Control of Photosynthesis and Stomatal Conductance in Ricinus communis L. (Castor Bean) by Leaf to Air Vapor Pressure Deficit

Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO₂ assimilation, stomatal conductance (g_s), quantum yield of photosystem II (_II), and nonphotochemical quenching of chlorophyll fluorescence (q_N) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO₂ assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g_s initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40°C, CO₂ assimilation and g_s were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g_s. The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO₂ and O₂) versus nonphotorespiring (700 microbars CO₂ and 2% O₂) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O₂ increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO₂ in the leaf (C_i) where C_i was estimated from gas exchange measurements. However, estimates of C_i from measurements of _II and q_N suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C_i. The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C_i from gas exchange measurements. Under low VPD, 30°C, high light, and saturating CO₂, castor bean (C₃ tropical shrub) has a rate of photosynthesis (61 micromoles CO₂ per square meter per second) that is about 50% higher than that of tobacco (C₃) or maize (C₄) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.     

Stomatal responses to light and humidity in sugarcane: prediction of daily time courses and identification of potential selection criteria

Abstract Stomatal sensitivities to light and VPD have potential as quantitative selection criteria in programs designed to enhance water-use efficiency of sugarcane and other crops. These responses were characterized using gas exchange techniques and then simulated by a mathematical relationship describing conductance as a function of photon fluence rates and VPD values. The same form of relationship simulated stomatal responses of well-watered greenhouse- and field-grown plants. A comparison between simulated and measured conductance values showed a close correlation, indicating that light and VPD responses of stomata are dominant input signals modulating stomatal conductance in sugarcane. Observed conductance of Hawaiian sugarcane in a commerical production area appeared larger than required to support prevailing rates of carbon assimilation, since predicted intercellular CO₂ was greater than required to saturate its C₄ photosynthesis. Manipulation of the relationship describing stomatal conductance allowed us to simulate the responses of plants with hypothetically altered stomatal sensitivities to VPD or to light, using micrometeorological data collected in the field. Further simulation indicated that selection for clones with altered stomatal sensitivity to either light or VPD could improve the water-use efficiency of sugarcane without inhibiting current high levels of productivity.     

Effects of CO2 concentration and irradiance on the stomatal behaviour of maize, barley and sunflower plants in the field

Abstract Maize, barley and sunflower plants were grown in the field, well supplied with water and nutrients. During growth, net CO₂ exchange and transpiration of the crops at varying ambient CO₂ concentrations and irradiance were determined by infra-red gas analysis. In maize the net photosynthetic rate (P_n) was linearly related to the irradiance (I) and independent of the ambient CO₂ concentration (C_a). The transpiration rate (E_T) was also linearly related to I but decreased strongly with increasing C_a. In sunflower and barley P_n increased and E_T decreased with increasing C_a. A mean stomatal conductance and intercellular CO₂ concentration (C_i) were calculated. In all three species the internal CO₂ concentration was independent of the irradiance. In maize it was also independent of C_a, but in sunflower and barley C_i was proportional to C_a with a ratio of 0.6. It is concluded that differences in stomatal behaviour are only partly species-specific and depend mainly on growing conditions. The importance of stomatal regulation for crop growth under conditions of water shortage and CO₂ depletion is discussed.     

A relationship between humidity response, growth form and photosynthetic operating point in C3 plants

Gas exchange experiments were performed with 13 plant species that differ from each other in growth-form and natural habitat. These comprised three herbaceous species, two ferns, two temperate deciduous trees, five rainforest trees and one liana from wet tropical forest. The aims were to investigate whether plants of similar growth-form and from similar habitats tended to respond similarly to a change in leaf-to-air vapour pressure difference (VPD), and to compare their ratio of intercellular to ambient partial pressures of CO₂ for given conditions. Leaves were subjected to a step change in VPD and the initial and final steady rates of transpiration were used to calculate an index of sensitivity, φ , which enabled comparison of species. The results suggest that species of similar growth-form and habitat respond similarly to increasing VPD, with the temperate deciduous trees undergoing a greater reduction in stomatal conductance than the herbaceous plants in well-watered soil. Also, for these experimental conditions, the ratio of leaf internal to ambient CO₂ partial pressure (p_i/p_a) was positively correlated with both CO₂ assimilation rate and stomatal insensitivity to VPD, across the 13 species. The results are discussed in terms of growth strategies and possible advantages and limitations of hydraulic systems in different plants.     

