Role of water transport in origin of water stress

Intensity of transpiration, intensity of water absorption, water saturation deficit (w.s.d.) in different parts of samples and rate of water transport was investigated in samples from leaf tissue of fodder cabbage and banana-tree. In all experiments (at initial w.s.d. 0% and 20%, in samples from upper, middle and lower leaves of fodder cabbage and from leaves of banana-tree) a distinct gradient of w.s.d. in the direction of transport of water was determined, therefore the limiting factor in the water balance was rate of water transport and not rate of water absorption. The lowest amount of water was always transported within transpiring part of sample. When the initial w.s.d. was 0% not only the water transported by tissue from the environment, but also the water of the leaf tissue itself took part in water lost by transpiration and therefore water stress originated in the whole sample. At an initial w.s.d. of 20%, the rate of water absorption was higher than the rate of water transport and therefore the increase of w.s.d. in the transpiring part of the sample was accompanied by a simultaneous decrease of w.s.d. in the transporting part. An increase in the value of w.s.d. in leaf tissue proportionally increased the resistance of water transport in the liquid phase (on the average from 1·7 . 10³ to 6·7 . 10³ atm min cm² g^?1) and also in the gaseous phase (on the average from 2·7 . 10^?2 to 14·0 . 10^?2 min cm^?1). It was proved that insufficient rate of water transport can be responsible for the origin of water stress. At the same time the rate of water transport was influenced by the value of the w.s.d. since every change of w.s.d. in leaf tissue not only the gradient of water potential changed but also the resistance to water transport.     

Water balance in leaf tissue

Samples of the leaf tissue (14cm²) were placed in a plexiglass chamber which consisted of three parts. Water absorbed by the leaf tissue on one side of the sample was transported through the middle part of the sample to the opposite side and was transpirated there. The intensity of transpiration the intensity of water absorption and water saturation deficit (w.s.d.) were determined simultaneously in this tissue by gravimetry. Water balance was studied either in saturated samples of leaf tissue or in tissue where w.s.d. (10%, 20%, 30%, 40%) was established in advance. Although conditions for water absorption in leaf segments were optimal, w.s.d. originated in the saturated leaf tissue under all given external conditions (evaporation from 41.7 to 17.8 mg cm^?2 h^?1). W.s.d. which was established in advance for the most part increased during the experiment and reached even high values (more than 60%). the equilibration was reached only under conditions of low evaporation and initial w.s.d. higher than 20% in young leaves and higher than 30% in adult leaves. A positive correlation between the ratio of the intensity of water absorption to the intensity of transpiration and w.s.d. was found only under conditions of lower evaporation (17.8 and 23.2mg cm^?2h^?1). The maximal values of w.s.d. were limited in this way. Water balance was studied: 1. in leaf tissue of upper, middle and lower leaves of fodder cabbage, 2. in leaf tissue of middle leaves of young and adult plants of fodder cabbage, 3. in leaf tissue of dicots (fodder cabbage) with different vessel orientation in respect to water transport, 4. in leaf tissue of monocots (banana-tree) with water transport upright to the vessel orientation. Considerable change of water balance was observed when the water transport was prolonged by two incisions in the middle part of the sample. Results of all these experiments revealed the possibility of water stress origin even in leaf tissue sufficiently supplied with water.     

Determination of relationships between water potential and water saturation deficit in leaf tissue

The mutual relationship between the water potential and water saturation deficit (w.s.d.) was studied on samples of leaf tissue of fodder cabage. Definite values of water potential were obtained by long-term exposure of plant material to an atmosphere with definite constant pressure of water vapour. The resulting w.s.d. values were determined gravimetrically. Water saturation deficit varies indirectly with the water potential. This dependence was linear for values of water potential from ?4·4 to ?43·9 atm. Since the stabilization of equilibrum of water potential between the leaf tissue and surrounding atmosphere was very slow the relationship between water potential and w.s.d. was influenced by the size of samples and by the length of exposure. Therefore this method was more suitable for relative than for absolute measurement.     

