Leaf magnesium alters photosynthetic response to low water potentials in sunflower

We grew sunflower (Helianthus annuus L.) plants in nutrient solutions having nutritionally adequate but low or high Mg²⁺ concentrations and determined whether photosynthesis was effected as leaf water potentials (ψ_w) decreased. Leaf Mg contents were 3- to 4-fold higher in the plants grown in high Mg²⁺ concentrations (10 millimolar) than in those grown in low concentrations (0.25 millimolar). These contents were sufficient to support maximum growth, plant dry weight, and photosynthesis, and the plants appeared normal. As low ψ_w developed, photosynthesis was inhibited but moreso in high Mg leaves than in low Mg leaves. The effect was particularly apparent under conditions of light- and CO₂-saturation, indicating that the chloroplast capacity to fix CO₂ was altered. The differential inhibition observed in leaves of differing Mg contents was not observed in leaves having differing K contents, suggesting that the effect may have been specific for Mg. Because Mg²⁺ inhibits photophosphorylation and coupling factor activities at concentrations likely to occur as leaves dehydrate, Mg may play a role in the inhibition of chloroplast reactions at low ψ_w, especially in leaves such as sunflower that markedly decrease in water content as ψ_w decreases.     

Very high CO2 partially restores photosynthesis in sunflower at low water potentials

We re-examined the question of whether the stomata limit photosynthesis in dehydrated sunflower (Helianthus annuus L.) plants having low leaf water potentials. A gas-exchange apparatus was modified to operate at external CO₂ partial pressures as high as 3000 Pa (3%), which were much higher than previously achieved. This allowed photosynthesis and stomatal behavior to be monitored simultaneously at very high CO₂ in the same leaf. The data were compared with those from leaves treated with abscisic acid (ABA) where effects on photosynthesis are entirely stomatal. Photosynthesis was inhibited at low water potential and was only slightly enhanced by increasing the external CO₂ partial pressure from 34 Pa (normal air) to 300 Pa. Photosynthesis in ABA-treated leaves was similarly inhibited but recovered fully at 300 Pa. In both cases, the stomata closed to the same extent as judged from the average conductance of the leaves. Because the ABA effect resulted from diffusion limitation for CO₂ caused by stomatal closure, the contrasting data show that most of the dehydration effect was nonstomatal at low water potentials. When CO₂ partial pressures were raised further to 3000 Pa, photosynthesis increased somewhat at low water potentials but not in ABA-treated leaves. This indicates that some nonstomatal component of photosynthesis responded differently in leaves at low water potential and leaves treated with ABA. Because this component was only partially restored by very high CO₂, it was likely to be metabolic and was an important source of photosynthetic inhibition.Abbreviations and Symbol ABA abscisic acid - Chl chlorophyll - p_a external partial pressure of CO₂ - P_i intercellular partial pressure of CO₂ - _w water potential This work was supported by grant DE-FG02-87ER13776 from the Department of Energy and a grant from E.I. DuPont de Nemours and Company.     

Osmotic adjustment, symplast volume, and nonstomatally mediated water stress inhibition of photosynthesis in wheat

At low water potential (ψ_w), dehydration reduces the symplast volume of leaf tissue. The effect of this reduction on photosynthetic capacity was investigated. The influence of osmotic adjustment on this relationship was also examined. To examine these relationships, comparative studies were undertaken on two wheat cultivars, one that osmotically adjusts in response to water deficits (`Condor'), and one that lacks this capacity (`Capelle Desprez'). During a 9-day stress cycle, when water was withheld from plants grown in a growth chamber, the relative water content of leaves declined by 30% in both cultivars. Leaf osmotic potential (ψ_s) declined to a greater degree in Condor plants. Measuring ψ_s at full turgor indicated that osmotic adjustment occurred in stressed Condor, but not in Capelle plants. Two methods were used to examine the degree of symplast (i.e. protoplast) volume reduction in tissue rapidly equilibrated to increasingly low ψ_w. Both techniques gave similar results. With well-watered plants, symplast volume reduction from the maximum (found at high ψ_w for each cultivar) was the same for Condor and Capelle. After a stress cycle, volume was maintained to a greater degree at low ψ_w in Condor leaf tissue than in Capelle. Nonstomatally controlled photosynthesis was inhibited to the same degree at low ψ_w in leaf tissue prepared from well-watered Condor and Capelle plants. However, photosynthetic capacity was maintained to a greater degree at low ψ_w in tissue prepared from stressed Condor plants than in tissue from stressed Capelle plants. Net CO₂ uptake in attached leaves was monitored using an infrared gas analyzer. These studies indicated that in water stressed plants, photosynthesis was 106.5% higher in Condor than Capelle at ambient [CO₂] and 21.8% higher at elevated external [CO₂]. The results presented in this report were interpreted as consistent with the hypothesis that there is a causal association between protoplast (and presumably chloroplast) volume reduction at low ψ_w and low ψ_w inhibition of photosynthesis. Also, the data indicate that osmotic adjustment allows for maintenance of relatively greater volume at low ψ_w, thus reducing low ψ_w inhibition of chloroplast photosynthetic potential.     

