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

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

Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd

Native scrub‐oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO₂] (approx. 350 μmol mol^?1) and to high [CO₂] (increased by 350 μmol mol^?1). Stomatal and photosynthetic acclimation to high [CO₂] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (g_s) was approximately 40% lower in the high‐ compared to low‐[CO₂]‐grown plants when measured at their respective growth concentrations. Reciprocal measurements of g_s in both high‐ and low‐[CO₂]‐grown plants showed that there was negative acclimation in the high‐[CO₂]‐grown plants (9–16% reduction in g_s when measured at 700 μmol mol^?1), but these were small compared to those for net CO₂ assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO₂] (c_i) which showed a reduction in stomatal sensitivity at low c_i in the high‐[CO₂]‐grown plants. Stomatal density showed no change in response to growth in high growth [CO₂]. Long‐term stomatal and photosynthetic acclimation to growth in high [CO₂] did not markedly change the 2·5‐ to 3‐fold increase in gas‐exchange‐derived water use efficiency caused by high [CO₂].     

Stomatal 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.     

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

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

Dynamic changes of canopy-scale mesophyll conductance to CO₂ diffusion of sunflower as affected by CO₂ concentration and abscisic acid

Leaf‐level measurements have shown that mesophyll conductance (g_m) can vary rapidly in response to CO₂ and other environmental factors, but similar studies at the canopy‐scale are missing. Here, we report the effect of short‐term variation of CO₂ concentration on canopy‐scale g_m and other CO₂ exchange parameters of sunflower (Helianthus annuus L.) stands in the presence and absence of abscisic acid (ABA) in their nutrient solution. g_m was estimated from gas exchange and on‐line carbon isotope discrimination (Δ_obs) in a ¹³CO₂/¹²CO₂ gas exchange mesocosm. The isotopic contribution of (photo)respiration to stand‐scale Δ_obs was determined with the experimental approach of Tcherkez et al. Without ABA, short‐term exposures to different CO₂ concentrations (C_a 100 to 900 µmol mol^?1) had little effect on canopy‐scale g_m. But, addition of ABA strongly altered the CO₂‐response: g_m was high (approx. 0.5 mol CO₂ m^?2 s^?1) at C_a < 200 µmol mol^?1 and decreased to <0.1 mol CO₂ m^?2 s^?1 at C_a >400 µmol mol^?1. In the absence of ABA, the contribution of (photo)respiration to stand‐scale Δ_obs was high at low C_a (7.2‰) and decreased to <2‰ at C_a > 400 µmol mol^?1. Treatment with ABA halved this effect at all C_a.     

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.     

Influence of stomatal distribution on transpiration in low-wind environments

Abstract According to computer energy balance simulations of horizontal thin leaves, the quantitative effects of stomatal distribution patterns (top vs. bottom surfaces) on transpiration (E) were maximal for sunlit leaves with high stomatal conductances (g_s) and experiencing low windspeeds (free or mixed convection regimes). E of these leaves decreased at windspeeds > 50 cm s^?1, despite increases in the leaf-to-air vapour density deficit. At 50 cm s^?1 wind-speed, rapidly transpiring leaves had greater E when one-half of the stomata were on each leaf surface (amphistomaty; 10.16 mmol H₂O m^?2 s^?1) than when all stomata were on either the top (hyperstomaty; 9.34 mmol m^?2s^?1) or bottom (hypostomaty; 7.02 mmol m^?2s^?1) surface because water loss occurred in parallel from both surfaces. Hyperstomatous leaves had larger E than hypostomatous leaves because free convection was greater on the top than on the bottom surface. Transpiration of leaves with large g, was greatest at windspeeds near zero when ～60–75% of the stomata were on the top surface, while at high windspeeds E was greatest with, 50% of the stomata on top. For leaves with low g_s, stomatal distribution exerted little influence on simulated E values. Laboratory measurements of water loss from simulated hypo-, hyper-, and amphistomatous leaf models qualitatively supported these predictions.     

Glucose biosensor based on polyaniline synthesized in ionic liquid

The use of good electro-active polyaniline at high pH for immobilizing glucose oxidase is reported. The response current increased with increasing potential from 0.35 to 0.70 V. The maximum response current occurred at about pH 7.0. The relationship between response current and the glucose concentration was linear from 0.005 to 10.0 mmol dm^?3. The Michaelis–Menten constant K_m', maximum response current i_max and the apparent activation energy (E_a) were 31.59 mmol dm^?3, 21.28 µA and 32.58 kJ mol^?1, respectively. The response currents of the biosensor increased with increasing temperature. The biosensor was characterized by FTIR, UV-Vis spectra and AC impedance.     

