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.     

Chloroplast Response to Low Leaf Water Potentials: IV. Quantum Yield Is Reduced

Quantum yields were measured for CO₂ fixation by sunflower (Helianthus annuus L.) leaves having various water potentials and for dichlorophenolindophenol photoreduction by chloroplasts isolated from similar leaves having various water potentials. In red radiation, the quantum yield for CO₂ was 0.076 for an attached sunflower leaf at a water potential of −3 to −4 bars but was 0.020 for the same leaf at −15.3 bars. After recovery to a water potential of −5 bars, the quantum yield rose to 0.060. Soybean (Glycine max L. [Merr.]) leaves behaved similarly. Chloroplasts from a sunflower leaf with a water potential of −3.6 bars had a quantum yield for 4 equivalents of 0.079, but when tissue from the same leaf had a water potential of −14.8 bars, the quantum yield of the chloroplasts decreased to 0.028. The decrease could not be attributed to differences in rates of respiration by the leaves or the chlorophyll content or absorption spectrum of the leaves and chloroplasts.     

Differing sensitivity of photosynthesis to low leaf water potentials in corn and soybean

Rates of net photosynthesis were studied in soil-grown corn (Zea mays) and soybean (Glycine max) plants having various leaf water potentials. Soybean was unaffected by desiccation until leaf water potentials were below −11 bars. Rates of photosynthesis in corn were inhibited whenever leaf water potentials dropped below −3.5 bars.     

Chloroplast Response to Low Leaf Water Potentials: III. Differing Inhibition of Electron Transport and Photophosphorylation

Cyclic and noncyclic photophosphorylation and electron transport by photosystem 1, photosystem 2, and from water to methyl viologen (“whole chain”) were studied in chloroplasts isolated from sunflower (Helianthus annus L. var Russian Mammoth) leaves that had been desiccated to varying degrees. Electron transport showed considerable inhibition at leaf water potentials of −9 bars when the chloroplasts were exposed to an uncoupler in vitro, and it continued to decline in activity as leaf water potentials decreased. Electron transport by photosystem 2 and coupled electron transport by photosystem 1 and the whole chain were unaffected at leaf water potentials of −10 to −11 bars but became progressively inhibited between leaf water potentials of −11 and −17 bars. A low, stable activity remained at leaf water potentials below −17 bars. In contrast, both types of photophosphorylation were unaffected by leaf water potentials of −10 to −11 bars, but then ultimately became zero at leaf water potentials of −17 bars. Although the chloroplasts isolated from the desiccated leaves were coupled at leaf water potentials of −11 to −12 bars, they became progressively uncoupled as leaf water potentials decreased to −17 bars. Abscisic acid and ribonuclease had no effect on chloroplast photophosphorylation. The results are generally consistent with the idea that chloroplast activity begins to decrease at the same leaf water potentials that cause stomatal closure in sunflower leaves and that chloroplast electron transport begins to limit photosynthesis at leaf water potentials below about −11 bars. However, it suggests that, during severe desiccation, the limitation may shift from electron transport to photophosphorylation.     

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.     

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.     

Response of carbon dioxide fixation to water stress: parallel measurements on isolated chloroplasts and intact spinach leaves

Application of water stress to isolated spinach (Spinacia oleracea) chloroplasts by redutcion of the osmotic potentials of CO₂ fixation media below −6 to −8 bars resulted in decreased rates of fixation regardless of solute composition. A decrease in CO₂ fixation rate of isolated chloroplasts was also found when leaves were dehydrated in air prior to chloroplast isolation. An inverse response of CO₂ fixation to osmotic potential of the fixation medium was found with chloroplasts isolated from dehydrated leaves—namely, fixation rate was inhibited at −8 bars, compared with −16 or −24 bars.     

Effect of carbon dioxide, osmotic potential of nutrient solution, and light intensity on transpiration and resistance to flow of water in pepper plants

The rate of transpiration, temperature of the leaves, and relative water content of leaves of pepper plants were measured in a small chamber in which the temperature, relative humidity, and carbon dioxide concentration of recirculated air were controlled and measured. The data reported were obtained by noting the response of pepper plants to all combinations of the following treatments: high light, 1.5 × 10⁶ ergs per square centimeter per second; low light, 3.0 × 10⁴ ergs per square centimeter per second; three levels of CO₂: 50, 268, and 730 parts per million; nutrient solution osmotic potentials of −0.5, −5.0, −7.5, and −9.5 bars.     

