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

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

Recent developments in mesophyll conductance in C3, C4, and crassulacean acid metabolism plants

The conductance of carbon dioxide (CO₂) from the substomatal cavities to the initial sites of CO₂ fixation (g_m) can significantly reduce the availability of CO₂ for photosynthesis. There have been many recent reviews on: (i) the importance of g_m for accurately modelling net rates of CO₂ assimilation, (ii) on how leaf biochemical and anatomical factors influence g_m, (iii) the technical limitation of estimating g_m, which cannot be directly measured, and (iv) how g_m responds to long‐ and short‐term changes in growth and measurement environmental conditions. Therefore, this review will highlight these previous publications but will attempt not to repeat what has already been published. We will instead initially focus on the recent developments on the two‐resistance model of g_m that describe the potential of photorespiratory and respiratory CO₂ released within the mitochondria to diffuse directly into both the chloroplast and the cytosol. Subsequently, we summarize recent developments in the three‐dimensional (3‐D) reaction‐diffusion models and 3‐D image analysis that are providing new insights into how the complex structure and organization of the leaf influences g_m. Finally, because most of the reviews and literature on g_m have traditionally focused on C₃ plants we review in the final sections some of the recent developments, current understanding and measurement techniques of g_m in C₄ and crassulacean acid metabolism (CAM) plants. These plants have both specialized leaf anatomy and either a spatially or temporally separated CO₂ concentrating mechanisms (C₄ and CAM, respectively) that influence how we interpret and estimate g_m compared with a C₃ plants.     

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

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

Stomatal responses to CO2 during a diel Crassulacean acid metabolism cycle in Kalanchoe daigremontiana and Kalanchoe pinnata

To investigate the diurnal variation of stomatal sensitivity to CO₂, stomatal response to a 30 min pulse of low CO₂ was measured four times during a 24 h time-course in two Crassulacean acid metabolism (CAM) species Kalanchoe daigremontiana and Kalanchoe pinnata , which vary in the degree of succulence, and hence, expression and commitment to CAM. In both species, stomata opened in response to a reduction in p CO₂ in the dark and in the latter half of the light period, and thus in CAM species, chloroplast photosynthesis is not required for the stomatal response to low p CO₂. Stomata did not respond to a decreased p CO₂ in K. daigremontiana in the light when stomata were closed, even when the supply of internal CO₂ was experimentally reduced. We conclude that stomatal closure during phase III is not solely mediated by high internal p CO₂, and suggest that in CAM species the diurnal variability in the responsiveness of stomata to p CO₂ could be explained by hypothesizing the existence of a single CO₂ sensor which interacts with other signalling pathways. When not perturbed by low p CO₂, CO₂ assimilation rate and stomatal conductance were correlated both in the light and in the dark in both species.     

Leaf responses to drought stress in Mediterranean accessions of Solanum lycopersicum: anatomical adaptations in relation to gas exchange parameters

In a previous study, important acclimation to water stress was observed in the Ramellet tomato cultivar (TR) from the Balearic Islands, related to an increase in the water‐use efficiency through modifications in both stomatal (g_s) and mesophyll conductances (g_m). In the present work, the comparison of physiological and morphological traits between TR accessions grown with and without water stress confirmed that variability in the photosynthetic capacity was mostly explained by differences in the diffusion of CO₂ through stomata and leaf mesophyll. Maximization of g_m under both treatments was mainly achieved through adjustments in the mesophyll thickness and porosity and the surface area of chloroplasts exposed to intercellular airspace (S_c). In addition, the lower g_m/S_c ratio for a given porosity in drought‐acclimated plants suggests that the decrease in g_m was due to an increased cell wall thickness. Stomatal conductance was also affected by drought‐associated changes in the morphological properties of stomata, in an accession and treatment‐dependent manner. The results confirm the presence of advantageous physiological traits in the response to drought stress in Mediterranean accessions of tomato, and relate them to particular changes in the leaf anatomical properties, suggesting specific adaptive processes operating at the leaf anatomical level.     

The evolutionary relationship between stomatal mechanism, crassulacean acid metabolism and C4 photosynthesis

Abstract. All of the features of crassulacean acid metabolism (CAM) and most characteristics of C₄ photosynthesis are exhibited by stomatal guard cells. It is proposed that CAM and possibly also C₄ photosynthesis result from the expression in photosynthetic cells of genetic information which is expressed only in guard cells of C₃ plants.     

