Stomatal, mesophyll conductance and biochemical limitations to photosynthesis as affected by drought and leaf ontogeny in ash and oak trees

Gas exchange measurements were carried out on ash and oak trees in a forest plantation during three whole growing seasons characterized by different water availability (2001, 2002 and 2003). A quantitative limitation analysis was applied to estimate the effects of drought and leaf ontogeny on stomatal (S_L) and non-stomatal limitations (NS_L) to light-saturated net photosynthesis (A_max), relative to the seasonal maximum rates obtained under conditions of optimal soil water content. Furthermore, based on combined gas exchange and chlorophyll fluorescence measurements, NS_L was partitioned into a diffusive (due to a decrease in mesophyll conductance, MC_L) and a biochemical component (due to a decrease in carboxylation capacity, B_L). During the wettest year (2002), the seasonal pattern of both A_max and stomatal conductance (g_sw) was characterized in both species by a rapid increase during spring and a slight decline over the summer. However, with a moderate (year 2001) or a severe (year 2003) water stress, the summer decline of A_max and g_sw was more pronounced and increased with drought intensity (30–40% in 2001, 60–75% in 2003). The limitation analysis showed that during the spring and the autumn periods S_L, MC_L and B_L were of similar magnitude. By contrast, from the summer data it emerged that all the limitations increased with drought intensity, but their relative contribution changed. At mild to moderate water stress (corresponding to values of g_sw > 100 mmol H₂O m⁻² s⁻¹) about two-thirds of the decline in A_max was attributable to S_L. However, with increasing drought intensity, NS_L increased more than S_L and nearly equalled it when the stress was very severe (i.e. with g_sw < 60 mmol H₂O m⁻² s⁻¹). Within NS_L, MC_L represented the main component, except at the most severe water stress levels when it was equalled by B_L. It is concluded that diffusional limitations (i.e. S_L + MC_L) largely affect net assimilation during most of the year, whereas biochemical limitations are quantitatively important only during leaf development and senescence or with severe droughts.     

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.     

Diffusive and metabolic limitations to photosynthesis under drought and salinity in C(3) plants

Drought and salinity are two widespread environmental conditions leading to low water availability for plants. Low water availability is considered the main environmental factor limiting photosynthesis and, consequently, plant growth and yield worldwide. There has been a long-standing controversy as to whether drought and salt stresses mainly limit photosynthesis through diffusive resistances or by metabolic impairment. Reviewing in vitro and in vivo measurements, it is concluded that salt and drought stress predominantly affect diffusion of CO(2) in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO(2), at mild to rather severe stress levels. The general failure of metabolism observed at more severe stress suggests the occurrence of secondary oxidative stresses, particularly under high-light conditions. Estimates of photosynthetic limitations based on the photosynthetic response to intercellular CO(2) may lead to artefactual conclusions, even if patchy stomatal closure and the relative increase of cuticular conductance are taken into account, as decreasing mesophyll conductance can cause the CO(2) concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO(2) concentration. Measurements based on the photosynthetic response to chloroplast CO(2) often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.     

Variability in mesophyll conductance between barley genotypes,and effects on transpiration efficiency and carbon isotope discrimination

Leaf internal, or mesophyll, conductance to CO₂ (g_m ) is a significant and variable limitation of photosynthesis that also affects leaf transpiration efficiency (TE). Genotypic variation in g_m and the effect of g_m on TE were assessed in six barley genotypes (four Hordeum vulgare and two H. bulbosum). Significant variation in g_m was found between genotypes, and was correlated with photosynthetic rate. The genotype with the highest g_m also had the highest TE and the lowest carbon isotope discrimination as recorded in leaf tissue (Δ_p). These results suggest g_m has unexplored potential to provide TE improvement within crop breeding programmes.     

Stomatal and non-stomatal limitations to photosynthesis in field-grown grapevine cultivars

Diurnal changes of photosynthesis in the leaves of grapevine (Vitis vinifera × V. labrusca) cultivars Campbell Early and Kyoho grown in the field were compared with respect to gas exchanges and actual quantum yield of photosystem 2 (Φ_PS2) in late May. Net photosynthetic rate (P_N) of the two cultivars rapidly increased in the morning, saturated at photosynthetic photon flux density (PPFD) from 1200 to 1500 μmol m⁻² s⁻¹ between 10:00 and 12:00 and slowly decreased after midday. Maximum P_N was 13.7 and 12.5 μmol m⁻² s⁻¹ in Campbell Early and Kyoho, respectively. The stomatal conductance (g_s) and transpiration rate changed in parallel with P_N, indicating that P_N was greatly affected by g_s. However, the decrease in P_N after midday under saturating PPFD was also associated with the observed depression of Φ_PS2 at high PPFD. The substantial increase in the leaf to air vapour pressure deficit after midday might also contribute to decline of g_s and P_N.     