Effects of sulfur on the photosynthesis of intact leaves and isolated chloroplasts of sugar beets

Effects of sulfur on photosynthesis in sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by inducing sulfur deficiency and determining changes in the photosynthesis of whole attached leaves and of isolated chloroplasts. The rates of photosynthetic CO₂ uptake by intact leaves, photoreduction of ferricyanide, cyclic and noncyclic photophosphorylation of isolated chloroplasts, and the rate of CO₂ assimilation by ribulose diphosphate carboxylase, decreased with decrease in total leaf sulfur from 2500 to about 500 μg g⁻¹ dry weight. Sulfur deficiency reduced photosynthesis through an effect on chlorophyll content, which decreased linearly with leaf sulfur, and by decreasing the rate of photosynthesis per unit chlorophyll. There was only a small effect of sulfur deficiency on stomatal diffusion resistance to CO₂ until leaf sulfur decreased below 1000 μg g⁻¹ when stomatal resistance became a more significant proportion of the total diffusion resistance to CO₂. Light respiration rates were positively correlated with photosynthesis rates and dark respiration was unchanged as leaf sulfur concentrations declined.     

Leaf enlargement and metabolic rates in corn, soybean, and sunflower at various leaf water potentials

Rates of photosynthesis, dark respiration, and leaf enlargement were studied in soil-grown corn (Zea mays), soybean (Glycine max), and sunflower (Helianthus annuus) plants at various leaf water potentials. As leaf water potentials decreased, leaf enlargement was inhibited earlier and more severely than photosynthesis or respiration. Except for low rates of enlargement, inhibition of leaf enlargement was similar in all three species, and was large when leaf water potentials dropped to about −4 bars.     

Water Relations and Photosynthesis of a Desert CAM Plant, Agave deserti

The water relations and photosynthesis of Agave deserti Engelm., a plant exhibiting Crassulacean acid metabolism, were measured in the Colorado desert. Although no natural stomatal opening of A. deserti occurred in the summer of 1975, it could be induced by watering. The resistance for water vapor diffusion from a leaf (R_WV) became less than 20 sec cm⁻¹ when the soil water potential at 10 cm became greater than −3 bars, as would occur after a 7-mm rainfall. As a consequence of its shallow root system (mean depth of 8 cm), A. deserti responded rapidly to the infrequent rains, and the succulent nature of its leaves allowed stomatal opening to continue for up to 8 days after the soil became drier than the plant. When the leaf temperature at night was increased from 5 to 20 C, R_WV increased 5-fold, emphasizing the importance of cool nighttime temperatures for gas exchange by this plant. Although most CO₂ uptake occurred at night, a secondary light-dependent rise in CO₂ influx generally occurred after dawn. The transpiration ratio (mass of water transpired/mass of CO₂ fixed) had extremely low values of 18 for a winter day, and approximately 25 for an entire year.     

Acclimation of photosynthesis to low leaf water potentials

Photosynthesis is reduced at low leaf water potentials (Ψ_l) but repeated water deficits can decrease this reduction, resulting in photosynthetic acclimation. The contribution of the stomata and the chloroplasts to this acclimation is unknown. We evaluated stomatal and chloroplast contributions when soil-grown sunflower (Helianthus annuus L.) plants were subjected to water deficit pretreatments for 2 weeks. The relationship between photosynthesis and Ψ_l, determined from gas-exchange and isopiestic thermocouple psychometry, was shifted 3 to 4 bars towards lower Ψ_l, in pretreated plants. Leaf diffusive resistance was similarly affected. Chloroplast activity, demonstrated in situ with measurements of quantum yield and the capacity to fix CO₂ at all partial pressures of CO₂, and in vitro by photosystem II activity of isolated organelles, was inhibited at low Ψ_l but less in pretreated plants than in control plants. The magnitude of this inhibition indicated that decreases in chloroplast activity contributed more than closure of stomata both to losses in photosynthesis and to the acclimation of photosynthesis to low Ψ_l.     

