Leaf water potentials measured with a pressure chamber

Leaf water potentials were estimated from the sum of the balancing pressure measured with a pressure chamber and the osmotic potential of the xylem sap in leafy shoots or leaves. When leaf water potentials in yew, rhododendron, and sunflower were compared with those measured with a thermocouple psychrometer known to indicate accurate values of leaf water potential, determinations were within ± 2 bars of the psychrometer measurements with sunflower and yew. In rhododendron. water potentials measured with the pressure chamber plus xylem sap were 2.5 bars less negative to 4 bars more negative than psychrometer measurements.

The discrepancies in the rhododendron measurements could be attributed, at least in part, to the filling of tissues other than xylem with xylem sap during measurements with the pressure chamber. It was concluded that, although stem characteristics may affect the measurements, pressure chamber determinations were sufficiently close to psychrometer measurements that the pressure chamber may be used for relative measurements of leaf water potentials, especially in sunflower and yew. For accurate determinations of leaf water potential, however, pressure chamber measurements must be calibrated with a thermocouple psychrometer.

     

Oxidation of proline by mitochondria isolated from water-stressed maize shoots

Proline oxidation and coupled phosphorylation were measured in mitochondria after isolation from shoots of water-stressed, etiolated maize (Zea mays L.) seedlings. Both state III and state IV rates of proline oxidation decreased as a logarithmic function of increased seedling water stress between −5 and −10 bars. Proline oxidation rates decreased 62% (state III) and 58% (state IV) as seedling water potentials were decreased from −5 to −10 bars. By comparison, oxidation of succinate, exogenous NADH, or malate + pyruvate decreased only 10 to 15% in this stress range. These decreases were a linear function of increased stress and were comparable to oxidation rates of mitochondria subjected to varying in vitro osmotic potentials. Osmotically induced in vitro stress reduced proline oxidation rates linearly with more negative osmotic potentials, a decrease that was similar to the responses of the other substrates to more negative osmotic potentials. Some decrease in coupling, with all substrates as determined by ADP/O ratios, was observed under osmotic stress. Mitochondria were also isolated from shoot tissue that had been stressed and then rewatered. On a percentage basis, the recovery of proline oxidation was greater than that of the other substrates.     

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.     

Kinetics of Adaptation to Osmotic Stress in Lentil (Lens culinaris Med.) Roots

When intact roots of lentil (Lens culinaris Med.) are subjected to severe osmotic stress by treatment with a solution of low water potential, they immediately begin to shrink. Within 10 to 15 minutes, shrinkage ceases, and within 20 minutes, the roots resume growth. The time lag between application of osmoticum and resumption of growth varies from about 10 to 30 minutes over the range of external water potentials of −2 to −12.4 bars. For external water potentials as low as −8.7 bars the new steady rate of growth in the presence of osmoticum is approximately equal to that prevailing before application of osmoticum. For external water potentials between −8.7 and −13 bars growth resumes, but the new rate is less than that prior to addition of osmoticum. Measurements of changes in the internal solute content during adaptation show that the solute content of the root increases but that the magnitude of the increase is, by itself, insufficient to account for the resumption of rapid growth.     

Comparative resistance of the soil and the plant to water transport

The resistances to liquid water transport in the soil and plant were determined directly and simultaneously from measurements of soil, root, and leaf water potentials and the flux of water through the soil-plant system to the sites of evaporation in the leaf. For soybean (Merr.) transporting water at a steady rate, water potential differences between soil and root were smaller than between root and leaf over the range of soil water potentials from −0.2 to −11 bars. As soil water was depleted, water flow through the soil and plant decreased to one-tenth the maximum rate, but both the soil resistance and plant resistance increased. The plant resistance remained larger than the soil resistance over the entire range of soil water availability. Previous suggestions that the soil is the major resistance have ignored the increase in plant resistance and/or assumed root densities that were too low.     