Reconciling the optimal and empirical approaches to modelling stomatal conductance

Models of vegetation function are widely used to predict the effects of climate change on carbon, water and nutrient cycles of terrestrial ecosystems, and their feedbacks to climate. Stomatal conductance, the process that governs plant water use and carbon uptake, is fundamental to such models. In this paper, we reconcile two long‐standing theories of stomatal conductance. The empirical approach, which is most commonly used in vegetation models, is phenomenological, based on experimental observations of stomatal behaviour in response to environmental conditions. The optimal approach is based on the theoretical argument that stomata should act to minimize the amount of water used per unit carbon gained. We reconcile these two approaches by showing that the theory of optimal stomatal conductance can be used to derive a model of stomatal conductance that is closely analogous to the empirical models. Consequently, we obtain a unified stomatal model which has a similar form to existing empirical models, but which now provides a theoretical interpretation for model parameter values. The key model parameter, g₁, is predicted to increase with growth temperature and with the marginal water cost of carbon gain. The new model is fitted to a range of datasets ranging from tropical to boreal trees. The parameter g₁ is shown to vary with growth temperature, as predicted, and also with plant functional type. The model is shown to correctly capture responses of stomatal conductance to changing atmospheric CO₂, and thus can be used to test for stomatal acclimation to elevated CO₂. The reconciliation of the optimal and empirical approaches to modelling stomatal conductance is important for global change biology because it provides a simple theoretical framework for analyzing, and simulating, the coupling between carbon and water cycles under environmental change.     

Acclimation to humidity modifies the link between leaf size and the density of veins and stomata

The coordination of veins and stomata during leaf acclimation to sun and shade can be facilitated by differential epidermal cell expansion so large leaves with low vein and stomatal densities grow in shade, effectively balancing liquid‐ and vapour‐phase conductances. As the difference in vapour pressure between leaf and atmosphere (VPD) determines transpiration at any given stomatal density, we predict that plants grown under high VPD will modify the balance between veins and stomata to accommodate greater maximum transpiration. Thus, we examined the developmental responses of these traits to contrasting VPD in a woody angiosperm (Toona ciliata M. Roem.) and tested whether the relationship between them was altered. High VPD leaves were one‐third the size of low VPD leaves with only marginally greater vein and stomatal density. Transpirational homeostasis was thus maintained by reducing stomatal conductance. VPD acclimation changed leaf size by modifying cell number. Hence, plasticity in vein and stomatal density appears to be generated by plasticity in cell size rather than cell number. Thus, VPD affects cell number and leaf size without changing the relationship between liquid‐ and vapour‐phase conductances. This results in inefficient acclimation to VPD as stomata remain partially closed under high VPD.     

Endogenous rhythms and chaos in crassulacean acid metabolism

Endogenous free-running regular circadian oscillations of net CO₂ exchange in the crassulacean-acidmetabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perrier de la Bâthie under constant external conditions in continuous light have been shown to change to irregular non-predictable (chaotic) time behaviour as irradiance or temperature are raised above a critical level. A model of CAM has been constructed with pools of major metabolites of varying concentrations, flows of metabolites leading to exchange between pools, metabolite transformations determined by chemical reactions, and feedback regulations. The model is described by a system of coupled non-linear differential equations. It shows stable rhythmicity in normal dark-light cycles and in continuous light and, like the K. daigremontiana leaves in the experiments, a change to chaos as irradiance is increased. The maintenance of endogenous oscillations in the model is brought about by a hysteresis switch or beat oscillator between two stable oscillation modes. In CAM these stable modes are vacuolar malate accumulation and remobilization. The model shows that the physical nature of the beat oscillator in the leaves can be explained by the balance between active and passive transport at the tonoplast.Abbreviations CAM crassulacean acid metabolism - D dark period - DL 12:12 h dark-light rhythm - L light period - LL constant illumination - PPFD photosynthetic photon flux density - T^L leaf temperature It is a great pleasure to thank Dr. G.-H. Vieweg, (Roßdorf, FRG) for his long-lasting efforts to have the phytotron in Darmstadt erected and for his persistent involvement during the various phases of planning and building. This made the present experiments possible. Dr. D. Kramer is thanked for all the time he spends to maintain functioning of the facility. Dr. P. Keller and Ms. Erika Ball assisted with the gas-exchange technology and helped with the surveillance of the long-running experiments, and Ms. Erika Ball performed all the integrations. Ms. Doris Schäfer is thanked for drawing the gas-exchange curves for publication. We are also most grateful to Professor Chr. Giersch and Professor M. Kluge (both Institut für Botanik, Technische Hochschule Darmstadt, FRG) for valuable discussions.     