Gas Exchange of Spring Barley and Wheat Grown under Mild Water Shortage

During mild water stress (decrease of full water capacity from 60 to 35 %) net photosynthetic rate (P _N) of four spring barley and wheat genotypes was about twice lower than that for unstressed plants and was mainly limited by non-stomatal factors. Availability of CO₂ from intercellular spaces did not change significantly when stomatal conductance (g _s) decreased from 0.25-0.35 to 0.15-0.20 mol(H₂O) m⁻² s⁻¹. There may be two main processes leading to similar intercellular CO₂ concentration (c _i) in stressed and unstressed seedlings despite of twice lower P _N under mild water stress: (a) lower diffusion of CO₂ through stomata represented by lower g _s, (b) lower consumption of CO₂ by photosynthetic apparatus of stressed plants. Last factor is partially pronounced by lower response of P _N to c _i observed for stressed than for control plants. This revised version was published online in August 2006 with corrections to the Cover Date.     

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

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

Kinetics and thermodynamics of catalysis and thermal inactivation of a novel α-amylase (Tp-AmyS) from Thermotoga petrophila

Abstract

This research is focussed on kinetic, thermodynamic and thermal inactivation of a novel thermostable recombinant α-amylase (Tp-AmyS) from Thermotoga petrophila. The amylase gene was cloned in pHIS-parallel1 expression vector and overexpressed in Escherichia coli. The steady-state kinetic parameters (V_max, K_m, k_cat and k_cat/K_m) for the hydrolysis of amylose (1.39?mg/min, 0.57?mg, 148.6?s^?1, 260.7), amylopectin (2.3?mg/min, 1.09?mg, 247.1?s^?1, 226.7), soluble starch (2.67?mg/min, 2.98?mg, 284.2?s^?1, 95.4) and raw starch (2.1?mg/min, 3.6?mg, 224.7?s^?1, 61.9) were determined. The activation energy (E_a), free energy (ΔG), enthalpy (ΔH) and entropy of activation (ΔS) at 98?°C were 42.9?kJ mol^?1, 74?kJ mol^?1, 39.9?kJ mol^?1 and ?92.3 J mol^?1 K^?1, respectively, for soluble starch hydrolysis. While ΔG of substrate binding (ΔG_E-S) and ΔG of transition state binding (ΔG_E-T) were 3.38 and ?14.1?kJ mol^?1, respectively. Whereas, EaD, Gibbs free energy (ΔG*), increase in the enthalpy (ΔH*) and activation entropy (ΔS*) for activation of the unfolding of transition state were 108, 107, 105?kJ mol^?1 and ?4.1 J mol^?1 K^?1. The thermodynamics of irreversible thermal inactivation of Tp-AmyS revealed that at high temperature the process involves the aggregation of the protein.     

Effect of Root Cooling on Photosynthesis of Artemisia tridentata Seedlings under Different Light Levels

Root chilling has been shown to inhibit shoot photosynthesis yet the mechanism for such an action is not clearly understood. A study was designed to elucidate the mechanism by which root cooling may affect net photosynthesis. Roots of Artemisia tridentata seedlings were cooled from 20°C to 5°C while their shoot temperature remained at 20°C. This was conducted at two light levels (700 and 1300 μmol m^?2 s^?1). The time course of shoot net photosynthesis (A), stomatal conductance to water vapor (g_s), intercellular CO₂ concentration (C_i) and root respiration (R_s) were determined on a whole-plant basis. Root cooling caused a 25% reduction in A at high PPFD, which was preceded by more than 50% reduction of g_s and about 10% reduction in C_i. A versus C_i curves for single branches showed no difference between cold and warm soil temperatures, although stomatal conductance was lower for the lower soil temperature. This suggests that a stomatal limitation may have been involved in the inhibition of A. Furthermore, a concomitant decrease of as much as 23% in leaf relative water content (RWC) indicated that root cooling affected stomatal closure due to decreased water supply to the foliage. At lower PPFD, root cooling did not cause a decrease in A of the whole plant despite a moderate drop in g_s, C_i and RWC. Cold soil also led to a substantial and rapid reduction in root respiration rate (R_s) regardless of the light level.     

