Water Deficit and Associated Changes in Some Photosynthetic Parameters in Leaves of ;Valencia' Orange (Citrus sinensis [L.] Osbeck)

Photosynthetic CO₂ assimilation, transpiration, ribulose-1,5-bisphosphate carboxylase (RuBPCase), and soluble protein were reduced in leaves of water-deficit (stress) `Valencia' orange (Citrus sinensis [L.] Osbeck). Maximum photosynthetic CO₂ assimilation and transpiration, which occurred before midday for both control and stressed plants, was 58 and 40%, respectively, for the stress (−2.0 megapascals leaf water potential) as compared to the control (−0.6 megapascals leaf water potential). As water deficit became more severe in the afternoon, with water potential of −3.1 megapascals for the stressed leaves vs. −1.1 megapascals for control leaves, stressed-leaf transpiration declined and photosynthetic CO₂ assimilation rapidly dropped to zero. Water deficit decreased both activation and total activity of RuBPCase. Activation of the enzyme was about 62% (of fully activated enzyme in vitro) for the stress, compared to 80% for the control. Water deficit reduced RuBPCase initial activity by 40% and HCO₃⁻/Mg²⁺-saturated activity by 22%. However, RuBPCase for both stressed and control leaves were similar in K_cat (25 moles CO₂ per mole enzyme per second) and K_m for CO₂ (18.9 micromolar). Concentrations of RuBPCase and soluble protein of stressed leaves averaged 80 and 85%, respectively, of control leaves. Thus, reductions in activation and concentration of RuBPCase in Valencia orange leaves contributed to reductions in enzyme activities during water-deficit periods. Declines in leaf photosynthesis, soluble protein, and RuBPCase activation and concentration due to water deficit were, however, recoverable at 5 days after rewatering.     

Mild Water Stress of Phaseolus vulgaris Plants Leads to Reduced Starch Synthesis and Extractable Sucrose Phosphate Synthase Activity

Mild water stress, on the order of −1.0 megapascals xylem water potential, can reduce the rate of photosynthesis and eliminate the inhibition of photosynthesis caused by O₂ in water-stress-sensitive plants such as Phaseolus vulgaris. To investigate the lack of O₂ inhibition of photosynthesis, we measured stromal and cytosolic fructose-1,6-bisphosphatase, sucrose phosphate synthase, and partitioning of newly fixed carbon between starch and sucrose before, during, and after mild water stress. The extractable activity of the fructose bisphosphatases was unaffected by mild water stress. The extractable activity of SPS was inhibited by more than 60% in plants stressed to water potentials of −0.9 megapascals. Water stress caused a decline in the starch/sucrose partitioning ratio indicating that starch synthesis was inhibited more than sucrose synthesis. We conclude that the reduced rate of photosynthesis during water stress is caused by stomatal closure, and that the restriction of CO₂ supply caused by stomatal closure leads to a reduction in the capacity for both starch and sucrose synthesis. This causes the reduced O₂ inhibition and abrupt CO₂ saturation of photosynthesis.     

Minor Physiological Response to Elevated CO(2) by the CAM Plant Agave vilmoriniana

One-year-old plants of the CAM leaf succulent Agave vilmoriniana Berger were grown outdoors at Riverside, California. Potted plants were acclimated to CO₂-enrichment (about 750 microliters per liter) by growth for 2 weeks in an open-top polyethylene chamber. Control plants were grown nearby where the ambient CO₂ concentration was about 370 microliters per liter. When the plants were well watered, CO₂-induced differences in stomatal conductances and CO₂ assimilation rates over the entire 24-hour period were not large. There was a large nocturnal acidification in both CO₂ treatments and insignificant differences in leaf chlorophyll content. Well watered plants maintained water potentials of −0.3 to −0.4 megapascals. When other plants were allowed to dry to water potentials of −1.2 to −1.7 megapascals, stomatal conductances and CO₂ uptake rates were reduced in magnitude, with the biggest difference in Phase IV photosynthesis. The minor nocturnal response to CO₂ by this species is interpreted to indicate saturated, or nearly saturated, phosphoenolpyruvate carboxylase activity at current atmospheric CO₂ concentrations. CO₂-enhanced diurnal activity of ribulose bisphosphate carboxylase activity remains a possibility.     