Leaf physiological responses to extreme droughts in Mediterranean Quercus ilex forest

Global climate change is expected to result in more frequent and intense droughts in the Mediterranean region. To understand forest response to severe drought, we used a mobile rainfall shelter to examine the impact of spring and autumn rainfall exclusion on stomatal (S_L) and non‐stomatal (NS_L) limitations of photosynthesis in a Quercus ilex ecosystem. Spring rainfall exclusion, carried out during increasing atmospheric demand and leaf development, had a larger impact on photosynthesis than autumn exclusion, conducted at a time of mature foliage and decreasing vapour pressure deficit. The relative importance of NS_L increased with drought intensity. S_L and NS_L were equal once total limitation (T_L) reached 60%, but NS_L greatly exceeded S_L during severe drought, with 76% NS_L partitioned equally between mesophyll conductance (MC_L) and biochemical (B_L) limitations when T_L reached 100%. Rainfall exclusion altered the relationship between leaf water potential and photosynthesis. In response to severe mid‐summer drought stress, A_n and V_cmax were 75% and 72% lower in the spring exclusion plot than in the control plot at the same pre‐dawn leaf water potential. Our results revealed changes in the relationship between photosynthetic parameters and water stress that are not currently included in drought parameterizations for modelling applications.     

Physiological and morphological adaptations in relation to water use efficiency in Mediterranean accessions of Solanum lycopersicum

The physiological traits underlying the apparent drought resistance of 'Tomàtiga de Ramellet' (TR) cultivars, a population of Mediterranean tomato cultivars with delayed fruit deterioration (DFD) phenotype and typically grown under non-irrigation conditions, are evaluated. Eight different tomato accessions were selected and included six TR accessions, one Mediterranean non-TR accession (NTR(M)) and a processing cultivar (NTR(O)). Among the TR accessions two leaf morphology types, normal divided leaves and potato-leaf, were selected. Plants were field grown under well-watered (WW) and water-stressed (WS) treatments, with 30 and 10% of soil water capacity, respectively. Accessions were clustered according to the leaf type and TR phenotype under WW and WS, respectively. Correlation among parameters under the different water treatments suggested that potential improvements in the intrinsic water-use efficiency (A(N)/g(s)) are possible without negative impacts on yield. Under WS TR accessions displayed higher A(N)/g(s), which was not due to differences in Rubisco-related parameters, but correlated with the ratio between the leaf mesophyll and stomatal conductances (g(m)/g(s)). The results confirm the existence of differential traits in the response to drought stress in Mediterranean accessions of tomato, and demonstrate that increases in the g(m)/g(s) ratio would allow improvements in A(N)/g(s) in horticultural crops.     

Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit

Responses of stomatal conductance (g_s) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease – the stomatal sensitivity – varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of g_s to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of g_s at low D ( ≤ 1 kPa). To test this relationship we used the function g_s = g_sref–m· lnD where m is the stomatal sensitivity and g_sref = g_s at D = 1 kPa. Regardless of species or methodology, m was highly correlated with g_sref (average r² = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of g_s in relation to leaf-surface D (D_s), or on sap flux-based stomatal conductance of whole trees (G_Si), or stand-level stomatal conductance (G_S) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential.     