Efficiency and regulation of water transport in some woody and herbaceous species

The efficiency with which plants transport water is related to the water potential differences required to drive water fluxes from the soil to the leaf. A comparative study of two woody and three herbaceous species (Citrus sinensis L. cv. Koethen, Pyrus kawakami L., Helianthus annuus L. cv. Mammoth Russian, Capsicum frutescens L. cv. Yolo Wonder, and Sesamum indicum L. cv. Glauca) indicated contrasts in water transport efficiency. Depression of leaf water potential in response to transpiration increases was found in the woody species; the herbaceous species, however, had more efficient water transport systems and presented no measurable response of leaf water potential to transpiration changes. Different maximum transpiration rates under the same climatic conditions were observed with different species and may be accounted for by stomatal response to humidity gradients between leaf and air. Leaf diffusion resistance in sesame increased markedly as the humidity gradient was increased, while leaf resistance of sunflower responded less to humidity. Stomata appeared to respond directly to the humidity gradient because changes in leaf water potential were not detected when leaf resistance increased or decreased.     

Errors in measuring water potentials of small samples resulting from water adsorption by thermocouple psychrometer chambers

The adsorption of water by thermocouple psychrometer assemblies is known to cause errors in the determination of water potential. Experiments were conducted to evaluate the effect of sample size and psychrometer chamber volume on measured water potentials of leaf discs, leaf segments, and sodium chloride solutions. Reasonable agreement was found between soybean (Glycine max L. Merr.) leaf water potentials measured on 5-millimeter radius leaf discs and large leaf segments. Results indicated that while errors due to adsorption may be significant when using small volumes of tissue, if sufficient tissue is used the errors are negligible. Because of the relationship between water potential and volume in plant tissue, the errors due to adsorption were larger with turgid tissue. Large psychrometers which were sealed into the sample chamber with latex tubing appeared to adsorb more water than those sealed with flexible plastic tubing. Estimates are provided of the amounts of water adsorbed by two different psychrometer assemblies and the amount of tissue sufficient for accurate measurements of leaf water potential with these assemblies. It is also demonstrated that water adsorption problems may have generated low water potential values which in prior studies have been attributed to large cut surface area to volume ratios.     

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.

     

Theoretical and experimental errors for in situ measurements of plant water potential

Errors in psychrometrically determined values of leaf water potential caused by tissue resistance to water vapor exchange and by lack of thermal equilibrium were evaluated using commercial in situ psychrometers (Wescor Inc., Logan, UT) on leaves of Tradescantia virginiana (L.). Theoretical errors in the dewpoint method of operation for these sensors were demonstrated. After correction for these errors, in situ measurements of leaf water potential indicated substantial errors caused by tissue resistance to water vapor exchange (4 to 6% reduction in apparent water potential per second of cooling time used) resulting from humidity depletions in the psychrometer chamber during the Peltier condensation process. These errors were avoided by use of a modified procedure for dewpoint measurement. Large changes in apparent water potential were caused by leaf and psychrometer exposure to moderate levels of irradiance. These changes were correlated with relatively small shifts in psychrometer zero offsets (−0.6 to −1.0 megapascals per microvolt), indicating substantial errors caused by nonisothermal conditions between the leaf and the psychrometer. Explicit correction for these errors is not possible with the current psychrometer design.     

Water stress development in kale leavesin situ and in water supplied cut leaves

The development of temporary water stress during the day-light hours, characterized by a decrease of the values of the water potential (?_w) and increase of the values of water saturation deficit (ΔW _sat) was found not only in the leaves of intact kale plants but also in cut leaves with their petioles immersed in water. These results indicate that the leaf resistance to water transport could not be supposed as negligible. The same decrease of ? _w was accompanied with the higher increase of ΔW _sat in cut leaves than in leavesin situ.     

