Effects of chloride and sodium on gas exchange parameters and water relations of Citrus plants

Gas exchange parameters, water relations and Na⁺/Cl^- content were measured on leaves of one-year-old sweet orange ( Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO₃) were used to separate the effects of Cl⁻ and Na⁺ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl⁻ in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na⁺ concentrations of up to 478 m M in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca²⁺ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl⁻ and Na⁺ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.     

Changes in leaf water status associated with salinity in mature, field grown Prunus salicina

Seasonal and diurnal measurements of leaf water potential (ψ₁), relative water content (RWC) and stomatal conductance (g_s) were made in the field on 19-year old Prunus salicina (L.) cv. Santa Rosa, a deciduous fruit tree species, irrigated with 3 different concentrations of saline water over a 3 year period (1985-1987). With the exception of stage III of fruit growth, little or no treatment difference in Φ₁, leaf turgor potential (Φ_p), or RWC was noted during the day. Seasonal averages of morning (0700-0900) and afternoon (1500-1700) Φ_p did not decline with increasing salinity, indicating long-term osmotic adjustment in this species. Maintenance of leaf water status under saline conditions was in part a consequence of increased stomatal closure, with a subsequent reduction in leaf transpiration rate. However, during stage III of fruit growth, an increase in mean afternoon (1200-1700) stomatal conductance of 26-117%, independent of salinity treatment, was observed in 1985 and again in 1987. Higher conductance values during this period may be associated with rapid fruit expansion and greater assimilate demand. The observed increase in conductance resulted in greater leaf water loss and larger measured differences in midday ψ₁ between salinity treatments. This research indicates that for Prunus salicina in the field, salinity stress resulted in leaf water deficits only during the final period of fruit expansion and ripening.     

Changes in free amino acids and osmotic adjustment in leaves of water-stressed bean

Bean ( Phaseolus vulgaris L. cv. Topcrop) plants were raised in a growth chamber in small pots or flower boxes and kept at full water regime until the full development of primary leaves (14–16 days). Both potted and flower box-grown plants were subjected to a gradually increased water stress of about 60–70 kPa day⁻¹ leaf water potential (stressed plants) or full water regime (control). The water potential, osmotic potential and turgor pressure in freshly detached primary leaves and osmotic potential at zero turgor were calculated using pressure/volume curves. Changes in free amino acids and amides were also measured in parallel trials. Water relation parameters documented that in the stressed leaves there was moderate osmotic adjustment, which was more evident in the potted plants. If considered 0% ionised, the accumulation of free amino acids and amides (μmol g⁻¹ H₂O) accounts for a van't Hoff's value of about 10.2 kPa in the small pot-grown and 5.5 kPa in the box-grown plants. The values are twice as high if considered 100% ionised. Proline accumulation accounted for about 6.4% of the pool enlargement in the potted plants and 22.3% in the flower box plants.     

Water relations and osmotic adjustment in Lycopersicon esculentum and L. pennellii during short-term salt exposure and recovery

Cultivated tomato Lycopersicon esculentum (L.) Mill. cv. P-73 and its wild salt-tolerant relative L. pennellii (Correll) D'Arcy accession PE-47 growing on silica sand in a growth chamber were exposed to 0, 70, 140 and 210 m M NaCl nutrient solutions 35 days after sowing. The saline treatments were imposed for 4 days, after which the plants were rinsed with distilled water. Salinity in L. esculentum reduced leaf area and leaf and shoot dry weights. The reductions were more pronounced when sodium chloride was removed from the root medium. Reduction in leaf area and weight in L. pennellii was only observed after the recovery period. In both genotypes salinity induced a progressive reduction in leaf water potential and leaf conductance. During the recovery period leaf water potential (ψ₁) and leaf conductance (g₁) reached levels similar to those of control plants in wild and cultivated species, respectively. Leaf osmotic potential at full turgor (ψ_os) decreased in the salt treated plants of both genotypes, whereas the bulk modulus of elasticity was not affected by salinity. Leaf water potential at turgor loss point (ψ_tlp) and relative water content at turgor loss point (RWC_tlp) appeared to be controlled by leaf osmotic potential at full turgor (ψ_os) and by bulk modulus of elasticity, respectively. At lowest salinity, the wild species carried out the osmotic adjustment based almost exclusively on Cl⁻ and Na⁺, with a marked energy savings. Under highest salinity, this species accommodate the stress through a higher expenditure of energy due to the contribution of organic solutes to the osmotic adjustment. The domesticated species carried out the osmotic adjustment based always on an important contribution of organic solutes.     