Effect of inhibition of abscisic Acid accumulation on the spatial distribution of elongation in the primary root and mesocotyl of maize at low water potentials

Previous work showed that accumulation of endogenous abscisic acid (ABA) acts both to maintain primary root growth and inhibit shoot growth in maize seedlings at low water potentials (ψ_w) (IN Saab, RE Sharp, J Pritchard, GS Voetberg [1990] Plant Physiol 93: 1329-1336). In this study, we have characterized the growth responses of the primary root and mesocotyl of maize (Zea mays L. cv FR27 × FRMo 17) to manipulation of ABA levels at low ψ_w with a high degree of spatial resolution to provide the basis for studies of the mechanism(s) of ABA action. In seedlings growing at low ψ_w and treated with fluridone to inhibit carotenoid (and ABA) biosynthesis, ABA levels were decreased in all locations of the root and mesocotyl growing zones compared with untreated seedlings growing at the same ψ_w. In the root, low ψ_w (−1.6 megapascals) caused a shortening of the growing zone, as reported previously. The fluridone treatment was associated with severe inhibition of root elongation rate, which resulted from further shortening of the growing zone. In the mesocotyl, low ψ_w (−0.3 megapascal) also resulted in a shortened growing zone. In contrast with the primary root, however, fluridone treatment prevented most of the inhibition of elongation and the shortening of the growing zone. Final cell length measurements indicated that the responses of both root and mesocotyl elongation to ABA manipulation at low ψ_w involve large effects on cell expansion. Measurements of the relative changes in root and shoot water contents and dry weights after transplanting to a ψ_w of −0.3 megapascal showed that the maintenance of shoot elongation in fluridone-treated seedlings was not attributable to increased water or seed-reserve availability resulting from inhibition of root growth. The results suggest a developmental gradient in tissue responsiveness to endogenous ABA in both the root and mesocotyl growing zones. In the root, the capacity for ABA to protect cell expansion at low ψ_w appears to decrease with increasing distance from the apex. In the mesocotyl, in contrast, the accumulation of ABA at low ψ_w appears to become increasingly inhibitory to expansion as cells are displaced away from the meristematic region.     

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.     

Nonstomatal inhibition of photosynthesis in sunflower at low leaf water potentials and high light intensities

The inhibition of photosynthesis at low leaf water potentials was studied in soil-grown sunflower to determine the degree to which photosynthesis under high light was affected by stomatal and nonstomatal factors. Below leaf water potentials of −11 to −12 bars, rates of photosynthesis at high light intensities were insensitive to external concentrations of CO₂ between 200 and 400 microliters per liter. Photosynthesis also was largely insensitive to leaf temperature between 10 and 30 C. Changes in CO₂ concentration and temperature had negligible effect on leaf diffusive resistance. The lack of CO₂ and temperature response for both photosynthesis and leaf diffuse resistance indicates that rates of photosynthesis were not limited by either CO₂ diffusion or a photosynthetic enzyme. It was concluded that photosynthesis under high light was probably limited by reduced photochemical activity of the leaves at water potentials below −11 to −12 bars.     

Effects of Water-Deficit Stress on Photosynthesis, Its Components and Component Limitations, and on Water Use Efficiency in Wheat (Triticum aestivum L.)