Differential response of stomata to air humidity in the parasitic mistletoe (Phthirusa pyrifolia) and its host,mandarin orange (Citrus resitulata)

Measurements of CO₂ and H₂O exchange rate and the calculated leaf conductance of attached leaves were conducted over a range of leaf-to-air vapour pressure difference (VPD) (1.5 to 5.5 kPa) to compare the response of the parasitic mistletoe, Phthirusa pyrifolia, with that of its host, the mandarin orange, Citrus reticulata. Seedlings of the host infected with the parasite were grown in well-watered and adequately fertilized large pots outdoors at the CIAT headquarters, Palmira, Colombia, South America. Observations of leaf anatomy of the parasite and nutrient analysis of young tissues of both the parasite and host were made. The photosynthetic rate of the host decreased linearly with increased VPD, whereas the parasite showed a constant rate. This trend coincided with similar responses in leaf conductance. Due to the insensitivity of the parasite stomata, the transpiration rate increased linearly with VPD as compared with an initial increase and then a decrease in the host transpiration rate. The higher photosynthetic rate and the closure of stomata of the host resulted in high water use efficiency as compared with that of the parasite. The parasite accumulated in its leaves more N, P, K and less Ca and Mg than the host. The significance of the host-parasite differential response to air humidity is discussed in relation to mechanism underlying stomatal sensitivity and in the context of host-parasite association.Visiting Scientist, Coordinator, and Research Assistant of the Cassava Physiology Program.     

The photosynthetic characteristics of papyrus in a tropical swamp

Summary Photosynthesis and transpiration was measured in the large emergent C₄ sedge Cyperus papyrus (papyrus) which occupies wide areas of wetland on the African continent. The maximum observed value of net assimilation was 35 mol CO₂ m^-2 s^-1 at full sunlight but light saturation of photosynthesis did not occur. The quantum yield of photosynthesis obtained from the initial slope of the light response curves (0.06 mol mol^-1 incident light) was relatively high and close to previously recorded values for some C₄ grasses. Measurements made over two days showed that stomatal conductance was sensitive to the ambient air vapour pressure deficit (VPD) and was consistently lower on the day when VPD's were higher. There was, however, no marked midday closure of the stomata. Photosynthesis was also reduced on the day when VPD's were higher. The relationship between net photosynthesis and stomatal conductance was close to linear over the range of measurement conditions, with the result that intercellular CO₂ concentrations (C _i ) did not vary markedly. There was some evidence that C _i decreased at high VPD's. The regulation of stomatal movement in papyrus appears to minimise excessive water loss while not severely limiting photosynthesis. The significance of this strategy for a wetland species with plentiful supplies of water is discussed.     

Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO₂ is evaluated from the results of over 40 studies focusing on the effect of long-term CO₂ enrichment on the short-term response of photosynthesis to intercellular CO₂ (the A/C_i response). The effect of CO₂ enrichment on the A/C_i response was dependent on growth conditions, with plants grown in small pots (< 5 L) or low nutrients usually exhibiting a reduction of A at a given C_i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C_i following a doubling or tripling of atmospheric CO₂ during growth. Using theoretical interpretations of A/C_i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C_i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO₂ enrichment usually had little effect or increased the value of A at all C_i. However, a minority of species grown at elevated CO₂ exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO₂ (the C_i/C_a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C_i/C_a exhibited no consistent change in a variety of C₃ species, indicating no stomatal acclimation. Under drought or humidity stress, C_i/C_a declined in high-CO₂ grown plants, indicating stomata will become more conservative during stress episodes in future high CO₂ environments.Abbreviations A net CO₂ assimilation rate - C_i (C_a) intercellular (ambient) partial pressure of CO₂ - operational C_i intercellular partial pressure of CO₂ at a given ambient partial pressure of CO₂ - g_s stomatal conductance - normal CO₂ current atmospheric mole fraction of CO₂ (330 to 355 mol mol^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

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.     