Stomatal and nonstomatal limitations of photosynthesis in 19 temperate tree species on contrasting sites during wet and dry years

A unique approach was used to evaluate stomatal and nonstomatal constraints to photosynthesis in 19 naturally occurring, deciduous tree species on xeric, mesic and wetmesic sites in central Pennsylvania, USA, during relatively wet (1990) and dry (1991) growing seasons. All species exhibited significantly decreased stomatal conductance to CO₂ (g_c) in 1991 compared to 1990. The mesic species had drought related decreases in photosynthesis (A) attributed primarily to increased absolute stomatal limitation to A (L_g), whereas in the wet-mesic species, the absolute mesophyll limitation (Lm) was at least as important as L_g in limiting A during drought. The xeric species maintained relatively high A during drought despite decreased g_c. In the xeric and mesic species, L_m decreased and L_g increased during drought due to stomatal closure. From xeric to mesic to wet-mesic, the relative stomatal limitation (I_g) generally decreased faster, and relative mesophyll limitations to A increased faster, with increasing g_c suggesting greater photosynthetic capacity (i.e. greater potential maximum A) with increasing drought tolerance rank of species. Few species exhibited a significant drought-related decrease in photosynthetic capacity. The results of this landscape-based study indicate that the interaction of stomatal and nonstomatal limitations of A vary in a manner consistent with species' drought tolerance and site conditions, and that nonstomatal constraints to A in field plants during a moderate, season-long drought were generally not as severe as reported in controlled studies.     

Estimating the sensitivity of stomatal conductance to photosynthesis: a review

A common approach for estimating fluxes of CO₂ and water in canopy models is to couple a model of photosynthesis (A_n) to a semi‐empirical model of stomatal conductance (g_s) such as the widely validated and utilized Ball–Berry (BB) model. This coupling provides an effective way of predicting transpiration at multiple scales. However, the designated value of the slope parameter (m) in the BB model impacts transpiration estimates. There is a lack of consensus regarding how m varies among species or plant functional types (PFTs) or in response to growth conditions. Literature values are highly variable, with inter‐species and intra‐species variations of >100%, and comparisons are made more difficult because of differences in collection techniques. This paper reviews the various methods used to estimate m and highlights how variations in measurement techniques or the data utilized can influence the resultant m. Additionally, this review summarizes the reported responses of m to [CO₂] and water stress, collates literature values by PFT and compiles nearly three decades of values into a useful compendium.     

Carbonyl sulfide (COS) as a tracer for canopy photosynthesis, transpiration and stomatal conductance: potential and limitations

The theoretical basis for the link between the leaf exchange of carbonyl sulfide (COS), carbon dioxide (CO(2)) and water vapour (H(2)O) and the assumptions that need to be made in order to use COS as a tracer for canopy net photosynthesis, transpiration and stomatal conductance, are reviewed. The ratios of COS to CO(2) and H(2)O deposition velocities used to this end are shown to vary with the ratio of the internal to ambient CO(2) and H(2)O mole fractions and the relative limitations by boundary layer, stomatal and internal conductance for COS. It is suggested that these deposition velocity ratios exhibit considerable variability, a finding that challenges current parameterizations, which treat these as vegetation-specific constants. COS is shown to represent a better tracer for CO(2) than H(2)O. Using COS as a tracer for stomatal conductance is hampered by our present poor understanding of the leaf internal conductance to COS. Estimating canopy level CO(2) and H(2)O fluxes requires disentangling leaf COS exchange from other ecosystem sources/sinks of COS. We conclude that future priorities for COS research should be to improve the quantitative understanding of the variability in the ratios of COS to CO(2) and H(2)O deposition velocities and the controlling factors, and to develop operational methods for disentangling ecosystem COS exchange into contributions by leaves and other sources/sinks. To this end, integrated studies, which concurrently quantify the ecosystem-scale CO(2), H(2)O and COS exchange and the corresponding component fluxes, are urgently needed.     