Effects of irradiance and leaf water deficit on net carbon dioxide assimilation and mesophyll and transport resistances

Rate of net CO₂ assimilation by soil-grown soybean plants were studied over a range of relative leaf water contents at each of four levels of irradiance. There was a large interaction between light level and leaf water deficit on the rate of CO₂ assimilation. The effect of leaf water deficit on assimilation became larger as irradiance increased. Both stomatal resistance to CO₂ transport and mesophyll resistance to CO₂ assimilation increased as leaf-water deficit increased. The increase in both resistance with changing leaf-water content was largest at high irradiance and became smaller as irradiance decreased. Relief of soil-moisture stress by watering induced large oscillations of CO₂ assimilation, stomatal resistance, and mesophyll resistance. The oscillation of the mesophyll resistance occurred in the absence of changes in relative water content and appeared to be related to oscillations in leaf temperature. The observed increase in mesophyll resistance with decreasing leaf-water content under nonoscillative conditions may be caused by changes in leaf temperature rather than leaf water content.     

Influence of water relations and temperature on leaf movements of rhododendron species

Rhododendron maximum L. and R. Catawbiense L. are subcanopy evergreen shrubs of the eastern United States deciduous forest. Field measurements of climate factors and leaf movements of these species indicated a high correlation between leaf temperature and leaf curling; and between leaf water potential and leaf angle. Laboratory experiments were performed to isolate the influence of temperature and cellular water relations on leaf movements. Significant differences were found between the patterns of temperature induction of leaf curling in the two species. Leaves of the species which curled at higher temperatures (R. catawbiense) also froze at higher leaf temperatures. However, in both cases leaf curling occurred at leaf temperatures two to three degrees above the leaf freezing point. Pressure volume curves indicated that cellular turgor loss was associated with a maximum of 45% curling while 100% or more curling occurred in field leaves which still had positive cell turgor. Moisture release curves indicated that 70% curling requires a loss of greater than 60% of symplastic water which corresponds to leaf water potentials far below those experienced in field situations. Conversely, most laboratory induced changes in leaf angle could be related to leaf cell turgor loss.     

Stomatal and nonstomatal regulation of water use in cotton, corn, and sorghum

Stomata of corn (Zea mays L.) and sorghum (Sorghum bicolor L.) responded to changes in leaf water potential during the vegetative growth phase. During reproductive growth, leaf resistances were minimal and stomata were no longer sensitive to bulk leaf water status even when leaf water potentials approached −27 bars. Stomata of corn, cotton (Gossypium hirsutum L.), and sorghum appear to respond to changes in the humidity deficit between the leaf and air and in this manner, regulated transpirational flux to some degree. Distinct differences in water transport efficiency were observed in the three species. Under nonlimiting soil water conditions, sorghum exhibited the greatest efficiency of water transport while under limiting soil moisture conditions, cotton appeared most efficient. Corn was the least efficient with respect to nonstomatal regulation of water use. Differences in drought tolerance among the three species are partially dependent on stomatal regulation of water loss, but efficiency of the water transport system may be more related to drought adaptation. This is particularly important since stomata of all three species did not respond to bulk leaf water status during a large portion of the growing season.     

Effects of manipulation of water and nitrogen regime on the water relations of the desert shrub Larrea tridentata

Summary Water and nitrogen regimes of Larrea tridentata shrubs growing in the field were manipulated during an annual cycle. Patterns of leaf water status, leaf water relations characteristics, and stomatal behavior were followed concurrently. Large variations in leaf water status in both irrigated and nonirrigated individuals were observed. Predawn and midday leaf water potentials of nonirrigated shrubs were lowest except when measurements had been preceded by significant rainfall. Despite the large seasonal variation in leaf water status, reasonably constant, high levels of turgor were maintained. Pressure-volume curve analysis suggested that changes in the bulk leaf osmotic potential at full turgor were small and that nearly all of the turgor adjustment was due to tissue elastic adjustment. The increase in tissue elasticity with increasing water deficit manifested itself as a decrease in the relative water content at zero turgor and as a decrease in the tissue bulk elastic modulus. Because of large hydration-induced displacement in the osmotic potential and relative water content at zero turgor, it was necessary to use shoots in their natural state of hydration for pressure-volume curve determinations. Large diurnal and seasonal differences in maximum stomatal conductance were observed, but could not easily be attributed to variations in leaf water potential or leaf water relations characteristics such as the turgor loss point. The single factor which seemed to account for most of the diurnal and seasonal differences in maximum stomatal conductance between individual shrubs was an index of soil/root/ shoot hydraulic resistance. Daily maximum stomatal conductance was found to decrease with increasing soil/root/ shoot hydraulic resistance. This pattern was most consistent if the hydraulic resistance calculation was based on an estimate of total canopy transpiration rather than the more commonly used transpiration per unit leaf area. The reasons for this are discussed. It is suggested that while stomatal aperture necessarily represents a major physical resistance controlling transpiration, plant hydraulic resistance may represent the functional resistance through its effects on stomatal aperture.     