It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO₂ and H₂O was determined in two growth-chamber-grown wheat cultivars (Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO₂-saturated A, and stomatal conductance (g_s) components. The limitations on A measured at ambient CO₂ concentration (A₃₅₀) were estimated. No cultivar difference was observed when A₃₅₀ was plotted versus leaf water potential (Ψ_w). Light- and CO₂-saturated A, c.e., and g_s decreased with decreasing leaf Ψ_w, but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and g_s increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A₃₅₀. WUE measured as A₃₅₀/g_s first increased with stomatal closure down to a g_s of about 0.25 mol H₂O m⁻² s⁻¹ (Ψ_w = −1.6 MPa). However, it was predicted that A₃₅₀/g_s would decrease with more severe stress due to inhibition of c.e.     

Photosynthesis at low water potentials in sunflower: lack of photoinhibitory effects

The losses in chloroplast capacity to fix CO₂ when photosynthesis is reduced at low leaf water potential (ψ₁) have been proposed to result from photoinhibition. We investigated this possibility in soil-grown sunflower (Helianthus annuus L. cv IS894) using gas exchange techniques to measure directly the influence of light during dehydration on the in situ chloroplast capacity to fix CO₂. The quantum yield for CO₂ fixation as well as the rate of light- and CO₂-saturated photosynthesis were strongly inhibited at low ψ₁. The extent of inhibition was the same whether the leaves were exposed to high or to low light during dehydration. When intercellular partial pressures of CO₂ were decreased to the compensation point, which was lower than the partial pressures resulting from stomatal closure, the inhibition of the quantum yield was also unaffected. Photoinhibition could be observed only after high light exposures were imposed under nonphysiological low CO₂ and O₂ where both photosynthesis and photorespiration were suppressed. The experiments are the first to test whether gas exchange at low ψ₁ is affected by potentially photoinhibitory conditions and show that the loss in chloroplast capacity to fix CO₂ was entirely the result of a direct effect of water availability on chloroplast function and not photoinhibition.     

Carbon Dioxide and Light Responses of Photosynthesis in Cowpea and Pigeonpea during Water Deficit and Recovery

Greenhouse-grown pigeonpea (Cajanus cajan, [L.] Millsp.; cultivar UW-10) and cowpea (Vigna unguiculata, [L.] Walp.; cultivar California No. 5) were well-watered (control) or subjected to low water potential by withholding water to compare their modes of adaptation to water-limited conditions. Leaf CO₂ exchange rate (CER), leaf diffusive conductance to CO₂ (g_l), and CO₂ concentration in the leaf intercellular air space (C_i) were determined at various CO₂ concentrations and photon flux densities (PFD) of photosynthetically active radiation (400 to 700 nanometer). In cowpea, g_l declined to less than 15% of controls and total water potential (ψ_w) at midafternoon declined to −0.8 megapascal after 5 days of withholding water, whereas g_l in pigeonpea was about 40% of controls even though midafternoon ψ_w was −1.9 megapascal. After 8 days of withholding water, ψ_w at midafternoon declined to −0.9 and −2.4 megapascals in cowpea and pigeonpea, respectively. The solute component of water potential (ψ_s) decreased substantially less in cowpea than pigeonpea. Photosynthetic CER at saturation photon flux density (PFD) and ambient external CO₂ concentration (360 microliters per liter) on day 5 of withholding decreased by 83 and 55% in cowpea and pigeonpea, respectively. When measured at external, CO₂ concentration in bulk air of 360 microliters per liter, the CER of cowpea had fully recovered to control levels 3 days after rewatering; however, at 970 microliters per liter the PFD-saturated CERs of both species were substantially lower than in controls, indicating residual impairment. In stressed plants of both species the CER responses to C_i from 250 to 600 microliters per liter indicated that a substantial nonstomatal inhibition of CER had occurred. Although the sensitivity of g_l to water limitation in cowpea suggested a dehydration avoidance response, parallel measurements of CER at various C_i and PFD indicated that photosynthetic activity of cowpea mesophyll was substantially inhibited by the water-limited treatment.     

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.     

