating no “water‐savings effect” of eCO₂. Periods of low V_SWC showed lower g_s (less than ≈ 0.12 mol m^?2 s^?1), higher relative S_lim (>30%) and decreased C_i under the ambient CO₂ concentration (aCO₂), with leaf photosynthesis strongly carboxylation‐limited. The alleviation of S_lim by eCO₂ was facilitated by increasing C_i, thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO₂, controls g_s and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO₂ has the potential to alter vegetation functioning in a periodically dry woodland understory through changes in stomatal limitation to photosynthesis, not by the “water‐savings effect” usually invoked in grasslands.     

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

Canopy gradients in leaf intercellular CO2 mole fractions revisited: interactions between leaf irradiance and water stress need consideration

Intercellular CO₂ mole fractions (C_i) are lower in the upper canopy relative to the lower canopy leaves. This canopy gradient in C_i has been associated with enhanced rates of carbon assimilation at high light, and concomitant greater draw‐downs in C_i. However, increases in irradiance in the canopy are generally also associated with decreases in leaf water availability. Thus, stress effects on photosynthesis rates (A) and stomatal conductance (G), may provide a further explanation for the observed C_i gradients. To test the hypotheses of the sources of canopy variation in C_i, and quantitatively assess the influence of within‐canopy differences in stomatal regulation on A, the seasonal and diurnal variation in G was studied in relation to seasonal average daily integrated quantum flux density (Q_int) in tall shade‐intolerant Populus tremula L. trees. Daily time‐courses of A were simulated using the photosynthesis model of Farquhar et al. (Planta 149, 78–90, 1980). Stable carbon isotope composition of a leaf carbon fraction with rapid turnover rate was used to estimate canopy gradient in C_i during the simulations. Daily maximum G (G_max) consistently increased with increasing Q_int. However, canopy differences in G_max decreased as soil water availability became limiting during the season. In water‐stressed leaves, there were strong mid‐day decreases in G that were poorly associated with vapour pressure deficits between the leaf and atmosphere, and the magnitude of the mid‐day decreases in G occasionally interacted with long‐term leaf light environment. Simulations indicated that the percentage of carbon lost due to mid‐day stomatal closure was of the order of 5–10%, and seasonal water stress increased this percentage up to 20%. The percentage of carbon lost due to stomatal closure increased with increasing Q_int. Canopy differences in light environment resulted in a gradient of daily average C_i of approximately 20 µmol mol^?1. The canopy variation in seasonal and diurnal reductions in G led to a C_i gradient of approximately 100 µmol mol^?1, and the actual canopy C_i gradient was of the same magnitude according to leaf carbon isotope composition. This study demonstrates that stress effects influence C_i more strongly than within‐canopy light gradients, and also that leaves acclimated to different irradiance and water stress conditions may regulate water use largely independent of foliar photosynthetic potentials.     

Steady-state stomatal responses of C3 and C4 species to blue light fraction: Interactions with CO2 concentration

Blue light induced stomatal opening has been studied by applying a short pulse (~5 to 60 s) of blue light to a background of saturating photosynthetic red photons, but little is known about steady-state stomatal responses. Here we report stomatal responses to blue light at high and low CO₂ concentrations. Steady-state stomatal conductance (g_s) of C₃ plants increased asymptotically with increasing blue light to a maximum at 20% blue (120 μmol m⁻² s⁻¹). This response was consistent from 200 to 800 μmol mol⁻¹ atmospheric CO₂ (C_a). In contrast, blue light induced only a transient stomatal opening (~5 min) in C₄ species above a C_a of 400 μmol mol⁻¹. Steady-state g_s of C₄ plants generally decreased with increasing blue intensity. The net photosynthetic rate of all species decreased above 20% blue because blue photons have lower quantum yield (moles carbon fixed per mole photons absorbed) than red photons. Our findings indicate that photosynthesis, rather than a blue light signal, plays a dominant role in stomatal regulation in C₄ species. Additionally, we found that blue light affected only stomata on the illuminated side of the leaf. Contrary to widely held belief, the blue light-induced stomatal opening minimally enhanced photosynthesis and consistently decreased water use efficiency.