Theory and practice of a portable photosynthesis instrument

Abstract. Theory and practice of non-steady-state portable photosynthesis instruments (LI-6000 and 6200, LI-COR Inc., Nebraska, U.S.A.) are presented. Mass balance equations for the time dependence of H₂O and CO₂ mol fractions within the leaf chamber were used to describe instrument function. Measurements for each run were fitted to an exponential function to estimate average rates of CO₂ assimilation and transpiration during the measurement period. Stomatal conductances and intercellular CO₂ mol fractions were also computed. Linear data analysis used in the LI-6200 produced similar results for assimilation rates, stomatal conductances and intercellular CO₂ concentrations compared to a more rigorous nonlinear analysis, provided humidity within the chamber was kept constant during the measurement period. Instrument performance for CO₂ fluxes was confirmed by injecting pure CO₂ at steady rates from a microsyringe into the chamber. Miniature evaporimeters were designed to check H₂O flux measurements. Significant discrepancies were observed between LI-6200 estimates of H₂O fluxes and direct measurement and errors were attributed to adsorption desorption of water vapour on chamber walls or to leaks. The leaf chamber should be stored at humidities and temperatures similar to those during measurement conditions for maximum reliability of results.     

Effect of excess Cu on the photosynthetic apparatus of runner bean leaves treated at two different growth stages

Variations in water binding strength, water relations and the accumulation of solutes during water stress of three durum wheat ( Triticum durum Desf.) cultivars are reported and discussed. Water binding strength was determined by constructing adsorption isotherms at 5 and 20°C and by calculating the differential enthalpy (ΔH) after van't Hoff.
Reducing sugars, proline, K⁺ and Cl⁻ were the major contributors to osmotic adjustment. Solutes, such as quaternary ammonium compounds, and non-reducing sugars contributed to osmotic potential at full turgor, but did not increase in proportion to water stress. Genotypic differences have been observed for K⁺ accumulation capability, the water-stressed leaves of cv. Capeiti 8 showing the largest increase. The same cultivar demonstrated the most negative ΔH values, indicative of strongly bound water, and the highest integrated enthalpy (ΔH_inf) values for leaf moisture below 0.1 g H₂O per g dry weight, i.e. in the isotherm region where water was presumably chemisorbed to the charged groups of macromolecules. The accumulation of ions (Cl⁻, K⁺) and proline was concurrent with an increase in the binding-strength of tightly and weakly bound water, respectively.     

Keeping a positive carbon balance under adverse conditions: responses of photosynthesis and respiration to water stress

Drought and salinity (i.e. soil water stress) are the main environmental factors limiting photosynthesis and respiration and, consequently, plant growth. This review summarizes the current status of knowledge on photosynthesis and respiration under water stress. It is shown that diffusion limitations to photosynthesis under most water stress conditions are predominant, involving decreased mesophyll conductance to CO₂, an important but often neglected process. A general failure of photochemistry and biochemistry, by contrast, can occur only when daily maximum stomatal conductance ( g _s) drops below 0.05–0.10 mol H₂O m⁻² s⁻¹. Because these changes are preceded by increased leaf antioxidant activities ( g _s below 0.15–0.20 mol H₂O m⁻² s⁻¹), it is suggested that metabolic responses to severe drought occur indirectly as a consequence of oxidative stress, rather than as a direct response to water shortage. As for respiration, it is remarkable that the electron partitioning towards the alternative respiration pathway sharply increases at the same g _s threshold, although total respiration rates are less affected. Despite the considerable improvement in the understanding of plant responses to drought, several gaps of knowledge are highlighted which should become research priorities for the near future. These include how respiration and photosynthesis interact at severe stress, what are the boundaries and mechanisms of photosynthetic acclimation to water stress and what are the factors leading to different rates of recovery after a stress period.     