High temperature acclimation of C4 photosynthesis is linked to changes in photosynthetic biochemistry

With average global temperatures predicted to increase over the next century, it is important to understand the extent and mechanisms of C4 photosynthetic acclimation to modest increases in growth temperature. To this end, we compared the photosynthetic responses of two C4 grasses (Panicum coloratum and Cenchrus ciliaris) and one C4 dicot (Flaveria bidentis) to growth at moderate (25/20 degrees C, day/night) or high (35/30 degrees C, day/night) temperatures. In all three C4 species, CO2 assimilation rates (A) underwent significant thermal acclimation, such that when compared at growth temperatures, A increased less than what would be expected given the strong response of A to short-term changes in leaf temperature. Thermal photosynthetic acclimation was further manifested by an increase in the temperature optima of A, and a decrease in leaf nitrogen content and leaf mass per area in the high- relative to the moderate-temperature-grown plants. Reduced photosynthetic capacity at the higher growth temperature was underpinned by selective changes in photosynthetic components. Plants grown at the higher temperature had lower amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase and cytochrome f and activity of carbonic anhydrase. The activities of photosystem II (PSII) and phosphoenolpyruvate carboxylase were not affected by growth temperature. Chlorophyll fluorescence measurements of F. bidentis showed a corresponding decrease in the quantum yield of PSII (phi(PSII)) and an increase in non-photochemical quenching (phi(NPQ)). It is concluded that through these biochemical changes, C4 plants maintain the balance between the various photosynthetic components at each growth temperature, despite the differing temperature dependence of each process. As such, at higher temperatures photosynthetic nitrogen use efficiency increases more than A. Our results suggest C4 plants will show only modest changes in photosynthetic rates in response to changes in growth temperature, such as those expected within or between seasons, or the warming anticipated as a result of global climate change.     

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

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

Responses of stomatal conductance to light, humidity and temperature in winter wheat and barley grown at three concentrations of carbon dioxide in the field

Responses of leaf stomatal conductance to light, humidity and temperature were characterized for winter wheat and barely grown at ambient (about 350 μmol mol^?1 in the daytime), ambient + 175 and ambient + 350 μmol mol^?1 concentrations of carbon dioxide in open‐topped chambers in field plots over a three year period. Stomatal responses to environment were determined by direct manipulation of single environmental factors, and those results were compared with responses derived from natural day to day variation in mid‐day stomatal conductance. The purpose of these experiments was to determine the magnitude of reduction in stomatal conductance at elevated [CO₂], and to assess whether the relative response of conductance to elevated [CO₂] was constant across light, humidity and temperature conditions. The results indicated that light, humidity and temperature all significantly affected the relative decrease in stomatal conductance at elevated [CO₂]. The relative decrease in conductance with elevated [CO₂] was greater at low light, low water vapour pressure difference, and high temperature in both species. For measurements made at saturating light near mid‐day, the ratio of mid‐day stomatal conductances at doubled [CO₂] to that at ambient [CO₂] ranged from 0.42 to 0.86, with a mean of 0.66 in barley, and from 0.33 to 0.80, with a mean of 0.56 in wheat. Day‐to‐day variation in the relative effect of elevated [CO₂] on conductance was correlated with the relative stimulation of [CO₂] assimilation rate and with temperature. Some limitations of multiple linear regression, multiplicative, and ‘Ball–Berry' models as summaries of the data are discussed. In barley, a better fit to the models occurred in individual years than for the combined data, and in wheat a better fit to the models occurred when data from near the end of the season were removed.     

Sensitivity to far-red radiation in stomata of Phaseolus vulgaris L.: Rhythmic effects on conductance and photosynthesis

The influence of far-red (FR; 700–800 nm) radiation on steady-state stomatal conductance and net photosynthesis in P. vulgaris has been studied. Whereas FR radiation alone was relatively ineffective, addition of FR to a background of white light (WL; predominantly 400–700 nm) resulted in increased stomatal conductance. Stomata exhibited a marked diurnal sensitivity to FR. The action maximum for enhancing stomatal conductance was near 714 nm. A combination of FR and infra-red (IR; >800 nm) enhanced net photosynthesis when added to a background of WL. When IR alone was added to WL, there was a net decrease in photosynthesis, indicating that it is the FR waveband which is responsible for the observed photosynthetic effects. Naturally occurring levels of FR radiation (235 mol·m^-2·s^-1) in vegetation-canopy shade enhanced net photosynthetic CO₂ gain by 28% when added to a background of 55 mol·m^-2·s^-1 WL.Abbreviations BL blue - FR far-red - IR infra-red - PAR photosynthetically active radiation - R red - WL white light     