Effect of water deficits on seed development in soybean : I. Tissue water status

Water deficits during seed filling often decrease seed size in soybean (Glycine max L.). The physiological basis for this response is not known but may result from direct effects of low seed water potential (Ψ_w) on the seed filling process. To determine whether low Ψ_w occurred in reproductive tissues of soybean, we monitored the water status (Ψ_w, Ψ_s, and Ψ_p) of leaf, pericarp, and seed (embryo and testa) tissue of greenhouse-grown plants subjected to a brief water deficit during the linear period of seed growth. Water deficits were imposed by withholding water and monitored in the reproductive tissues by thermocouple psychrometry. When water was abundant, leaf, pericarp, and seed Ψ_w were −0.5 to −0.7 megapascal at midday. When water was withheld, leaf Ψ_w decreased to −2.3 megapascals within 6 days. Pericarp Ψ_w also decreased to −1.9 megapascal during this time. Pericarp Ψ_s followed the decline in Ψ_w, but osmotic adjustment was not evident as the pericarp lost turgor completely by day 6. However, seed Ψ_w, Ψ_s, and Ψ_p were not significantly different from the controls. These results indicate that the water status of the developing seeds of soybean is not altered by short-term water deficits severe enough to inhibit the metabolic activity of the maternal plant. Maintenance of a favorable water status may be important for the conservation of seed growth rate exhibited by soybean under dry conditions.     

Mapping Intercellular CO2 Mole Fraction (Ci) in Rosa rubiginosa Leaves Fed with Abscisic Acid by Using Chlorophyll Fluorescence Imaging : Significance of Ci Estimated from Leaf Gas Exchange

Imaging of photochemical yield of photosystem II (PSII) computed from leaf chlorophyll fluorescence images and gas-exchange measurements were performed on Rosa rubiginosa leaflets during abscisic acid (ABA) addition. In air ABA induced a decrease of both the net CO₂ assimilation (A_n) and the stomatal water vapor conductance (g_s). After ABA treatment, imaging in transient nonphotorespiratory conditions (0.1% O₂) revealed a heterogeneous decrease of PSII photochemical yield. This decline was fully reversed by a transient high CO₂ concentration (7400 μmol mol⁻¹) in the leaf atmosphere. It was concluded that ABA primarily affected A_n by decreasing the CO₂ supply at ribulose-1,5-bisphosphate carboxylase/oxygenase. Therefore, the A_n versus intercellular mole fraction (C_i) relationship was assumed not to be affected by ABA, and images of C_i and g_s were constructed from images of PSII photochemical yield under nonphotorespiratory conditions. The distribution of g_s remained unimodal following ABA treatment. A comparison of calculations of C_i from images and gas exchange in ABA-treated leaves showed that the overestimation of C_i estimated from gas exchange was only partly due to heterogeneity. This overestimation was also attributed to the cuticular transpiration, which largely affects the calculation of the leaf conductance to CO₂, when leaf conductance to water is low.     

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: I. At High Soil Water Potential

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), stomatal resistance (r_s), leaf water potential (ψ), osmotic potential (π), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. var. Pa 602A), sorghum (Sorghum bicolor [L.] Moench var. RS610), and tobacco (Nicotiana tabacum L. var. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were high. In all three crops the r_s, measured with a ventilated diffusion porometer, the ψ, measured with the pressure chamber, the π, measured with a vapor pressure osmometer, and the calculated P, decreased from sunrise to reach minimum values near midday and then increased again in the afternoon. The diurnal range of all the variables was greater for leaves in the upper canopy than for those in the lower canopy. P was observed to decrease with decreasing ψ, but never became zero. Sorghum had a higher P at a ψ of, say −10 bars, than did maize, and maize had a higher P than tobacco at the same ψ. Moreover, at the same ψ the upper leaves in all canopies had a higher P than the lower leaves. When compared at high irradiances, r_s did not increase as ψ declined to −13, −15, and −10 bars or as P declined to 0.3, 3.5, and 1.2 bars in maize, sorghum, and tobacco, respectively. The relation between r_s and I in the upper, nonsenescent leaves of all three crops fits a hyperbolic curve, but the response varied with species and leaf senescence. The adaxial and abaxial epidermises had the same response of r_s to I in maize and sorghum, whereas in tobacco the adaxial epidermis had a higher r_s than the abaxial epidermis at all values of I. At equal values of I, tobacco had the lowest leaf resistance (r_l) and maize had the highest r_l. Senescent maize leaves had nonfunctional stomata, whereas the lowermost sorghum leaves had higher stomatal resistances on average than the other leaves.     