Diurnal variations of photosynthesis and dew absorption by leaves in two evergreen shrubs growing in Mediterranean field conditions

The effects of summer drought, dew deposition on leaves and autumn rainfall on plant water relations and diurnal variations of photosynthesis were measured in two evergreen shrubs, rosemary ( Rosmarinus officinalis ) and lavender ( Lavandula stoechas ), grown in Mediterranean field conditions. Withholding water for 40 d caused a similar decrease in predawn shoot water potential (ψ_pd) from c. −0.4 to c. −1.3 MPa in both species, but a 50% decrease in the relative leaf water content in L. stoechas compared with 22% in R. officinalis . A similar decrease in CO₂ assimilation rates by c. 75% was observed in water-stressed plants of both species, although L. stoechas showed smaller photosynthesis: stomatal conductance ratio than R. officinalis (35 vs 45 μmol CO₂:mol H₂O). The relative quantum efficiency of photosystem II photochemistry also decreased by c. 45% at midday in water- stressed plants of both species. Nevertheless, neither L. stoechas nor R. officinalis suffered drought-induced damage to photosystem II, as indicated by the maintenance of the ratio F _v: F _m throughout the experiment, associated with an increase in the carotenoid content per unit of chlorophyll by c. 62% and c. 30%, respectively, in water-stressed plants. Only L. stoechas absorbed dew by leaves. In this species the occurrence of 6 d of dew over a 15-d period improved relative leaf water content by c. 72% and shoot water potential by c. 0.5 MPa throughout the day in water-stressed plants, although the photosynthetic capacity was not recovered until the occurrence of autumn rainfall. The ability of leaves to absorb dew allowed L. stoechas to restore plant water status, which is especially relevant in plants exposed to prolonged drought.     

Influence of leaf age and light environment on the gas exchange of lupins and wheat

Rates of net photosynthesis (A), transpiration (E) and leaf conductance to water vapour transfer (gH₂O) were measured on leaves of Lupinus angustifolius L. cv. Ritson's and L. cosentinii Guss. cv. Eregulla throughout development and on flag leaves of wheat ( Triticum aestivum L. cvs Gutha, Gamenya and Warigal) after full expansion. Plants were grown in large containers of soil, in a naturally-lit, temperature controlled glasshouse. Throughout most of their life, lupin leaves had higher photosynthetic rates and leaf conductances than found for wheat. During leaf ageing in lupins, photosynthesis and conductance changed proportionately such that leaf intercellular CO₂ concentration was maintained relatively constant at about 200 ppm. Under continuously cloudy conditions, leaf conductance at midday of lupins and wheat was higher than at similar photon flux densities at other times of day on cloudless days. On cloudy days the relationship between gH₂O and photon flux density in lupins was very different from that derived from diurnal measurements on clear days. The potentially low water use efficiency under cloud, evident as decreases in the A/gH₂O ratio, was rarely realised in practise due to a reduction in leaf-to-air water vapour concentration difference on cloudy days. The possible reasons for the high conductance on cloudy days are discussed.     

Implications of water stress-induced changes in the levels of endogenous ascorbic acid and hydrogen peroxide in Vigna seedlings

Vigna cutjang Endl. cv. Pusa Barsati seedlings, subjected to increasing degrees of water stress (−0.5, −1.0, −1,5 MPa), produced an approximately proportional increase in glycolate oxidase activity, hydrogen peroxide (H₂O₂) and proline content but a decrease in catalase activity, ascorbic acid and protein content. Leaf water potential (leaf ψ) and relative water content (RWC) were also lowered with increasing stress. Pretreatment with l -cysteine and reduced glutathione (10-3 M) decreased glycolate oxidase activity, H₂O₂ content, ascorbic acid oxidase activity, proline content and also slightly improved the water status of leaves stressed (−1.0 MPa) for 2 days. Pretreatment of non-stressed seedlings with these antioxidants had little or no effect. These studies indicate that treatment with antioxidants makes the plant tolerant against water stress by modulating the endogenous levels of H₂O₂ and ascorbic acid in stressed tissue.     

Photosynthesis affects following night leaf conductance in Vicia faba

Night-time stomatal opening in C₃ plants may result in significant water loss when no carbon gain is possible. The objective of this study was to determine if endogenous patterns of night-time stomatal opening, as reflected in leaf conductance, in Vicia faba are affected by photosynthetic conditions the previous day. Reducing photosynthesis with low light or low CO₂ resulted in reduced night-time stomatal opening the following night, irrespective of the effects on daytime stomatal conductance. Likewise, increasing photosynthesis with enriched CO₂ levels resulted in increased night-time stomatal opening the following night. Reduced night-time stomatal opening was not the result of an inability to regulate stomatal aperture as leaves with reduced night-time stomatal opening were capable of greater night-time opening when exposed to low CO₂. After acclimating plants to long or short days, it was found that night-time leaf conductance was greater in plants acclimated to short days, and associated with greater leaf starch and nitrate accumulation, both of which may affect night-time guard cell osmotic potential. Direct measurement of guard cell contents during endogenous night-time stomatal opening will help identify the mechanism of the effect of daytime photosynthesis on subsequent night-time stomatal regulation.     