Tolerance and physiological responses of Phragmites australis to water deficit

The water stress tolerance of Phragmites australis (Cav.) Trin ex. Steud. grown in the laboratory were investigated by examining effects of different levels of imposed water deficits on growth, photosynthesis and various physiological traits related to water stress. Individual plants were grown under conditions of unrestricted water supply and compared with groups of plants receiving 60, 30, 15 or 5% of previous daily water requirements, respectively.Water deficit was found to reduce the leaf area and the leaf biomass per plant due to decreased production of new leaves, increased leaf shedding and reduced average leaf size. Leaf production and leaf expansion growth were very sensitive to water availability and were reduced when plants were subjected to fairly mild water deficit. Osmolality in sap expressed from leaves and the concentration of proline in leaves were only significantly increased in severely stressed plants, indicating that osmotic adjustment was of minor importance until a critical stress level was reached. Photosynthetic parameters were rather unaffected until the water availability was very low and led to the assertion that reduced CO₂ assimilation was mainly due to stomatal closure and not biochemical changes. Water stress had no effect on the activity of Rubisco. The CO₂ assimilation rate and stomatal conductance decreased in such a way that the intrinsic water use efficiency (A/g_s) increased, indicating efficient CO₂ utilization in water stressed plants. The apparent quantum yield (φ_i) was reduced in leaves of the most stressed plants, probably due to a decrease in the CO₂ molar fraction in the chloroplasts following stomatal closure.The initial response of P. australis to water deficit is a reduction in leaf area, the remaining leaves staying physiological rather well functioning until they are severely stressed. A high intrinsic water use efficiency and the ability to maintain some capacity for photosynthesis under severe water stress can undoubtedly contribute to the survival of P. australis under dry conditions. Taken together with its well-developed adaptations to flooding, P. australis seems very well adapted to grow in wetland areas with a widely fluctuating hydroperiod. P. australis grows very well in rather deep water, but can also tolerate extensive periods of drought with reduced availability of water.     

The Response of Leaf Water Potential and Crassulacean Acid Metabolism to Prolonged Drought in Sedum rubrotinctum

Plants of Sedum rubrotinctum R. T. Clausen were studied in a green-house over a 2-year period without watering. Only the apical leaves survived and were turgid at the end of the experiment. The midday leaf water potential of these apical leaves was −1.20 megapascals, while the leaf water potential of comparable leaves on well-watered control plants was −0.20 megapascals. The unwatered plants appear to have maintained turgor by means of an osmotic adjustment. After 2 years without water the plants no longer exhibited a nocturnal accumulation of titratable acidity. However, the daytime levels of titratable acidity of the unwatered plants were more than 2-fold greater than the levels in well-watered control plants. Well-watered plants of S. rubrotinctum exhibited seasonal shifts in biomass stble carbon isotope ratios, indicating a greater proportion of day versus night CO₂ uptake in the winter than in the summer. The imposition of water stress prevented the expression of this seasonal rhythm and restricted the plants to dark CO₂ uptake.     

Light Energy Dissipation under Water Stress Conditions: Contribution of Reassimilation and Evidence for Additional Processes

Using ¹⁴CO₂ gas exchange and metabolite analyses, stomatal as well as total internal CO₂ uptake and evolution were estimated. Pulse modulated fluorescence was measured during induction and steady state of photosynthesis. Leaf water potential of Digitalis lanata EHRH. plants decreased to −2.5 megapascals after withholding irrigation. By osmotic adjustment, leaves remained turgid and fully exposed to irradiance even at severe water stress. Due to the stress-induced reduction of stomatal conductance, the stomatal CO₂ exchange was drastically reduced, whereas the total CO₂ uptake and evolution were less affected. Stomatal closure induced an increase in the reassimilation of internally evolved CO₂. This `CO<sub>2</sub> recycling' consumes a significant amount of light energy in the form of ATP and reducing equivalents. As a consequence, the metabolic demand for light energy is only reduced by about 40%, whereas net photosynthesis is diminished by about 70% under severe stress conditions. By CO<sub>2</sub> recycling, carbon flux, enzymatic substrate turnover and consumption of light energy were maintained at high levels, which enabled the plant to recover rapidly after rewatering. In stressed D. lanata plants a variable fluorescence quenching mechanism, termed `coefficient of actinic light quenching,' was observed. Besides water conservation, light energy dissipation is essential and involves regulated metabolic variations.     