Effect of abscisic Acid on the gain of the feedback loop involving carbon dioxide and stomata

Gains of the feedback loops involving intercellular CO₂ concentration on one hand, and CO₂ assimilation and stomata on the other (= assimilation loop with gain [G_A] and conductance loop with gain [G_g]) were determined in detached leaves of Amaranthus powelli S. Wats., Avena sativa L., Gossypium hirsutum L., Xanthium strumarium L., and Zea mays in the absence and presence of 10⁻⁵ m (±) abscisic acid (ABA) in the transpiration stream. Determinations were made for an ambient CO₂ concentration of 300 microliters per liter. In the absence of ABA, stomata were insensitive to CO₂ (G_g between 0.00 and −0.02) in A. sativa, G. hirsutum, and X. strumarium, sensitive in A powelli (G_g = −0.46), and very sensitive in Z. mays (G_g = −3.6). Addition of ABA increased the absolute values of the gain of the conductance loop in A. powelli (G_g = −2.0), G. hirsutum (G_g = −0.31), and X. strumarium (G_g = −1.14). Stomata closed completely in A. sativa. In Z. mays, G_g decreased after application of ABA to a value of −0.86, but stomatal sensitivity to CO₂ increased for intercellular CO₂ concentrations < 100 microliters per liter. The gain of the assimilation loop increased after application of ABA in all cases, from values between 0.0 (A. powelli) and −0.21 (Z. mays) in the absence of ABA to values between −0.19 (A. powelli) and −0.43 (Z. mays) in the presence of ABA. In none of the species examined did ABA affect the photosynthetic capacity of the leaves.     

Correlation between the Maintenance of Photosynthesis and in Situ Protoplast Volume at Low Water Potentials in Droughted Wheat

Studies were undertaken to examine the relationship between water deficit effects on photosynthesis and the extent of protoplast volume reduction which occurs in leaves at low water potential (Ψ_w). This relationship was monitored in two cultivars (`Condor' and `Capelle Desprez') of cultivated wheat (Triticum aestivum) that differed in sensitivity to drought, and in a wild relative of cultivated wheat (Triticum kotschyi) that has been previously found to be `drought resistant.' When subjected to periods of water stress, Condor and T. kotschyi plants underwent osmotic adjustment; Capelle plants did not. Photosynthetic capacity was maintained to different extents in the three genotypes as leaf Ψ_w declined during stress; Capelle plants were most severely affected. Calculations of internal leaf [CO₂] and stomatal conductance from gas exchange measurements indicated that differences in photosynthetic inhibition at low Ψ_w among the genotypes were primarily due to nonstomatal effects. The extent of protoplast volume reduction that occurred in leaves at low Ψ_w was also found to be different in the three genotypes; maintenance of protoplast volume and photosynthetic capacity in stressed plants of the genotypes appeared to be correlated. When the extent of water stress-induced inhibition of photosynthesis was plotted as a function of declining protoplast volume, this relationship appeared identical for the three genotypes. It was concluded that there is a correlative association between protoplast volume and photosynthetic capacity in leaves of wheat plants subjected to periods of water stress.     

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.     

Spatial Distribution of Photosynthesis during Drought in Field-Grown and Acclimated and Nonacclimated Growth Chamber-Grown Cotton

Inhomogeneous photosynthetic activity has been reported to occur in drought-stressed leaves. In addition, it has been suggested that these water stress-induced nonuniformities in photosynthesis are caused by “patchy” stomatal closure and that the phenomenon may have created the illusion of a nonstomatal component to the inhibition of photosynthesis. Because these earlier studies were performed with nonacclimated growth chamber-grown plants, we sought to determine whether such “patches” existed in drought-treated, field-grown plants or in chamber-grown plants that had been acclimated to low leaf water potentials (ψ_leaf). Cotton (Gossypium hirsutum L.) was grown in the field and subjected to drought by withholding irrigation and rain from 24 d after planting. The distribution of photosynthesis, which may reflect the stomatal aperture distribution in a heterobaric species such as cotton, was assayed by autoradiography after briefly exposing attached leaves of field-grown plants to ¹⁴CO₂. A homogeneous distribution of radioactive photosynthate was evident even at the lowest ψ_leaf of −1.34 MPa. “Patchiness” could, however, be induced by uprooting the plant and allowing the shoot to air dry for 6 to 8 min. In parallel studies, growth chamber-grown plants were acclimated to drought by withholding irrigation for three 5-d drought cycles interspersed with irrigation. This drought acclimation lowered the ψ_leaf value at which control rates of photosynthesis could be sustained by approximately 0.7 MPa and was accompanied by a similar decline in the ψ_leaf at which patchiness first appeared. Photosynthetic inhomogeneities in chamber-grown plants that were visible during moderate water stress and ambient levels of CO₂ could be largely removed with elevated CO₂ levels (3000 μL L⁻¹), suggesting that they were stomatal in nature. However, advanced dehydration (less than approximately 2.0 MPa) resulted in “patches” that could not be so removed and were probably caused by nonstomatal factors. The demonstration that patches do not exist in drought-treated, field-grown cotton and that the presence of patches in chamber-grown plants can be altered by treatments that cause an acclimation of photosynthesis leads us to conclude that spatial heterogeneities in photosynthesis probably do not occur frequently under natural drought conditions.     