Predicting leaf-level fluxes of O3 and NO2: the relative roles of diffusion and biochemical processes

Pollutants like O₃ and NO₂ enter leaves through the stomata and cause damage during reactions with components of biological cell membranes. The steady-state flux rates of these gases into the leaf are determined by a series of physical and biochemical resistances including stomatal aperture, reactions occurring within the cell wall and the ability of the leaf to remove the products of apoplastic reactions. In the present study, multiple regression models incorporating stomatal conductance, apoplastic and symplastic ascorbate concentrations, and nitrate reductase (NR) activities were generated to explain the observed variations in leaf-level flux rates of O₃ and NO₂. These measurements were made on the plant Catharanthus roseus (Madagascar periwinkle). The best-fit model explaining NO₂ flux included stomatal conductance, apoplastic ascorbate and NR activity. This model explained 89% of the variation in observed leaf fluxes and suggested physical resistances, reaction between NO₂ and apoplastic ascorbate, and the removal rate of nitrate (generated by reactions of NO₂ and water) from the apoplast all play controlling roles in NO₂ flux to leaves. O₃ flux was best explained by stomatal conductance and symplastic ascorbate explaining 66% of the total variation in leaf flux. Both models demonstrate the importance of measuring processes other than stomatal conductance to explain steady-state leaf-level fluxes of pollutant gases.     

外源Ca2+对PEG处理下转C4型PEPC基因水稻光合生理的调节

磷酸烯醇式丙酮酸羧化酶(PEPC)通过固定二氧化碳参与光合作用, 是关键的C₄植物光合作用酶。为了揭示高光效转C₄ PEPC基因水稻(Oryza sativa)对干旱胁迫的适应机理, 以高表达转C₄ PEPC水稻(PC)和野生型水稻Kitaake (WT)为供试材料, 在植株的4-5叶期, 使用不同浓度外源CaCl₂溶液处理, 测定在15%聚乙二醇6000 (polyethylene glycol-6000, PEG-6000)胁迫下叶片相对含水量、光合参数、内源钙总含量、叶片总蛋白激酶活性、PEPC酶活性以及相关基因表达和蛋白质含量。结果表明, 0.5 mmol∙L^-1 CaCl₂明显提高PC叶片相对含水量(P<0.05), 2 mmol∙L^-1和10 mmol∙L^-1 CaCl₂则作用不显著, 对WT则影响不显著。不同浓度钙处理对PEG处理PC的净光合速率影响不显著, 而通过维持气孔导度减少水分胁迫。内源总钙浓度的数据显示, 在PEG6000处理下, PC具有维持稳定内源Ca²⁺浓度的能力, 过高浓度(10 mmol∙L^-1 CaCl₂)钙处理反而降低了PEPC酶活性、PEPC基因表达和可溶性蛋白的含量。     

Pressure-volume curves and drought resistance in two wheat genotypes

The water relations of two durum wheat cultivars ( Triticum durum Desf.) were studied throughout the growing season. Irrigated and unirrigated plants were compared from booting to milk stage; a period where water stress occurred naturally in the field. Modulus of elasticity (ε), turgid weight/dry weight ratio (TW/DW), relative water content at zero turgor (RWC_o) and osmotic potential at full turgor (ε) declined throughout the season while average turgor (ψ_p) increased. Water stress induced a further decrease in ψ_π¹⁰⁰ and the TW/DW ratio. The elastic modulus varied greatly. During the first stages of growth, cv. Appulo (the more resistant cultivar) showed lower ε values than cv. Valforte. At the milk stage, ε was lower for the unirrigated than the irrigated plants. Correlation coefficients between the TW/DW ratio and the osmotic potential were significant for both cultivars. In cv. Valforte, TW/DW was also correlated with the average turgor and the bulk modulus of elasticity. Structural changes that affect the TW/DW ratio seem to be important factors influencing water relations and drought tolerance in durum wheat.     