Limitations to photosynthesis inCoffea canephoraas a result of nitrogen and water availability

Plants of C. canephora grown in pots under low nitrogen (LN) or high nitrogen (HN) applications were submitted to either cyclic water stress or daily irrigation. Water deficit led to marked decreases in net carbon assimilation rate (A) and, to a lesser extent, in stomatal conductance (g_s), regardless of the N treatments. In well-watered plants, A appreciably increased in HN plants relative to LN plants without significant changes in g_s. As a whole, changes in internal CO₂ concentration predominantly reflected changes in A rather than in g_s. Under irrigated conditions, A, but not g_s, correlated with leaf N concentration in a curvilinear way. Photosynthetic nitrogen-use efficiency was considerably low, and decreased with increasing leaf N concentration. Limited N, but not water, slightly decreased the maximum photochemical efficiency of photosystem II (PSII). Under continuous irrigation, LN plants had a smaller quantum yield of electron transport (_PSII) through slight decreases of photochemical quenching (q_p) and capture efficiency of excitation energy by open PSII reaction centres, and increases in Stern-Volmer non-photochemical quenching. Under water-stressed conditions, changes in PSII photochemistry were apparent only in HN plants, with a 25 % decrease in _PSII, due mainly to variations in q_p. Biochemical constraints, rather than stomatal or photochemical limitations, provoked the decreases in A under limited supply of either N or water.     

Stomatal and nonstomatal limitations of photosynthesis in relation to the drought and shade tolerance of tree species in open and understory environments

 Light saturated photosynthesis (A) in field saplings of shade tolerant, intermediate, and intolerant tree species was analyzed for stomatal and nonstomatal limitations to test differences between species and sun and shade phenotypes during drought. Throughout the study, photosynthesis was highest and mesophyll limitations of A (L_m) lowest in the intolerant species in both open and understory habitats. The shade tolerant species exhibited the only drought-related decreased A and increased L_m in the open, and the greatest drought-related decreased A and increased L_m in the understory. Few species exhibited significant habitat or drought-related differences in stomatal conductance to CO₂ (g_c), but even slight decreases in g_c during drought were associated with large increases in stomatal limitations to A (L_g). Combined changes in L_m and L_g resulted in increased relative stomatal limitation to A (l _g) in several species during drought. Nevertheless, the overall lack of stomatal closure allowed for nonstomatal limitations to play a major role in reduced A during drought. Higher leaf N was associated with shallower slope of the l _g versus g_c relationship, an indication of greater A capacity. Photosynthetic capacity tended to be greater in the intolerant species than the tolerant species, and it tended to decrease during drought primarily in the shade tolerant species in the understory. Findings in the literature suggest that carbon reduction reactions may be more susceptible to drought than photosynthetic light reactions. If so, reduced carbon reduction capacity of shade tolerant species or shade phenotypes may predispose them to drought conditions, which suggests a mechanism behind the well-recognized tradeoff between drought tolerance and shade tolerance of temperate tree species. Received: 20 October 1995 / Accepted: 20 February 1996     

Photosynthetic limitations in response to water stress and recovery in Mediterranean plants with different growth forms

* Whether photosynthesis is limited during water stress and recovery because of diffusive or biochemical factors is still open to debate, and apparent contradictions appear when various studies on species with different growth forms are compared. * Ten Mediterranean species, representing different growth forms, were subjected to different levels of water stress, the most severe followed by rewatering. A quantitative limitation analysis was applied to estimate the effects of water stress on stomatal (S(L)), mesophyll conductance (MC(L)) and biochemical limitations (B(L)). * Results confirmed a general pattern of photosynthetic response to water stress among C(3) plants when stomatal conductance (g(s)) is used as a reference parameter. As g(s) values decreased from a maximum to approx. 0.05 mol H(2)O m(-2) s(-1), the total photosynthetic limitation rose from 0 to approx. 70%, and this was caused by a progressive increase of both S(L) and MC(L) limitations, while B(L) remained negligible. When lower values of g(s) were achieved (total photosynthetic limitation increased from 70 to 100%), the contribution of S(L) declined, while MC(L) still increased and B(L) contributed significantly (20-50%) to the total limitation. * Photosynthetic recovery of severely stressed plants after rewatering showed a dominant role of MC(L), irrespective of the degree of photosynthesis recovery.     