Altered ultrastructure of cells of sunflower leaves having low water potentials

Summary Desiccation-induced alterations in cell structure were investigated in sunflower (Helianthus annum L.) leaves using light and electron microscopy. Desiccation was imposed by withholding water from the tissue, and all tissue fixation was carried out under isosmotic conditions. In addition to shrinkage of the vacuoles and intercellular spaces caused by water loss, the significant features of cell desiccation were the appearance of lipid droplets and vesicles close to dictyosomes, and plasmalemma and/or tonoplast breakage in the mesophyll cells. Breakage was followed by massive loss of cell organelles except for the thylakoid membranes of the chloroplasts, which retained much of their integrity even in the air-dried state. Plasmalemma and tonoplast disruption began in a few cells at water potentials of — 15 bars (relative water contents of 47%) and went to completion below —26 bars (relative water contents less than 28%) in the leaf mesophyll. Typically in this tissue, net photosynthesis becomes zero and the tissue becomes increasingly incapable of full rehydration at water potentials below — 20 bars. By contrast, water potentials of — 26 bars had no detectable effects on the phloem tissue. Structural alterations were little influenced by the rapidity of desiccation (a few minutes to as long as four days). It was concluded that desiccation-induced changes in cell structure are tissue-specific and occur on a cell-by-cell basis rather than in all cells of a tissue at once. The concentration of the cytoplasm and the disruption of the plasmalemma and/or tonoplast seem to be central events in the alteration of cell ultrastructure by desiccation.This research was supported by NSF grant GB41314.     

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.     

Soil water potential as a factor in the ecology ofFusarium roseum f. sp.cerealis ‘Culmorum’

Summary The soil water potential (inferred from vapor pressure measurements by thermocouple psychrometry) influenced both chlamydospore germination and continuing growth of germlings ofFusarium roseum f. sp.cerealis ‘Culmorum’ the same way in two different soils. Chlamydospore germination in both Ritzville silt loam (RSL) and Palouse silt loam (PSL) amended with about 2,500 ppm C (as glucose) and 250 ppm N (as ammonium sulfate) was 40–50 per cent in 24 hours at water potentials down to −50 to −60 bars. Some germination occurred by 72 hours at −80 to −85 bars in both soils but not at lower potentials. At a potential of −10 bars or higher, germ tubes lysed or converted into new chlamydospores within 48–72 hours after germination, whereas at lower potentials germlings branched and appeared to grow for at least 6 days. Bacterial numbers/g of RSL, 24 and 72 hours after adding nutrients, were 200 to 300 times greater in soil at water potentials of −5 bars or more than in comparably treated soil at about −14 to −17 bars or less. Markedly reduced bacterial activity appeared to coincide with a water potential of about −9 to −10 bars. When streptomycin and neomycin (300 ppm each) were mixed into the soil in addition to nutrients, the survival of germlings of Culmorum was greatly enhanced, even in soil at potentials of less than −1 bar. Indications were that soil water potentials of −10 bars or more favored bacterial activity, and that this in turn repressed growth of germlings of Culmorum. Culmorum infections of below-ground parts of wheat are serious primarily in drier soils, possibly because the fungus escapes bacterial antagonism but can still extract water for growth. Cooperative investigations, Crops Research and the Water and Soil Conservation Research Divisions, Agricultural Research Service, U.S. Department of Agriculture and the Agricultural Experiment Stations of Idaho, Montana, Oregon, Utah, and Washington. Scientific Paper No.3152, College of Agriculture, Washington State University, Pullman.     

Inhibition of oxygen evolution in chloroplasts isolated from leaves with low water potentials

Chloroplasts were isolated from pea and sunflower leaves having various water potentials. Oxygen evolution by the chloroplasts was measured under identical conditions for all treatments with saturating light and with dichloroindophenol as oxidant. Evolution was inhibited when leaf water potentials were below -12 bars in pea and -8 bars in sunflower and the inhibition was proportional to leaf water potential below these limits. Inhibition was more severe in sunflower than in pea chloroplasts. In sunflower, it could be detected after 5 minutes of leaf desiccation, and, up to 1 hour, the effect was independent of the duration of low leaf water potential.     