Pressure-volume analyses on shoots of Picea abies and leaves of Coffea liberica at various temperatures

Shoots from two ecologically different evergreen tree species, Picea abies (L.) Karst and Coffea liberica Hiern, were used to carry out pressure volume (PV) measurements at 5–35°C. For this purpose a pressure chamber was equipped with thermoelectric temperature regulation. The non-linear sections of the resultant PV curves were sigmoidal for both species, with recognizable points of inflexion. These points, at around ψ= 1.12 MPa and relative water content (RWC) = 88.5% for Picea and at ψ= 0.92 MPa and RWC = 95.5% for Coffea , were characterised by a temporary increase in the resistance to water flow of the entire shoot (R₅).
The maximum value of the bulk modulus of elasticity (ɛ max) was also in the region of the point of inflexion. This value was considerably higher in Coffea than in Picea . The osmotic pressures at full water saturation (π₀) and at turgor loss point (π_p) showed a clear temperature dependence between 15 and 35°C differing only slightly from the theoretically expected situation. At 25°C these values were 1.72 and 2.48 MPa. respectively, for Picea and 1.58 and 1.87 MPa. respectively, for Coffea . The turgor loss point occurred at 76–77% RWC in Picea and at 86% RWC in Coffea , the proportion of apoplastic water varied between 22 and 25% in Picea but was only 9–10% in Coffea . The ecological differences between the two species are reflected in their temperature dependence for R₅, which was much steeper for Coffea than for Picea . The energy of activation for the water conductance of the whole shoots was 13.0–14.4 kJ mol⁻¹ in Picea and about 23 kJ mol⁻¹ in Coffea .     

Salinity effects on water relations in Lycopersicon esculentum and its wild salt-tolerant relative species L. pennellii

Cultivated tomato Lycopersicon esculentum (L.) Mill. cv. P-73 and its wild salt tolerant relative L. pennellii (Correll) D'Arcy accession PE-47, were grown during spring-summer 1989 under unheated plastic greenhouse conditions. Plants were submitted to two different salt treatments using 0 and 140 mM NaCI irrigation water. In both tomato species, salinity caused a proportionally larger reduction in leaf area than in leaf weight and, in L. esculentum , a proportionally larger decrease in stem weight than in leaf weight. Daily variations in leaf water potential (Ψ₁) were fundamentally due to changes in the evaporative demand of the atmosphere. Reductions in Ψ₁ due to salinity were consistent only in L. esculentum . In all the conditions studied, leaf turgor was maintained. Leaf conductance (g₁)was higher in L. esculentum than in L. pennellii .Salinity induced a clear reduction in g₁ levels in L. esculentum whereas, in L. pennellii , this reduction was noted only in May. In both species the Ψ^o_s (leaf osmotic potential at full turgor) levels were reduced by salinity. The bulk modulus of elasticity (E) and relative water content at turgor loss point (RWC_tlp) were not affected by salinity. The RWC_tlp values in L. pennellii seem to be controlled by E values.     

Drought acclimation confers oxidative stress tolerance by inducing co-ordinated antioxidant defense at cellular and subcellular level in leaves of wheat seedlings

Wheat ( Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods (drought acclimation). The antioxidant defense in terms of redox metabolites and enzymes in leaf cells, chloroplasts, and mitochondria was examined in relation to ROS-induced membrane damage. Drought-acclimated seedlings modulated growth by maintaining favorable turgor potential and RWC and were able to limit H₂O₂ accumulation and membrane damage as compared with non-acclimated plants during severe water stress conditions. This was due to systematic upregulation of H₂O₂-metabolizing enzymes especially ascorbate peroxidase (APX, EC 1.11.1.11) and by maintaining ascorbate–glutathione redox pool in acclimated plants. By contrast, failure in the induction of APX and ascorbate–glutathione cycle enzymes makes the chloroplast susceptible to oxidative stress in non-acclimated plants. Non-acclimated plants protected the leaf mitochondria from oxidative stress by upregulating superoxide dismutase (SOD, EC 1.15.1.1), APX, and glutathione reductase (GR, EC 1.6.4.2) activities. Rewatering led to rapid enhancement in all the antioxidant defense components in non-acclimated plants, which suggested that the excess levels of H₂O₂ during severe water stress conditions might have inhibited or downregulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced oxidative stress tolerance by well-co-ordinated induction of antioxidant defense both at the chloroplast and at the mitochondrial level.     