Growth and water relations of Tamarix ramosissima and Populus euphratica on Taklamakan desert dunes in relation to depth to a permanent water table

The hypothesis that water relations and growth of phreatophytic Tamarix ramosissima Ledeb. and Populus euphratica Oliv. on dunes of varying height in an extremely arid Chinese desert depend on vertical distance to a permanent water table was tested. Shoot diameter growth of P. euphratica was inversely correlated with groundwater depth (GD) of 7 to 23 m (adj. R² = 0.69, P = 0.025); growth of T. ramosissima varied independent of GD between 5 and 24 m (P = 0.385). Pre‐dawn (pd) and midday (md) water potentials were lower in T. ramosissima (minimum pd ?1.25 MPa, md ?3.6 MPa at 24 m GD) than in P. euphratica (minimum pd ?0.9 MPa, md ?3.05 MPa at 23 m GD) and did not indicate physiologically significant drought stress for either species. Midday water potentials of P. euphratica closely corresponded to GD throughout the growing season, but those of T. ramosissima did not. In both species, stomatal conductance was significantly correlated with leaf water potential (P. euphratica: adj. R² = 0.84, P < 0.0001; T. ramosissima: adj. R² = 0.64, P = 0.011) and with leaf‐specific hydraulic conductance (P. euphratica: adj. R² = 0.79, P = 0.001; T. ramosissima: adj. R² = 0.56, P = 0.019); the three variables decreased with increasing GD in P. euphratica. Stomatal conductance of P. euphratica was more strongly reduced (> 50% between ?2 and ?3 MPa) in response to decreasing leaf water potential than that of T. ramosissima (30% between ?2 and ?3 MPa). Tolerance of lower leaf water potentials due to higher concentrations of leaf osmotically active substances partially explains why leaf conductance, and probably leaf carbon gain and growth, of T. ramosissima was less severely affected by GD. Additionally, the complex below‐ground structure of large clonal T. ramosissima shrub systems probably introduces variability into the assumed relationship of xylem path length with GD.     

Latitudinal variation in the degree of crassulacean acid metabolism in Puya chilensis

Crassulacean acid metabolism (CAM) is a photosynthetic pathway found in many plant species from arid and semiarid environments. Few studies aiming to characterise plant species as CAM or C₃ account for inter‐population differences in photosynthetic pathway, often relying on samples taken from herbarium material and/or a single plant or population. This may be especially problematic for species growing under contrasting climate conditions, as is the case for species with a wide geographic range. We used Puya chilensis, a species previously reported as CAM and C₃, to study among‐population variation in expression of the CAM pathway within its distribution range, which spans a significant climate gradient. We carried out a wide sampling scheme, including five populations and a combination of analytical methods (quantification of nocturnal acidification and stable isotope measurements). The study populations of P. chilensis encompass the entire latitudinal distribution range, from semi‐arid to temperate oceanic climates. Our results indicate that CAM decreased with latitude. However, even in the southern (wetter) populations, where δ¹³C values were indicative of C₃ metabolism, we found some nocturnal acidification. We stress the value of using two methods along with the use of samples from different populations, as this allows more reliable conclusions on the photosynthetic pathway for ‘probable’ CAM species that face varying climate conditions within their distribution ranges.     

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.     

C4 grasses adapted to low precipitation habitats show traits related to greater mesophyll conductance and lower leaf hydraulic conductance

In habitats with low water availability, a fundamental challenge for plants will be to maximize photosynthetic C-gain while minimizing transpirational water-loss. This trade-off between C-gain and water-loss can in part be achieved through the coordination of leaf-level photosynthetic and hydraulic traits. To test the relationship of photosynthetic C-gain and transpirational water-loss, we grew, under common growth conditions, 18 C₄ grasses adapted to habitats with different mean annual precipitation (MAP) and measured leaf-level structural and anatomical traits associated with mesophyll conductance (g_m) and leaf hydraulic conductance (K_leaf). The C₄ grasses adapted to lower MAP showed greater mesophyll surface area exposed to intercellular air spaces (S_mes) and adaxial stomatal density (SD_ada) which supported greater g_m. These grasses also showed greater leaf thickness and vein-to-epidermis distance, which may lead to lower K_leaf. Additionally, grasses with greater g_m and lower K_leaf also showed greater photosynthetic rates (A_net) and leaf-level water-use efficiency (WUE). In summary, we identify a suite of leaf-level traits that appear important for adaptation of C₄ grasses to habitats with low MAP and may be useful to identify C₄ species showing greater A_net and WUE in drier conditions.     