Drought stress effects on three cultivars of Eragrostis curvula: photosynthesis and water relations

Water stress effects were studied on three cultivars ofEragrostis curvula. Leaf water potential, RWC, total plantleaf area, green dry weight mass percentage and CO₂ gas-exchangeweremeasured during the onset of stress and after recovery. After 3 days of waterstress, RWC of cv Tanganyika plants was around 30–40% of controls,while RWC of cvs Ermelo and Consol was around 50–60% of controls.However midday and predawn water potentials were lower in cvs Tanganyka andErmelo than in cv Consol. After re-watering, RWC and water potentials recoveredonly in Consol plants. A strong decrease of leaf area was recorded in cvsErmeloand Consol during water stress (about 91–94% less than the leafarea of controls). Photosynthesis decreased as a function of the degree ofwaterstress severity in all cultivars. Also, light saturated photosynthesis,CO₂ quantum yield and light at which saturated photosynthesisoccurred, were strongly reduced by water stress. Recovery of photosynthesis wasfound in cv Consol after five days re-watering. Cv Consol showed a betterconservation of water and higher resistance to water stress than the other twocvs.     

Effects of light intensity and oxygen on photosynthesis and translocation in sugar beet

The mass transfer rate of ¹⁴C-sucrose translocation from sugar beet (Beta vulgaris, L.) leaves was measured over a range of net photosynthesis rates from 0 to 60 milligrams of CO₂ decimeters⁻² hour⁻¹ under varying conditions of light intensity, CO₂ concentration, and O₂ concentration. The resulting rate of translocation of labeled photosynthate into total sink tissue was a linear function (slope = 0.18) of the net photosynthesis rate of the source leaf regardless of light intensity (2000, 3700, or 7200 foot-candles), O₂ concentration (21% or 1% O₂), or CO₂ concentration (900 microliters/liter of CO₂ to compensation concentration). These data support the theory that the mass transfer rate of translocation under conditions of sufficient sink demand is limited by the net photosynthesis rate or more specifically by sucrose synthesis and this limitation is independent of light intensity per se. The rate of translocation was not saturated even at net photosynthesis rates four times greater than the rate occurring at 300 microliters/liter of CO₂, 21% O₂, and saturating light intensity.     

Recovery of photosynthesis in sunflower after a period of low leaf water potential

Photosynthesis was studied in sunflower plants subjected to 1 to 2 days of desiccation and then permitted to recover. The leaf water potential to which leaves returned after rewatering was dependent on the severity of desiccation and the evaporative conditions. Under moderately evaporative conditions, leaf water potential returned to predesiccation levels after 3 to 5 hours when desiccation was slight. Leaf water potentials remained below predesiccation levels for several days after rewatering when leaf water potentials decreased to −13 to −19 bars during desiccation. Leaf water potential showed no sign of recovery when leaf water potentials decreased to −20 bars or below during desiccation. The lack of full recovery of leaf water potential was attributable to increased resistance to water transport in the roots and stem. The resistance ultimately became large enough to result in death of the leaves because net water loss continued even after the soil had been rewatered.     

Influence of certain environmental factors on photosynthesis and photorespiration in Simmondsia chinensis

The response of net photosynthesis and apparent light respiration to changes in [O₂], light intensity, and drought stress was determined by analysis of net photosynthetic CO₂ response curves. Low [O₂] treatment resulted in a large reduction in the rate of photorespiratory CO₂ evolution. Lightintensity levels influenced the maximum net photosynthetic rate at saturating [CO₂]. These results indicate that [CO₂], [O₂] and light intensity affect the levels of substrates involved in the enzymatic reactions of photosynthesis and photorespiration. Intracellular resistance to CO₂ uptake decreased in low [O₂] and increased at low leaf water potentials. This response reflects changes in the efficiency with which photosynthetic and photorespiratory substrates are formed and utilized. Water stress had no effect on the CO₂ compensation point or the [CO₂] at which net photosynthesis began to saturate at high light intensity. The relationship between these data and recently published in-vitro kinetic measurements with ribulose-diphosphate carboxylase is discussed.Abbreviations C _w intracellular CO₂ concentration - F _gross gross photosynthesis - F _net net photosynthesis - I light intensity - R _L light respiration rate - r _c carboxylation resistance - r ₈ leaf gas-phase resistance - r _i intracellular resistance; to CO₂ uptake - r _t resistance to CO₂ flux between the intercellular spaces and the carboxylation sites - T _L leaf temperature - _t leaf water potential - CO₂ compensation point     

Effects of magnesium deficiency on the photosynthesis and respiration of leaves of sugar beet