Simultaneous and independent effects of abscisic acid on stomata and the photosynthetic apparatus in whole leaves

(±)-Abscisic acid (ABA) at 10^-5 M was added to the transpiration stream of leaves of 16 species (C₃ and C₄, monocotyledons and dicotyledons). Stomatal responses followed one of three patterns: i) stomata that were wide and insensitive to CO₂ initially, closed partially and became sensitive to CO₂; ii) for stomata that were sensitive to CO₂ before the application of ABA, the range of highest sensitivity to CO₂ shifted from high to low intercellular partial pressures of CO₂, for instance in leaves of Zea mays from 170–350 to 70–140 bar; iii) when stomata responded strongly to ABA, their conductance was reduced to a small fraction of the initial conductance, and sensitivity to CO₂ was lost. The photosynthetic apparatus was affected by applications of ABA to various degrees, from no response at all (in agreement with several previous reports on the absence of effects of ABA on photosynthesis) through a temporary decrease of its activity to a lasting reduction. Saturation curves of photosynthesis with respect to the partial pressure of CO₂ in the intercellular spaces indicated that application of ABA could produce three phenomena: i) a reduction of the initial slope of the saturation curve (which indicates a diminished carboxylation efficiency); ii) a reduction of the level of the CO₂-saturated rate of assimilation (which indicates a reduction of the ribulose-1,5-bisphosphate regeneration capacity); and iii) an increase of the CO₂ compensation point. Photosynthesis of isolated mesophyll cells was not affected by ABA treatments. Responses of the stomatal and photosynthetic apparatus were usually synchronous and often proportional to each other, with the result that the partial pressure of CO₂ in the intercellular spaces frequently remained constant in spite of large changes in conductance and assimilation rate. Guard cells and the photosynthetic apparatus were able to recover from effects of ABA applications while the ABA supply continued. Recovery was usually partial, in the case of the photosynthetic apparatus occasionally complete. Abscisic acid did not cause stomatal closure or decreases in the rate of photosynthesis when it was applied during a phase of stomatal opening and induction of photosynthesis that followed a transition from darkness to light.Abbreviations and symbols A rate of CO₂ assimilation - ABA (±)-abscisic acid - c _a partial pressure of CO₂ in the ambient air or in the gas supplied to the leaf chambers - c _i partial pressure of CO₂ in the intercellular spaces of a leaf - e _a partial pressure of H₂O in the air - g conductance for water vapor - J quantum flux - T ₁ leaf temperature     

The effect of vapor pressure on stomatal control of gas exchange in Douglas fir (Pseudotsuga menziesii) saplings

Summary Increasing leaf to air vapor pressure deficit (VPD) caused reductions in stomatal conductance of both current year and previous season needles of Pseudotsuga menziesii saplings. The stomata of current year needles were found to be more responsive to changes in VPD than those of previous season needles. The reductions in stomatal conductance of current year needles were not associated with decreases in xylem pressure potential. In fact, the reductions in stomatal conductance of current year needles were sometimes sufficient to reduce transpiration and thus raise xylem pressure potential even though VPD was increasing. There was a decline in stomatal responsiveness to VPD in current year needles between early and late summer. Pressure-volume curves determined for different age needles at different times of the year suggested that differences and changes in stomatal responsiveness to VPD may have been caused in part by differences and changes in needle water potential components. Hexane washes of current year needles during the late summer succeeded in partially restoring their VPD sensitivity, suggesting that changes in the water permeability of the external cuticle during needle maturation may also have played a role in causing the summer decline in VPD responsiveness.In both current and previous year needles VPD-induced changes in stomatal conductance had a greater relative effect on transpiration (q _w) than on net photosynthesis (Ph_N). In maturing needles the ratio of the sensitivities of transpiration and net photosynthesis to changes in stomatal conductance, (q _w/g _s)/Ph_N/g _s), remained nearly constant as VPD was varied. This provides experimental support for a recent hypothesis that stomata respond to environmental fluctuations in such a manner as to maintain the above ratio constant, which optimizes CO₂ uptake with respect to water loss.     