Comparisons of Photosynthetic Responses of Xanthium strumarium and Helianthus annuus to Chronic and Acute Water Stress in Sun and Shade

We have examined the effects of mild, chronic water stress and acute water stress on two water stress sensitive plants, Xanthium strumarium and Helianthus annuus. Using a combination of the leaf disc O₂ electrode to measure the light responses of photosynthesis and 77 K fluorescence to monitor damage to the primary photochemistry, we have found the following: (a) The CO₂ saturated rate of photosynthesis at high light is the most water stress sensitive parameter measured. (b) The apparent quantum yield (moles O₂ per mole photons) was slightly, if at all, affected by mild water stress (>−1.5 megapascals). (c) Severe water stress (<−1.5 megapascals) reduced the quantum yield of photosynthesis regardless of whether the stress was applied in sun or shade. The light independent reduction of quantum yield was not associated with a reduction in 77 K fluorescence (F_v/F_m) indicating that the quantum yield reduction was not the result of damage to primary photochemistry. (d) The diel fluctuation in 77 K fluorescence seen in sun-exposed control leaves was greatly exaggerated in water stressed leaves because of enhanced decline in 77 K fluorescence in the morning. The rate of recovery was similar in both control and water stressed leaves. Shaded leaves showed no change in 77 K fluorescence regardless of whether water stress was imposed or not. (e) The water stress sensitive plants used in these experiments did not recover from acute water stress severe enough to reduce the quantum yield or chronic water stress which lasted long enough that light dependent damage to primary photochemistry occurred.     

Mechanism of Photosynthesis Decrease by Verticillium dahliae in Potato

Young, visually symptomless leaves from potato (Solanum tuberosum) plants infected with Verticillium dahliae exhibited reduced carbon assimilation rate, stomatal conductance, and intercellular CO₂, but no increase in dark respiration, no change in the relationship between carbon assimilation rate versus intercellular CO₂, and no change in light use efficiency when intercellular CO₂ was held constant. Therefore, the initial decrease in photosynthesis caused by V. dahliae was caused by stomatal closure. Errors in the intercellular CO₂ calculation caused by uneven distribution of carbon assimilation rate across the leaf were tested by ¹⁴CO₂ autoradiography. Patchiness was found at a low frequency. Low stomatal conductance was correlated with low leaf water potentials. Infection did not affect leaf osmotic potentials.     

Mild water stress effects on carbon-reduction-cycle intermediates, ribulose bisphosphate carboxylase activity, and spatial homogeneity of photosynthesis in intact leaves

We have examined the effect of mild water stress on photosynthetic chloroplast reactions of intact Phaseolus vulgaris leaves by measuring two parameters of ribulose bisphosphate (RuBP) carboxylase activity and the pool sizes of RuBP, 3-phosphoglycerate (PGA), triose phosphates, hexose monophosphates, and ATP. We also tested for patchy stomatal closure by feeding ¹⁴CO₂. The k_cat of RuBP carboxylase (moles CO₂ fixed per mole enzyme per second) which could be measured after incubating the enzyme with CO₂ and Mg²⁺ was unchanged by water stress. The ratio of activity before and after incubation with CO₂ and Mg²⁺ (the carbamylation state) was slightly reduced by severe stress but not by mild stress. Likewise, the concentration of RuBP was slightly reduced by severe stress but not by mild stress. The concentration of PGA was markedly reduced by both mild and severe water stress. The concentration of triose phosphates did not decline as much as PGA. We found that photosynthesis in water stressed leaves occurred in patches. The patchiness of photosynthesis during water stress may lead to an underestimation of the effect of stomatal closure. We conclude that reductions in whole leaf photosynthesis caused by mild water stress are primarily the result of stomatal closure and that there is no indication of damage to chloroplast reactions.     