Day-to-night variations of cytoplasmic pH in a crassulacean acid metabolism plant

Summary In crassulacean acid metabolism (CAM) large amounts of malic acid are redistributed between vacuole and cytoplasm in the course of night-to-day transitions. The corresponding changes of the cytoplasmic pH (pH_cyt) were monitored in mesophyll protoplasts from the CAM plantKalanchoe daigremontiana Hamet et Perrier by ratiometric fluorimetry with the fluorescent dye 2′,7′-bis-(2-carboxyethyl)-5-(and-6-)carboxyfluorescein as a pH_cyt indicator. At the beginning of the light phase, pH_cyt was slightly alkaline (about 7.5). It dropped during midday by about 0.3 pH units before recovering again in the late-day-to-early-dark phase. In the physiological context the variation in pH_cyt may be a component of CAM regulation. Due to its pH sensitivity, phosphoenolpyruvate carboxylase appears as a likely target enzyme. From monitoring ΔpH_cyt in response to loading the cytoplasm with the weak acid salt K-acetate a cytoplasmic H⁺-buffer capacity in the order of 65 mM H+ per pH unit was estimated at a pH_cyt of about 7.5. With this value, an acid load of the cytoplasm by about 10 mM malic acid can be estimated as the cause of the observed drop in pH_cyt. A diurnal oscillation in pH_cyt and a quantitatively similar cytoplasmic malic acid is predicted from an established mathematical model which allows simulation of the CAM dynamics. The similarity of model predictions and experimental data supports the view put forward in this model that a phase transition of the tonoplast is an essential functional element in CAM dynamics.     

Mesophyll conductance to CO2 and Rubisco as targets for improving intrinsic water use efficiency in C3 plants

Water limitation is a major global constraint for plant productivity that is likely to be exacerbated by climate change. Hence, improving plant water use efficiency (WUE) has become a major goal for the near future. At the leaf level, WUE is the ratio between photosynthesis and transpiration. Maintaining high photosynthesis under water stress, while improving WUE requires either increasing mesophyll conductance (g_m) and/or improving the biochemical capacity for CO₂ assimilation—in which Rubisco properties play a key role, especially in C₃ plants at current atmospheric CO₂. The goals of the present analysis are: (1) to summarize the evidence that improving g_m and/or Rubisco can result in increased WUE; (2) to review the degree of success of early attempts to genetically manipulate g_m or Rubisco; (3) to analyse how g_m, g_sw and the Rubisco's maximum velocity (V_cmax) co‐vary across different plant species in well‐watered and drought‐stressed conditions; (4) to examine how these variations cause differences in WUE and what is the overall extent of variation in individual determinants of WUE; and finally, (5) to use simulation analysis to provide a theoretical framework for the possible control of WUE by g_m and Rubisco catalytic constants vis‐à‐vis g_sw under water limitations.     

Soil water deficits decrease the internal conductance to CO2 transfer but atmospheric water deficits do not

The internal conductance to CO₂ supply from substomatal cavitiesto sites of carboxylation poses a large limitation to photosynthesis.It is known that internal conductance is decreased by soil waterdeficits, but it is not known if it is affected by atmosphericwater deficits (i.e. leaf to air vapour pressure deficit, VPD).The aim of this paper was to examine the responses of internalconductance to atmospheric and soil water deficits in seedlingsof the evergreen perennial Eucalyptus regnans F. Muell and theherbaceous plants Solanum lycopersicum (formerly Lycopersiconesculentum) Mill. and Phaseolus vulgaris L. Internal conductancewas estimated with the variable J method from concurrent measurementsof gas exchange and fluorescence. In all three species steady-statestomatal conductance decreased by 30% as VPD increased from1 kPa to 2 kPa. In no species was internal conductance affectedby VPD despite large effects on stomatal conductance. In contrast,soil water deficits decreased stomatal conductance and internalconductance of all three species. Decreases in stomatal andinternal conductance under water deficit were proportional,but this proportionality differed among species, and thus therelationship between stomatal and internal conductance differedamong species. These findings indicate that soil water deficitsaffect internal conductance while atmospheric water deficitsdo not. The reasons for this distinction are unknown but areconsistent with soil and atmospheric water deficits having differingeffects on leaf physiology and/or root–shoot communication. Key words: Carbon dioxide, drought, internal conductance, mesophyll conductance, photosynthesis, stomatal conductance, transfer conductance, vapour pressure deficit, water deficit Received 11 October 2007; Revised 9 November 2007 Accepted 15 November 2007