Stomatal response to blue light: water use efficiency in sugarcane and soybean*

Abstract. The significance of blue light-stimulated stomatal conductance for carbon assimilation (A), stomatal conductance (g), intercellular CO₂ (C_i), stomatal limitation of A (L), transpiration (E) and water use efficiency (W = A/E), was determined in a C₄ and a C₃ species. W and L were evaluated for steady-state gas exchange with constant, saturating red light (A_s, g_s, E_s), and for the integrated gas exchange above the steady state baseline induced by a single, brief pulse of blue light (A_p, g_p, E_p). Sugarcane (Saccharum spp. hybrid), a C₄ grass, and soybean (Glycine max) a C₃ dicot, were compared. Sugarcane exhibited typical C₄ behaviour, with A saturing at C_i of ca. 200 μmol mol^?1, compared to >500 μmol mol^?1 in soybean. Steady-state W was also considerably higher in sugarcane. The extent of stomatal opening in response to a blue light pulse, from baseline (g_s) to the maximum value of conductance during the opening response (g_m), was similar in the two species. More rapid opening and closing of stomata in sugarcane resulted in a smaller integrated magnitude of the conductance response (g_p) than in soybean. At the peak of the blue light response, both species exhibited similar levels of L. During the response to the pulse of blue light, A and C_i increased and L decreased to a greater extent in sugarcane than in soybean. As a result, the gas exchange attributed to the stomatal response to blue light exhibited a higher ratio of A_p to E_p (W_p) in sugarcane than in soybean. This W_p was lower in both species than was the W_s associated with the steady state gas exchange. The two species did not differ in the rate of induction of photosynthetic utilization of elevated C_i. The greater stimulation of A in sugarcane was attributed to its C₄ attributes of greater carboxylation efficiency (slope of the A versus C_i relationship), lower g_s and prevailing C_i,s, and greater L_s under steady-state red illumination. Despite saturation of A at low levels of C_i in C₄ species, the gas exchange attributed to the stomatal response to blue light decreased L and contributed considerably to carbon acquisition, while maintaining the high level of W associated with C₄ metabolism.     

Co-ordination of physiological and morphological responses of stomata to elevated [CO2] in vascular plants

Plant stomata display a wide range of short-term behavioural and long-term morphological responses to atmospheric carbon dioxide concentration ([CO₂]). The diversity of responses suggests that plants may have different strategies for controlling gas exchange, yet it is not known whether these strategies are co-ordinated in some way. Here, we test the hypothesis that there is co-ordination of physiological (via aperture change) and morphological (via stomatal density change) control of gas exchange by plants. We examined the response of stomatal conductance (G _s) to instantaneous changes in external [CO₂] (C _a) in an evolutionary cross-section of vascular plants grown in atmospheres of elevated [CO₂] (1,500 ppm) and sub-ambient [O₂] (13.0 %) compared to control conditions (380 ppm CO₂, 20.9 % O₂). We found that active control of stomatal aperture to [CO₂] above current ambient levels was not restricted to angiosperms, occurring in the gymnosperms Lepidozamia peroffskyana and Nageia nagi. The angiosperm species analysed appeared to possess a greater respiratory demand for stomatal movement than gymnosperm species displaying active stomatal control. Those species with little or no control of stomatal aperture (termed passive) to C _a were more likely to exhibit a reduction in stomatal density than species with active stomatal control when grown in atmospheres of elevated [CO₂]. The relationship between the degree of stomatal aperture control to C _a above ambient and the extent of any reduction in stomatal density may suggest the co-ordination of physiological and morphological responses of stomata to [CO₂] in the optimisation of water use efficiency. This trade-off between stomatal control strategies may have developed due to selective pressures exerted by the costs associated with passive and active stomatal control.     

Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of <Emphasis Type="Italic">Cucumis sativus</Emphasis> seedlings under different CO<Subscript>2</Subscript> concentrations

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO₂ concentrations ([CO₂]; 400 and 1,000 μmol mol^–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO₂ concentrations. Under ambient [CO₂], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO₂] (C_i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P_N) at low salinity, but P_N was potentially limited by C_i at high salinity. At high [CO₂], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C_i occurred where slope of P_N–C_i curve was almost flat.     

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO<Subscript>2</Subscript>

Using a free-air CO₂ enrichment (FACE) experiment, poplar trees (Populus × euramericana clone I214) were exposed to either ambient or elevated [CO₂] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm⁻²) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (g _s, μmol m⁻² s⁻¹), photosynthetic CO₂ fixation (A, mmol m⁻² s⁻¹), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO₂ (C_i:C_a). Elevated [CO₂] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO₂] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO₂], g _s was reduced, A/E was stimulated, and C_i:C_a was reduced relative to ambient [CO₂]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO₂]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO₂], although the mechanism for this remains obscure.     