The effects of Mg deficiency on the photosynthesis and respiration of sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by withholding Mg from the culture solution and by following changes in CO₂ and water vapor exchange of attached leaves. Leaf blade Mg concentration decreased from about 1200 to less than 200 meq kg⁻¹ dry matter without change in the rate of photosynthetic CO₂ uptake per unit leaf area, while from 200 to 50 meq kg⁻¹ the rate decreased to one-third. Rates of photorespiratory evolution of CO₂ into CO₂-free air responded to Mg like those of photosynthetic CO₂ uptake, the rates decreasing to one-half, below 200 meq kg⁻¹. Respiratory CO₂ evolution in the dark increased almost 2-fold in low Mg leaves. Magnesium deficiency had less effect on leaf (mainly stomatal) diffusion resistance (r₁) than on mesophyll resistance (r_m); in Mg-deficient plants r_m increased from 2.9 to 7.1 sec cm⁻¹, whereas r₁ became significantly greater than the control value only in the most severe instances of Mg deficiency.     

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.     

Modulation of water stress effects on photosynthesis by altered leaf k

Wheat irrigated with nutrient solutions containing 0, 0.2, 0.5, 1, 2, or 6 millimolar K⁺ had maximum photosynthetic rates at 1 to 2 millimolar K⁺ concentrations. Rates in the 6 millimolar K⁺-grown plants were not higher than the 2 millimolar K⁺-grown wheat, and rates were inhibited below 0.5 millimolar K⁺. Photosynthesis was measured by both attached whole leaf CO₂ uptake and by ¹⁴CO₂ fixation of leaf slices in solution. Exposure of leaf slices from 0.2, 2, and 6 millimolar K⁺-grown wheat to various assay media water potentials showed that photosynthesis of the 0.2 millimolar K⁺-grown wheat decreased from control (high water potential) rates by 35%, that of the 2 millimolar K⁺-grown wheat by 20.4%, and that of the 6 millimolar K⁺-grown wheat by only 8.3% at −3.11 megapascals. Also, photosynthesis of the 6 millimolar K⁺-grown wheat was enhanced by 28% over that of the 2 millimolar K⁺ wheat at the most severe water stress (−3.11 megapascals), indicating that the excess leaf K⁺ in the 6 millimolar K⁺-grown wheat partially reversed dehydration effects on photosynthesis. Oligomycin eliminated the protective effects of high K⁺ on photosynthesis in dehydrated leaf slices. These results suggest that the protective effect of high K⁺ under water stress may involve the exchange of K⁺ in the cytoplasm for stroma H⁺, thus altering stromal pH and restoring photosynthesis. The protective effect of high K⁺ was also observed in attached whole leaf photosynthesis of in situ water-stressed wheat grown on 0.2, 2, and 6 millimolar K⁺. Under water stress, rates of the 6 millimolar K⁺-grown wheat were enhanced by 66.2% and 113.9% over that of 2 millimolar K⁺-grown wheat in two separate experiments. Internal CO₂ concentration of the 6 millimolar K⁺-grown wheat was lower than that of the 0.2 and 2 millimolar K⁺-grown wheat. These results suggest that the high K⁺ effects on chloroplast photosynthesis seen in leaf slices also occur at the whole plant level.     

SEASONAL PATTERNS OF CO2 AND WATER VAPOR EXCHANGE OF JUNCUS ROEMERIANUS SCHEELE IN A GEORGIA SALT MARSH

CO₂ and water vapor exchange studies of intact plants of black needle rush (Juncus roemerianus Scheele) were conducted in an undisturbed marsh community on Sapelo Island, Georgia. The seasonal patterns of the light and temperature responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration were determined five times over the year. Internal resistances to CO₂ uptake were also evaluated. Net photosynthesis was highest in early spring, but declined only slightly through the year. A distinct and moderate temperature optimum of net photosynthesis was observed with decreasing rates above 30 C. Leaf conductances to water vapor were similar at all seasons and were high at cooler temperatures and decreased with increasing temperature. Transpiration was relatively high and constant during all seasons. The water-use efficiency of photosynthesis was high below 25 C, but decreased sharply above that temperature. Dark respiration was relatively low. Seasonal changes reflected changes in leaf density. Decreasing stomatal conductances and increasing respiration rates reduced net photosynthesis at higher temperatures. The stomatal resistance increased and internal resistances to CO₂ uptake decreased over the year, but the total resistance remained constant. The internal resistance to CO₂ uptake was consistently higher than the stomatal resistance. These seasonal response patterns show that J. roemerianus is well adapted to the seasonal changes in ambient temperature and irradiance and other microenvironmental factors in the high marsh. These physiological characteristics permit this C₃ species to maintain a high productivity in a seasonally hot and stressful environment.