The Interactive Effects of Temperature and Osmotic Potential on the Growth of Aquatic Isolates of Fusarium culmorum

The mycelial growth of 10 Fusarium culmorum strains isolated from water of the Andarax riverbed in the provinces of Granada and Almeria in southeastern Spain was tested on potato-dextrose-agar adjusted to different osmotic potentials with either KCl or NaCl (?1.50 to ?144.54 bars) at 10°C intervals ranging from 15° to 35°C. Fungal growth was determined by measuring colony diameter after 4 d of incubation. Mycelial growth was maximal at 25°C. The quantity and capacity of mycelial growth of F. culmorum were similar at 15 and 25°C, with maximal growth occurring at ?13.79 bars water potential and a lack of growth at 35°C. The effect of water potential was independent of salt composition. The general growth pattern of Fusarium culmorum growth declined at potentials below ?13.79 bars. Fungal growth at 25°C was always greater than growth at 15°C, at all of the water potentials tested. Significant differences were observed in the response of mycelia to water potential and temperature as main and interactive effects. The number of isolates that showed growth was increasingly inhibited as the water potential dropped, but some growth was still observable at ?99.56 bars. These findings could indicate that F. culmorum strains isolated from water have a physiological mechanism that permits survival in environments with low water potential. Propagules of Fusarium culmorum are transported long distances by river water, which could explain the severity of diseases caused by F. culmorum on cereal plants irrigated with river water and its interaction under hydric stress or moderate soil salinity. The observed differences in growth magnitude and capacity could indicate that the biological factors governing potential and actual growth are affected by osmotic potential in different ways.     

Relationship of water potential to growth of leaves

A thermocouple psychrometer that measures water potentials of intact leaves was used to study the water potentials at which leaves grow. Water potentials and water uptake during recovery from water deficits were measured simultaneously with leaves of sunflower (Helianthus annuus L.), tomato (Lycopersicon esculentum Mill.), papaya (Carica papaya L.), and Abutilon striatum Dickson. Recovery occurred in 2 phases. The first was associated with elimination of water deficits; the second with cell enlargement. The second phase was characterized by a steady rate of water uptake and a relatively constant leaf water potential. Enlargement was 70% irreversible and could be inhibited by puromycin and actinomycin D. During this time, leaves growing with their petioles in contact with pure water remained at a water potential of —1.5 to —2.5 bars regardless of the length of the experiment. It was not possible to obtain growing leaf tissue with a water potential of zero. It was concluded that leaves are not in equilibrium with the potential of the water which is absorbed during growth. The nonequilibrium is brought about by a resistance to water flow which requires a potential difference of 1.5 to 2.5 bars in order to supply water at the rate necessary for maximum growth.

Leaf growth occurred in sunflower only when leaf water potentials were above —3.5 bars. Sunflower leaves therefore require a minimum turgor for enlargement, in this instance equivalent to a turgor of about 6.5 bars. The high water potentials required for growth favored rapid leaf growth at night and reduced growth during the day.

     

Growth-induced Water Potentials in Plant Cells and Tissues

A physical analysis of water movement through elongating soybean (Glycine max L. Merr.) hypocotyls was made to determine why significant water potentials persist in growing tissues even though the external water potentials were zero and transpiration is virtually zero. The analysis was based on a water transport theory modified for growth and assumed that water for growing cells would move through and along the cells in proportion to the conductivity of the various pathways.