Alfalfa leaf senescence induced by drought stress: photosynthesis, hydrogen peroxide metabolism, lipid peroxidation and ethylene evolution

Exchange rates of CO₂ and H₂O and metabolism of hydrogen peroxide have been measured in leaves of alfalfa ev. Aragón) under drought stress. The inhibitory effect of drought upon photosynthesis depended on the severity of the stress treatment. Leaf water potential (Ψ_leaf) down to,-2.8 MPa reduced CO₂ availability due to stomatal closure and inhibited the rate of photosynthesis. Leaf water potential lower than,-2.8 MPa directly affected CO₂ fixation, although CO₂ was not limiting. Transpiration was more affected by stornatal closure than photosynthesis, which led to am apparent improvement in WUE (water use efficiency). Alfalfa leaves with Ψ_leaf lower than,-2.0 MPa had an increased quantum requirement, probably due to the severe stress effect on photoenergetic reactions.
Ethylene evolution from alfalfa leaves increased when they were subjected to Ψ_leaf of,- 1.6 MPa. Under more severe stress, the leaves showed low or almost no ethylene production. In parallel with the increase in ethyiene production, alfalfa leaves exhibited an increased membrane lipid peroxidation index (maloridialdehyde content) and an increased peroxide content. Superoxide disinutase activity (SOD; EC 1.15.1.1) was not affected by drought stress. Catalase (EC 1.11.1.6) was inhibited at slight stress, but significantly increased at a Ψ_leaf of -2.0 MPa. Peroxidase (EC 1.11.1.7) was progressively inhibited as drought stress developed. The possible implication of reactive O₂ intermediates in drought stress-induced senescence of alfalfa leaves is discussed in the light of the pattern of enzymatic scavenging systems.     

Direct and indirect effects of light on stomata II. In Commelina communis L.

Abstract. Two experiments are described which test the normal correlations that arise between stomatal conductance, net CO₂ assimilation rate, and intercellular CO₂ concentration (C_i), using whole shoots of Commelina communis L. In the first, conductance increased with decreasing C_i, at four different quantum flux densities, such that there was no unique relationship between conductance and quantum flux density or C_i, In the second, conductance increased hyperbolically with increasing quantum flux density while C_i was held constant at 466, 302, and 46 μmiolmol⁻¹, and the response differed at each C_i. In neither experiment was conductance consistently related to net CO₂ assimilation rate in the mesophyll. In both experiments high C_i suppressed the response of conductance to light, while there was a large response of conductance to light at low C_i, indicating an interaction between the effects of light and CO₂ on stomata. The results show that the parallel responses of assimilation and conductance to light result in constant intercellular CO₂ concentrations, and not that stomata maintain a 'constant C_i'.     

Physiological effects of long-term exposure to low and moderate concentrations of atmospheric NH3 on poplar leaves

Abstract. Poplar shoots ( Populus euramericana L.) obtained from cuttings were exposed for 6 or 8 weeks to NH₃ concentrations of 50 and 100 μgm⁻³ or filtered air in fumigation chambers. After this exposure the rates of NH₃ uptake, transpiration, CO₂ assimilation and respiration of leaves were measured using a leaf chamber. During the long-term exposure also modulated chlorophyll fluorescence measurements were carried out to obtain information about the photosynthetic performance of individual leaves. Both fluorescence and leaf chamber measurements showed a higher photosynthetic activity of leaves exposed to 100 μg NH₃ m⁻³. These leaves showed also a larger leaf conductance and a larger uptake rate of NH₃ than leaves exposed to 50 μg m⁻³ NH₃ or filtered air. The long-term NH₃ exposure did not induce an internal resistance against NH₃ transport in the leaf, nor did it affect the leaf cuticle. So, not only at a short time exposure, but also at a long-term exposure NH₃ uptake into leaves can be calculated from data on the boundary layer and stomatal resistance for H₂O and ambient NH₃-concentration. Furthermore, the NH₃ exposure had no effect on the relation between CO₂-assimilation and stomatal conductance, indicating that NH₃ in concentrations up to 100 μg m⁻³ has no direct effect on stomatal behaviour; for example, by affecting the guard or contiguous cells of the stomata.