Intrinsic changes in photosynthetic parameters of carrot leaves under increasing CO<Subscript>2</Subscript> concentrations and soil moisture regimes

A controlled growth chamber experiment was conducted to investigate the short-term water use and photosynthetic responses of 30-d-old carrot seedlings to the combined effects of CO₂ concentration (50–1 050 μmol mol⁻¹) and moisture deficits (−5, −30, −55, and −70 kPa). The photosynthetic response data was fitted to a non-rectangular hyperbola model. The estimated parameters were compared for effects of moisture deficit and elevated CO₂ concentration (EC). The carboxylation efficiency (α) increased in response to mild moisture stress (−30 kPa) under EC when compared to the unstressed control. However, moderate (−55 kPa) and extreme (−70 kPa) moisture deficits reduced α under EC. Maximum net photosynthetic rate (P _Nmax) did not differ between mild water deficit and unstressed controls under EC. Moderate and extreme moisture deficits reduced P _Nmax by nearly 85 % compared to controls. Dark respiration rate (R _D) showed no consistent response to moisture deficit. The CO₂ compensation concentration (Γ) was 324 μmol mol⁻¹ for −75 kPa and ranged 63–93 μmol mol⁻¹ for other moisture regimes. Interaction between moisture deficit and EC was noticed for P _N, ratio of intercellular and ambient CO₂ concentration (C _i/C _a), stomatal conductance (g _s ), and transpiration rate (E). P _N was maximum and C _i/C _a was minimum at −30 kPa moisture deficit and at C _a of 350 μmol mol⁻¹. The g _s and E showed an inverse relationship at all moisture deficit regimes and EC. Water use efficiency (WUE) increased with moisture deficit up to −55 kPa and declined thereafter. EC showed a positive influence towards sustaining P _N and increasing WUE only under mild moisture stress, and no beneficial effects of EC were noticed at moderate or extreme moisture deficits.     

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     

Short-term dynamics of stomatal response to sudden increase in CO<Subscript>2</Subscript> concentration in maize supplied with different amounts of water

Environmental factors that influence stomatal conductance (g _s) interact through a complex network of signal transduction and have therefore highly interdependent effect. In the present study we examined how plant water status affects stomatal sensitivity to the change of CO₂ concentration ([CO₂]). We investigated the short-term dynamic of stomatal response to a sudden [CO₂] increase (from 400 to 700 μmol(CO₂) mol⁻¹) in maize supplied with different amounts of water (resulting ψ_w = −0.35, −0.52 and −0.75 MPa). Gas exchange measurements were performed in short logging intervals and the response was monitored under two different levels of water vapour pressure deficit (VPD) of 1 and 2 kPa in order to observe the impact of air humidity. Generalized logistic curves were fitted to standardized stomatal response data, which enabled us to objectively estimate the level (relative decrease of g _s) and the dynamics of the response.     

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.     

Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

In order to separate the net effect of growth at elevated [CO₂] on stomatal conductance (g_s) into direct and acclimatory responses, mid‐day values of g_s were measured for plants grown in field plots in open‐topped chambers at the current ambient [CO₂], which averaged 350 μmol mol^?1 in the daytime, and at ambient + 350 μmol mol^?1[CO₂] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing g_s measured at 700 μmol mol^?1[CO₂] for plants grown at the two [CO₂]. The direct effect of increasing [CO₂] from 350 to 700 μmol mol^?1 was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with g_s. For all species, growth at the higher [CO₂] significantly reduced g_s measured at 700 μmol mol^?1[CO₂]. The reduction in g_s caused by growth at the higher [CO₂] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO₂] on conductance. For barley, there was no other evidence for stomatal acclimation, despite consistent down‐regulation of photosynthetic rate in plants grown at the higher [CO₂]. In wheat and potato, in addition to the vapour pressure difference interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down‐regulation of photosynthetic rate through the season. In sorghum, g_s consistently exhibited acclimation, but there was no down‐regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in g_s when averaged over measurement dates. The net effect of growth at elevated [CO₂] on mid‐day g_s resulted from unique combinations of direct and acclimatory responses in the various species.     

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.     

Influence of salinity on Na+ and K+ accumulation, and gas exchange in Avicennia germinans

We analysed plant growth, ion accumulation, leaf water relations, and gas exchange of Avicennia germinans (L.) L. subjected to a long-term, controlled salinity gradient from 0 to 55 ‰. Growth and leaf area were affected by salinity higher than 10 ‰. As salinity increased, the predawn leaf water potential (Ψ_w) and leaf osmotic potential (Ψ_s) decreased. Leaf Ψ_w was at least −0.32 MPa lower than the Ψ_w of solution. Na⁺ and K⁺ ions explained about 78 % of decrease in Ψ_s. K⁺ tissue water concentration decreased by more than 60 % in all salinity treatments as compared with those grown at 0 ‰. Inversely, Na⁺ concentration in tissue water increased with nutrient solution salinity. The maximum net photosynthetic rate (P _N) and stomatal conductance (g _s) decreased by 68 and 82 %, respectively, as salinity increased from 0 to 55 ‰; the intercellular CO₂ concentration (C _i) followed the same trend. The P _N as a function of C _i showed that both the initial linear slope and upper plateau of the P _N vs. C _i curve were markedly affected by high salinity (40 and 55 ‰).