Stomatal conductance and leaf water potential responses to hydraulic conductance variation in <Emphasis Type="Italic">Pinus pinaster</Emphasis> seedlings

In this study, tree hydraulic conductance (K _tree) was experimentally manipulated to study effects on short-term regulation of stomatal conductance (g _s), net photosynthesis (A) and bulk leaf water potential (Ψ_leaf) in well watered 5–6 years old and 1.2 m tall maritime pine seedlings (Pinus pinaster Ait.). K _tree was decreased by notching the stem and increased by progressively excising the root system and stem. Gas exchange was measured in a chamber at constant irradiance, vapour pressure deficit, leaf temperature and ambient CO₂ concentration. As expected, we found a strong and positive relationship between g _s and K _tree (r = 0.92, P = 0.0001) and between A and K _tree (r = 0.9, P = 0.0001). In contrast, however, we found that the response of Ψ_leaf to K _tree depended on the direction of change in K _tree: increases in K _tree caused Ψ_leaf to decrease from around −1.0 to −0.6 MPa, but reductions in K _tree were accompanied by homeostasis in Ψ_leaf (at −1 MPa). Both of these observations could be explained by an adaptative feedback loop between g _s and Ψ_leaf, with Ψ_leaf prevented from declining below the cavitation threshold by stomatal closure. Our results are consistent with the hypothesis that the observed stomatal responses were mediated by leaf water status, but they also suggest that the stomatal sensitivity to water status increased dramatically as Ψ_leaf approached −1 MPa.     

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.     

Apparent carboxylation efficiency and relative stomatal and mesophyll limitations of photosynthesis in an evergreen cerrado species during water stress

Miconia albicans, a common evergreen cerrado species, was studied under field conditions. Leaf gas exchange and pre-dawn leaf water potential (Ψ_pd) were determined during wet and dry seasons. The potential photosynthetic capacity (P _Npmax) and the apparent carboxylation efficiency (ε) dropped in the dry season to 28.0 and 0.7 %, respectively, of the maximum values in the wet season. The relative mesophyll (L_m) and stomatal (L_s) limitations of photosynthesis increased, respectively, from 24 and 44 % in the wet season to 79 and 57 % at the peak of the dry season when mean Ψ_pd reached −5.2 MPa. After first rains, the P _Npmax, ε, and L_m recovered reaching the wet season values, but L_s was maintained high (63 %). The shallow root system growing on stonemason limited by lateral concrete wall to a depth of 0.33 m explained why extreme Ψ_pd was brought about. Thus M. albicans is able to overcome quickly the strains imposed by severe water stress.     

Evaluation of the potential to measure photosynthetic rates in C3 plants (Flaveria pringlei and Oryza sativa) by combining chlorophyll fluorescence analysis and a stomatal conductance model

The response curves of leaf photosynthesis to varying light, temperature and leaf-to-air vapour pressure deficit were measured in the C₃ plants Flaveria pringlei and Oryza sativa in normal air with a computerized open infrared gas analysis (IRGA) system, and the photochemical efficiency of photosystem II, described as (1–F,/F′_m) after Genty. Briantais & Baker (1989, Biochimica et Biophysica Acta 990, 87–92), was simultaneously measured with a modulated fluorometer. A model was written for rates of CO₂ fixation as a function of the true rate of O₂ evolution measured by fluorescene analysis (Jo₂), mesophyll conductance and intercellular CO₂ partial pressure. A second model was developed for rates of CO₂ fixation as a function of Jo₂, mesophyll conductance and stomatal conductance. In the latter case, leaf stomatal conductance was simulated using the stomatal model proposed by Leuning (1995, Plant, Cell and Environment 18 , 339–355). The rates of CO₂ fixation predicted from the models were similar to rates measured by IRGA. The results indicate that there is potential to measure CO₂ fixation in C₃ plants by combining the non-invasive measurement of Jo₂ by chlorophyll fluorescence analysis with the stomatal conductance model.