Water potentials calculated for individual cells were nearly in local equilibrium with the water potentials of the immediate cell surroundings during growth. However, water potentials calculated for growing tissue were 1.2 to 3.3 bars below the water potential of the vascular supply in those cells farthest from the xylem. Only cells closest to the xylem had water potentials close to that of the vascular supply. Gradients in water potential were steepest close to the xylem because all of the growth-sustaining water had to move through this part of the tissue. Average water potentials calculated for the entire growing region were −0.9 to −2.2 bars depending on the tissue diffusivity.

For comparison with the calculations, average water potentials were measured in elongating soybean hypocotyls using isopiestic thermocouple psychrometers for intact and excised tissue. In plants having virtually no transpiration and growing in Vermiculite with a water potential of −0.1 bar, rapidly growing hypocotyl tissue had water potentials of −1.7 to −2.1 bars when intact and −2.5 bars when excised. In mature, nongrowing hypocotyl tissue, average water potentials were −0.4 bar regardless of whether the tissue was intact or excised.

The close correspondence between predicted and measured water potentials in growing tissue indicates that significant gradients in water potential are required to move growth-associated water through and around cells over macroscopic distances. The presence of such gradients during growth indicates that cells must have different cell wall and/or osmotic properties at different positions in the tissue in order for organized growth to occur. The mathematical development used in this study represents the philosophy that would have to be followed for the application of contemporary growth theory when significant tissue water potential gradients are present.

     

Soil water regimes and water extraction patterns under two semi-arid shrub (Atriplex spp.) communities

Soil water regimes and water extraction patterns estimated over a period of two years are described for two plantation communities of semi-arid shrubs, Atriplex vesicaria Hew. ex Benth. and A. nummularia Lindl., growing on the same soil type under identical climatological conditions near Deniliquin, New South Wales. In spite of poor water flow properties of the soil, surface run-off was negligible.About 90% of the extractable water was stored in the top (45 cm) soil layer. Both species withstood exceedingly low water potentials, although A, vesicaria reduced soil water to a much lower water potential than did A. nummularia. Water potentials at depths below 60 cm were always – 15 bars and remained constant. Water extracted beyond –15 bars amounted to 41% more than the water available within conventionally accepted water potential limits (between –0.3 to – 15 bars). During Slimmer, the plant water potential of A. vesicaria fell to much lower values than that of A. nummularia. Relationships between relative leaf water content and plant water potential differed between the two species, and the suggestion is made that at low plant water potential, leaf targidity of A. vesicaria would be higher, and thus this species would have a higher tolerance to desiccation. On a yearly, half-yearly and even a quarterly basis, evapotranspiration (FT) of the two communities did not differ. Fortnightly FT rates were similar during winter but during early summer, the initial ET rate of A. vesicaria was higher than that of A. nummularia; A, nummularia can therefore conserve water for later use. These differences in water extraction patterns and evapotranspiration were associated with differential rooting characteristics and probably differential stomatal functioning. The relationships between fortnightly ET/FO (ratio of actual evapotranspiration to that from a Class A pan) and profile water content, for both communities, were linear but different.     

Effect of water potential on seed germination

The response of seed germination to substrate water potential was determined for several plant species of the arctic tundra. Seeds were collected from Cape Thompson and Eagle Summit, Alaska and germinated on dialysis membranes over water solutions of polyethylene glycol with osmotic potentials of 0 to −6 bars. Germination did not occur with potentials below −3 bars, except for three fellfield species. Germination was delayed at lower osmotic potentials. Because the response of most species was similar, substrate water potential is probably not a factor affecting the establishment of most tundra plant species from seeds.     

Effects of salt secretion on psychrometric determinations of water potential of cotton leaves

Thermocouple psychrometers gave lower estimates of water potential of cotton leaves than did a pressure chamber. This difference was considerable for turgid leaves, but progressively decreased for leaves with lower water potentials and fell to zero at water potentials below about −10 bars. The conductivity of washings from cotton leaves removed from the psychrometric equilibration chambers was related to the magnitude of this discrepancy in water potential, indicating that the discrepancy is due to salts on the leaf surface which make the psychrometric estimates too low. This error, which may be as great as 400 to 500%, cannot be eliminated by washing the leaves because salts may be secreted during the equilibration period. Therefore, a thermocouple psychrometer is not suitable for measuring the water potential of cotton leaves when it is above about −10 bars.     