Leaf water potential, stomatal resistance, and photosynthetic response to water stress in peach seedlings

Individual groups of peach (Prunus persica [L.] Batsch) seedlings stressed to −17, −26 and −36 bars recovered to control levels within 1, 3, and 4 days, respectively. Stomatal resistance was significantly correlated with both leaf water potential and net photosynthesis. In seedlings stressed to −52 bars, leaf water potential and stomatal resistance recovered sooner than net photosynthesis, despite recovery of 0₂ evolution at a rate similar to leaf water potential. Therefore, some nonstomatal factor other than reduction in photochemical activity must be responsible for the lag in recovery of CO₂ assimilation following irrigation.     

Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis

In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO₂ assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO₂ loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO₂ assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36°C and 50 millibars bar⁻¹, respectively). Studies on the relationship between CO₂ assimilation rate and intercellular CO₂ partial pressure at various temperatures and humidities showed that this decrease in CO₂ assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO₂ diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher ¹³C content of Welwitschia compared to the majority of C₃ species.     

Acclimation of CO(2) Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO₂ assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO₂ assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO₂ partial pressure. The slopes of net CO₂ assimilation versus intercellular CO₂ partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of −0.3 and −0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at −0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately −0.5 megapascal. The accumulation of Cl⁻ and Na⁺ in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO₂ partial pressure, the declined inhibition in CO₂ assimilation of stress-acclimated plants was found for both salinity and water stress.     

Interaction of water supply and N in wheat

The purpose of this study was to investigate effects of N nutrition and water stress on stomatal behavior and CO₂ exchange rate in wheat (Triticum aestivum L. cv Olaf). Wheat plants were grown hydroponically with high (100 milligrams per liter) and low (10 milligrams per liter) N. When plants were 38 days old, a 24-day water stress cycle was begun. A gradual increase in nutrient solution osmotic pressure from 0.03 to 1.95 mega Pascals was achieved by incremental additions of PEG-6,000. Plants in both N treatments adjusted osmotically, although leaf water potential was consistently lower and relative water content greater for low N plants in the first half of the stress cycle. Leaf conductance of high N plants appeared greater than that of low N plants at high water potentials, but showed greater sensitivity to reductions in water potential as indicated by earlier stomatal closure during the stress cycle. The apparent greater stomatal sensitivity of high N plants was associated with a curvilinear relationship between leaf conductance and leaf water potential; low N plants exhibited more of a threshold response. Trends in [CO₂]_INT throughout the stress cycle indicated nonstomatal effects of water stress on CO₂ exchange rate were greater in high N plants. Although estimates of [CO₂]_INT were generally lower in high N plants, they were relatively insensitive to leaf water potential-induced changes in leaf conductance. In contrast, [CO₂]_INT of low N plants dropped concomitantly with leaf conductance at low leaf water potentials. Oxygen response of CO₂ exchange rate for both treatments was affected less by reductions in water potential than was CO₂ exchange rate at 2.5% O₂, suggesting that CO₂ assimilation capacity of the leaves was affected more by reductions in leaf water potential than were processes related to photorespiration.     

Internal CO(2) Measured Directly in Leaves : Abscisic Acid and Low Leaf Water Potential Cause Opposing Effects

Observations of nonuniform photosynthesis across leaves cast doubt on internal CO₂ partial pressures (p_i) calculated on the assumption of uniformity and can lead to incorrect conclusions about the stomatal control of photosynthesis. The problem can be avoided by measuring p_i directly because the assumptions of uniformity are not necessary. We therefore developed a method that allowed p_i to be measured continuously in situ for days at a time under growth conditions and used it to investigate intact leaves of sunflower (Helianthus annuus L.), soybean (Glycine max L. Merr.), and bush bean (Phaseolus vulgaris L.) subjected to high or low leaf water potentials (ψ_w) or high concentrations of abscisic acid (ABA). The leaves maintained a relatively constant differential (Δp) between ambient CO₂ and measured p_i throughout the light period when water was supplied. When water was withheld, ψ_w decreased and the stomata began to close, but measured p_i increased until the leaf reached a ψ_w of −1.76 (bush bean), −2.12 (sunflower) or −3.10 (soybean) megapascals, at which point Δp = 0. The increasing p_i indicated that stomata did not inhibit CO₂ uptake and a Δp of zero indicated that CO₂ uptake became zero despite the high availability of CO₂ inside the leaf. In contrast, when sunflower leaves at high ψ_w were treated with ABA, p_i did not increase and instead decreased rapidly and steadily for up to 8 hours even as ψ_w increased, as expected if ABA treatment primarily affected stomatal conductance. The accumulating CO₂ at low ψ_w and contrasting response to ABA indicates that photosynthetic biochemistry limited photosynthesis at low ψ_w but not at high ABA.     