Stomatal sensitivities to changes in leaf water potential, air humidity, CO2 concentration and light intensity, and the effect of abscisic acid on the sensitivities in six temperate deciduous tree species

To characterise the stomata of six temperate deciduous tree species, sets of stomatal sensitivities to all the most important environmental factors were measured. To compare the importance of abscisic acid (ABA) in the different stomatal responses, the effect of exogenous ABA on all the stomatal sensitivities was determined.Almost all the stomatal sensitivities: the sensitivity to a decrease in leaf water potential, air humidity, CO₂ concentration ([CO₂]) and light intensity, and to an increase in [CO₂] and light intensity were the highest in the slow-growing species, and the lowest in the fast-growing species. Drought increased the sensitivity to the environmental changes that induce a decrease in the stomatal conductance, and decreased the sensitivity to the changes that induce an increase in this conductance. The sensitivities of the slow-growers were most strongly affected by drought and ABA. Therefore the success of the slow-growers in their ecological niches can be based on the highly sensitive and strictly regulated responses of their stomata. The fast-growers had the highest sensitivity to an increase in leaf water potential and this sensitivity was sharply reduced by drought and ABA. Thus, the dominance of the trees in riparian areas can be based on the ability of their stomata to quickly reach high conductance in well-watered conditions and to efficiently decrease this rate during drought.Stomatal sensitivities to the hydraulic environmental factors (water potentials in plant and air) had higher values in well-watered trees and a more pronounced response to drought than the sensitivities to the photosynthetic environmental factors ([CO₂] and light intensity). Thus, the hydraulic factors most likely prevail over the photosynthetic factors in determining stomatal conductance in these species.In response to exogenous ABA, the rates of stomatal closure, following a decrease in air humidity and light intensity, and an increase in [CO₂], were accelerated. Stomatal opening following an increase in air humidity and light intensity and a decrease in [CO₂] was replaced by slow closing. The rate of stomatal opening following an increase in leaf water potential was reduced. As the sensitivities to changes in light were modified less by the ABA than the other stomatal sensitivities, the prediction of stomatal responses on the basis of the sensitivity to light alone should be excluded in stomatal models.     

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.     

Maintenance of a high photosynthetic performance is linked to flooding tolerance in citrus

Citrus trees have been considered as flooding-sensitive although important differences in tolerance among species have been reported. The tolerance to flooding has been linked to optimal photosynthetic performance in other woody plants. To test whether there was a relationship between photosynthetic performance and flooding tolerance, leaf damage, chlorophyll content, net photosynthetic rate, stomatal conductance, the ratio of internal to ambient CO₂ concentration (C_i/C_a), water use efficiency and chlorophyll fluorescence parameters were studied in leaves of three citrus genotypes differing in their tolerance to flooding during continuous substrate flooding and alternate cycles of flooding and recovery. In Cleopatra and Citrumelo genotypes, marked reductions in net photosynthetic rate and stomatal conductance as well as increases in C_i/C_a in response to flooding stress were observed although with differences in the magnitude of the variation. In contrast, in Carrizo, a relatively flooding-tolerant genotype, there were no changes in net photosynthetic rate or in C_i/C_a and only a slight decrease in stomatal conductance occurred in response to flooding. Significant correlation between net photosynthetic rate and chlorophyll fluorescence parameters during flooding indicated a biochemical impairment of photosynthetic activity. This effect was apparently linked to damage in the PSII light-harvesting complexes induced by flooding and a subsequent effect on PSII to PSI electron flow that may alter the redox status in cells. Such biochemical impairment could lead to an increase in oxidative damage in Cleopatra and Citrumelo. The maintenance of good photosynthetic performance together with mechanisms to adjust electron flow in the photosynthetic apparatus could be linked to flooding tolerance in these woody plants.     

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.     

Specialized stomatal humidity responses underpin ecological diversity in C3 bromeliads

The Neotropical Bromeliaceae display an extraordinary level of ecological variety, with species differing widely in habit, photosynthetic pathway and growth form. Divergences in stomatal structure and function, hitherto understudied in treatments of bromeliad evolutionary physiology, could have been critical to the generation of variety in ecophysiological strategies among the bromeliads. Because humidity is a key factor in bromeliad niches, we focussed on stomatal responses to vapour pressure deficit (VPD). We measured the sensitivity of stomatal conductance and assimilation rate to VPD in eight C₃ bromeliad species of contrasting growth forms and ecophysiological strategies and parameterised the kinetics of stomatal responses to a step change in VPD. Notably, three tank‐epiphyte species displayed low conductance, high sensitivity and fast kinetics relative to the lithophytes, while three xeromorphic terrestrial species showed high conductance and sensitivity but slow stomatal kinetics. An apparent feedforward response of transpiration to VPD occurred in the tank epiphytes, while water‐use efficiency was differentially impacted by stomatal closure depending on photosynthetic responses. Differences in stomatal responses to VPD between species of different ecophysiological strategies are closely linked to modifications of stomatal morphology, which we argue has been a pivotal component of the evolution of high diversity in this important plant family.