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: II. At Low Soil Water Potential

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), leaf resistance (r₁), leaf water potential (ψ), osmotic potential (π), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. cv. Pa602A), sorghum (Sorghum bicolor [L.] Moench cv. RS 610), and tobacco (Nicotiana tabacum L. cv. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were low. The r₁, measured with a ventilated diffusion porometer, of the leaves in the upper canopy decreased temporarily after sunrise [~0530 hours Eastern Standard Time] as I increased, but then r₁ increased again between 0700 and 0830 hr Eastern Standard Time as the ψ, measured with a pressure chamber, decreased rapidly from the values of −7, −4 and −6 bars at sunrise to minimal values of −18, −22 and −15 bars near midday in the maize, sorghum, and tobacco, respectively. The π, measured with a vapor pressure osmometer, also decreased after sunrise, but not to the same degree as the decrease in ψ, so that a P of zero was reached in some leaves between 0730 and 0800 hours. The lower (more negative) π of leaves in the upper canopy than those in the lower canopy gave the upper leaves a higher P at a given ψ than the lower leaves in all three species; leaves at intermediate heights had an intermediate P. This difference between leaves at the three heights in the canopy was maintained at all values of ψ. The r₁ remained unchanged over a wide range of P and then increased markedly at a P of 2 bars in maize, −1 bar in sorghum, and near zero P in tobacco: r₁ also remained constant until ψ decreased to −17, −20, and −13 bars in leaves at intermediate heights in maize, sorghum, and tobacco, respectively. In all three species r₁ of leaves in the upper canopy increased at more negative values of ψ than those at the base of the canopy, and in tobacco, leaves in the upper canopy wilted at more negative values of ψ than those in the lower canopy.     

Water deficits and reproduction in maize : response of the reproductive tissue to water deficits at anthesis and mid-grain fill

Reproductive development in maize (Zea mays L.) is vulnerable to plant water deficits during anthesis but becomes less sensitive as reproduction progresses. To determine whether changes in tissue water status correlated with the change in sensitivity, we examined the water potential (Ψ_w), osmotic potential (Ψ_s), and turgor of reproductive tissues during a short-term water deficit imposed at anthesis or mid-grain fill. Plants were grown in controlled environments in soil. At anthesis, leaf, husk, silk, and ovary Ψ_w of control plants was similar (−0.5 to −0.65 megapascal) at midday. When water was withheld, Ψ_w decreased to −1.75, −1.3, −1.2, and −1.0 megapascal in these tissues. Net water uptake by the ovaries was inhibited, but final dry weight, solute content, and total extractable carbohydrates were similar to the controls. At mid-grain fill, leaf, husk, grain, and embryo Ψ_w of control plants were −0.55, −0.35, −0.75, and −0.80 megapascal at midday. When water was withheld, leaf and husk Ψ_w decreased to −2.4 and −1.4 megapascal within 6 days. However, grain and embryo Ψ_w remained within 0.15 megapascal of control values. The grain continued to accumulate dry matter despite a net loss of water and a reduction in total solute content. These results indicate that the response of the reproductive tissues to plant water deficits varies with stage of grain development. The maintenance of a favorable water status only after grain filling is under way may explain, at least in part, the high sensitivity to plant water deficits early in reproductive development and the decrease in sensitivity as reproduction progresses.