Plant morphological and biochemical responses to field water deficits: I. Responses of glutathione reductase activity and paraquat sensitivity

The effects of water deficits on plant morphology and biochemistry were analyzed in two photoperiodic strains of field-grown cotton (Gossypium hirsutum L.). Plants grown under dryland conditions exhibited a 40 to 85% decrease in leaf number, leaf area index, leaf size, plant height, and total weight per plant. Gross photosynthesis decreased from 0.81 to 0.47 milligram CO₂ fixed per meter per second and the average midday water, osmotic, and turgor potentials decreased to −2.1, −2.4, and 0.3 megapascals, respectively.

There was a progressive increase in glutathione reductase activity and in the cellular antioxidant system in the leaves of stressed plants compared to the irrigated controls. The stress-induced increases in enzyme activity occurred at all canopy positions analyzed.

Irrigation of the dryland plots following severe water stress resulted in a 50% increase in leaf area per gram fresh weight in newly expanded leaves of both strains over the leaves which had expanded under the dryland conditions. Paraquat resistance (a relative measure of the cellular antioxidant system) decreased in the strain T25 following irrigation. Glutathione reductase activities remained elevated in the T25 and T185 leaves which were expanded fully prior to irrigation and in the leaves which expanded following the irrigation treatment.

     

Induction of Acid Metabolism in Portulacaria afra

Portulacaria afra, a succulent plant, shifts from a predominantly C₃ mode of gas exchange to a typical Crassulacean acid metabolism type CO₂ uptake in response to water or NaCl stress. Control plants in the absence of water stress assimilated CO₂ during the light (about 7-8 mg CO₂ dm⁻² hr⁻¹), transpiration (about 1.5 g dm⁻² hr⁻¹) was predominantly during the day, stomates were open during the day, and there was little diurnal organic acid fluctuation. Stressed plants showed only dark CO₂ uptake and dark water loss, nocturnal stomatal opening, and an increased diurnal fluctuation of titratable acidity. Within 2 weeks after rewatering, stressed plants returned to the control acid fluctuation levels indicating that the response to stress was reversible.     

Asymmetric patchy stomatal closure for the two surfaces of Xanthium strumarium L. leaves at low humidity

Images of chlorophyll fluorescence were used to demonstrate patchy stomatal closure at low humidities in leaves of well-watered Xanthium strumarium plants. The pattern and extent of patchy stomatal closure were shown to be different for the two surfaces of amphistomatous leaves by taking images of leaves with CO₂ available to only one leaf was exposed to low humidity, patchiness was more extensive on that surface. Gas-exchange experiments were also conducted to determine the apparent photosynthetic capacity of the mesophyll (photosynthesis rate at constant c_i when it was supplied with CO₂ through both surfaces or through each surface alone. These experiments showed declines in the apparent photosynthetic capacity of the mesophyll at low humidities that were consistent with patchy stomatal closure on one or both surfaces. The results suggest that patchy stomatal closure can be a factor in the steady-state reponses of stomata to humidity. In amphistomatous leaves this is further complicated by the fact that patches on one epidermis may not coincide with those of the other surface.     

Stomatal and Nonstomatal Components to Inhibition of Photosynthesis in Leaves of Capsicum annuum during Progressive Exposure to NaCl Salinity