Comparison of the dye method with the thermocouple psychrometer for measuring leaf water potentials

The dye method for measuring water potential was examined and compared with the thermocouple psychrometer method in order to evaluate its usefulness for measuring leaf water potentials of forest trees and common laboratory plants. Psychrometer measurements are assumed to represent the true leaf water potentials. Because of the contamination of test solutions by cell sap and leaf surface residues, dye method values of most species varied about 1 to 5 bars from psychrometer values over the leaf water potential range of 0 to −30 bars. The dye method is useful for measuring changes and relative values in leaf potential. Because of species differences in the relationships of dye method values to true leaf water potentials, dye method values should be interpreted with caution when comparing different species or the same species growing in widely different environments. Despite its limitations the dye method has a usefulness to many workers because it is simple, requires no elaborate equipment, and can be used in both the laboratory and field.     

The Effect of Soil Water Regimes on Leaf Water Potential, Growth and Development of Soybeans

The growth and development of soybeans (Glycine max L. cv. Amsoy) was studied at soil matric potentials of ?0.1 to ?1.0 bars. Chlorophyll, photosynthesis, and leaf nitrogen per plant was greatest at ?4 bars leaf water potential. Leaf area, number of internodes, plant height and dry weight of vegetative parts declined as leaf water potential decreased from ?2 to ?19 bars. Nitrogen content and nitrate reductase activity per g fresh weight determined the percentage protein of individual seeds but nitrogen content and nitrate reductase activity per plant determined the amount of total seed protein. The protein synthesized in the seed changed little in amino acid composition with changes in leaf water potential. Leaf water potentials above or below ?4 bars decreased yield, total protein and total lipid but plants produced the largest percentage of individual seed protein at ?19 bars leaf water potential.     

Water Potential and Stomatal Resistance of Sunflower and Soybean Subjected to Water Stress during Various Growth Stages

Plants of two varieties of soybean (Glycine max (L.) Merr.) and two varieties of sunflower (Helianthus annuus L.) were grown in controlled environments and subjected to water stress at various stages of growth. Leaf resistances and leaf water potentials were measured as stress developed. In soybeans the upper leaf surface had a higher resistance than the lower surface at all leaf water potentials and growth stages. Resistance of the upper surface began to increase at a higher water potential and increased more than the resistance of the lower surface. Resistances returned to prestress values 4 days after rewatering. In sunflowers upper and lower leaf surfaces had similar resistances at all water potentials and growth stages. Leaf resistances were higher in sunflower plants stressed before flowering than in those stressed later. Sunflower plants stressed to −16 bars recovered their prestress leaf resistance and water potential a few days after rewatering, but leaves of sunflower plants stressed to −23 bars died. Leaves of soybean and sunflower plants stressed before flowering suffered less injury than those of older plants and sunflowers stressed after flowering suffered more injury than soybeans.     

Chloroplast response to low leaf water potentials: I. Role of turgor

The effect of decreases in turgor on chloroplast activity was studied by measuring the photochemical activity of intact sunflower (Helianthus annuus L. cv. Russian Mammoth) leaves having low water potentials. Leaf turgor, calculated from leaf water potential and osmotic potential, was found to be affected by the dilution of cell contents by water in the cell walls, when osmotic potentials were measured with a thermocouple psychrometer. After the correction of measurements of leaf osmotic potential, both the thermocouple psychrometer and a pressure chamber indicated that turgor became zero in sunflower leaves at leaf water potentials of −10 bars. Since most of the loss in photochemical activity occurred at water potentials below −10 bars, it was concluded that turgor had little effect on the photochemical activity of the leaves.     

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.