Young bell pepper (Capsicum annuum L.) plants grown in nutrient solution were gradually acclimated to 50, 100, or 150 moles per cubic meter NaCl, and photosynthetic rates of individual attached leaves were measured on several occasions during the salinization period at external CO₂ concentrations ranging from approximately 70 to 1900 micromoles per mole air. Net CO₂ assimilation (A) was plotted against computed leaf internal CO₂ concentration (C_i), and the initial slope of this A-C_i curve was used as a measure of photosynthetic ability. During the 10 to 14 days after salinization began, leaves from plants exposed to 50 moles per cubic meter NaCl showed little change in photosynthetic ability, whereas those treated to 100 or 150 moles per cubic meter NaCl had up to 85% inhibition, with increase in CO₂ compensation point. Leaves appeared healthy, and leaf chlorophyll content showed only a 14% reduction at the highest salinity levels. Partial stomatal closure occurred with salinization, but reductions in photosynthesis were primarily nonstomatal in origin. Photosynthetic ability was inversely related to the concentration of either Na⁺ or Cl⁻ in the leaf laminas sampled at the end of the experimental period. However, the concentration of Cl⁻ expressed on a tissue water basis was greater, exceeding 300 moles per cubic meter, and Cl⁻ was more closely associated (R² = 0.926) with the inhibition of photosynthetic ability. Leaf turgor was not reduced by salinization and leaf osmotic potential decreased to a slightly greater extent than the osmotic potential decreases of the nutrient solutions. Concentration of accumulated Na⁺ and Cl⁻ (on a tissue water basis) accounted quantitatively for maintenance of leaf osmotic balance, assuming that these ions were sequestered in the vacuoles.     

Effects of Salinity on Stomatal Conductance, Photosynthetic Capacity, and Carbon Isotope Discrimination of Salt-Tolerant (Gossypium hirsutum L.) and Salt-Sensitive (Phaseolus vulgaris L.) C(3) Non-Halophytes

The effects of salinity on growth, stomatal conductance, photosynthetic capacity, and carbon isotope discrimination (Δ) of Gossypium hirsutum L. and Phaseolus vulgaris L. were evaluated. Plants were grown at different NaCl concentrations from 10 days old until mature reproductive structures were formed. Plant growth and leaf area development were strongly reduced by salinity, in both cotton and bean. Stomatal conductance also was reduced by salinity. The Δ always declined with increasing external salinity concentration, indicating that stomatal limitation of photosynthesis was increased. In cotton plant dry matter, Δ correlated with the ratio of intercellular to atmospheric CO₂ partial pressures (p_l/p_a) calculated by gas exchange. This correlation was not clear in bean plants, although Δ showed a more pronounced salt induced decline in bean than in cotton. Possible effects of heterogeneity of stomatal aperture and consequent overestimation of p_l as determined from gas exchange could explain these results. Significant differences of Δ between leaf and seed material were observed in cotton and bean. This suggests different patterns of carbon allocation between leaves and seeds. The photon yield of O₂ evolution determined at rate-limiting photosynthetic photon flux density was insensitive to salinity in both species analyzed. The light- and CO₂-saturated rate of CO₂ uptake and O₂ evolution showed a salt induced decline in both species. Possible explanations of this observation are discussed. O₂ hypersensitivity was observed in salt stressed cotton plants. These results clearly demonstrate that the effect of salinity on assimilation rate was mostly due to the reduction of stomatal conductance, and that calculation of p_l may be overestimated in salt stressed plants, because of heterogeneity of stomatal aperture over the leaf surface.     

Low CO(2) Prevents Nitrate Reduction in Leaves

The correlation between CO₂ assimilation and nitrate reduction in detached spinach (Spinacia oleracea L.) leaves was examined by measuring light-dependent changes in leaf nitrate levels in response to mild water stress and to artificially imposed CO₂ deficiency. The level of extractable nitrate reductase (NR) activity was also measured. The results are: (a) In the light, detached turgid spinach leaves reduced nitrate stored in the vacuoles of mesophyll cells at rates between 3 and 10 micromoles per milligram of chlorophyll per hour. Nitrate fed through the petiole was reduced at similar rates as storage nitrate. Nitrate reduction was accompanied by malate accumulation. (b) Under mild water stress which caused stomatal closure, nitrate reduction was prevented. The inhibition of nitrate reduction observed in water stressed leaves was reversed by external CO₂ concentrations (10-15%) high enough to overcome stomatal resistance. (c) Nitrate reduction was also inhibited when turgid leaves were kept in CO₂-free air or at the CO₂-compensation point or in nitrogen. (d) When leaves were illuminated in CO₂-free air, activity of NR decreased rapidly. It increased again, when CO₂ was added back to the system. The half-time for a 50% change in activity was about 30 min. It thus appears that there is a rapid inactivation/activation mechanism of NR in leaves which couples nitrate reductase to net photosynthesis.