Leaf water relations and maintenance of gas exchange in coffee cultivars grown in drying soil

Plant water status, leaf tissue pressure-volume relationships, and photosynthetic gas exchange were monitored in five coffee (Coffea arabica L.) cultivars growing in drying soil in the field. There were large differences among cultivars in the rates at which leaf water potential (Ψ_L) and gas exchange activity declined when irrigation was discontinued. Pressure-volume curve analysis indicated that increased leaf water deficits in droughted plants led to reductions in bulk leaf elasticity, osmotic potential, and in the Ψ_L at which turgor loss occurred. Adjustments in Ψ_L at zero turgor were not sufficient to prevent loss or near loss of turgor in three of five cultivars at the lowest values of midday Ψ_L attained. Maintenance of protoplasmic volume was more pronounced than maintenance of turgor as soil drying progressed. Changes in assimilation and stomatal conductance were largely independent of changes in bulk leaf turgor, but were associated with changes in relative symplast volume. It is suggested that osmotic and elastic adjustment contributed to maintenance of gas exchange in droughted coffee leaves probably through their effects on symplast volume rather than turgor.     

Detrimental effect of rust infection on the water relations of bean

Bean plants (Phaseolus vulgaris L.) infected with the rust Uromyces phaseoli became unusually susceptible to drought as sporulation occurred. Under the conditions used (1,300 ft-c, 27 C, and 55% relative humidity) such plants wilted at soil water potentials greater than −1 bar, whereas healthy plants did not wilt until the soil water potential fell below −3.4 bars. Determinations of leaf water and osmotic potentials showed that an alteration in leaf osmotic potential was not responsible for the wilting of diseased plants. When diffusive resistance was measured as a function of decreasing leaf water content, the resistance of healthy leaves increased to 50 sec cm⁻¹ by the time relative water content decreased to 70%, whereas the resistance of diseased leaves remained less than 8 sec cm⁻¹ down to 50% relative water content. Apparently, water vapor loss through cuticle damaged by the sporulation process, together with the reduction in root to shoot ratio which occurs in diseased plants, upset the water economy of the diseased plant under mild drought conditions.     

Effects of NaCl salinity and water stress on growth and leaf water relations of Asteriscus maritimus plants

Potted plants of Asteriscus maritimus (L.) Less were submitted to water stress (during two consecutive cycles, irrigation water was withheld for 5 days followed by a recovery period of 25 days) and saline stress (150 days of exposure to 0, 70 and 140 mM NaCl daily irrigation) in order to assess the effect on leaf water relations and growth parameters. Plants under saline and water stress conditions showed lower biomass and an early reduction in leaf expansion growth. Both stresses promoted a substantial degree of stomatal regulation; but, in spite of this, the plants showed signs of leaf tissue dehydration, decreases in RWC and Ψ_pd values. However, salt-treated plants, developed a NaCl inclusion mechanisms, underwent osmotic adjustment, which was able to maintain leaf turgor. Under both stress conditions g_l was independent to plant water status in the range between –0.8 and 1.0 MPa. Under water stress conditions, midday leaf water potential showed a threshold value (around −1.1 MPa), below which leaf conductance remained constant. In the salt-treated plants, the gradual closure of the stomata over a wide range of Ψ_md may be important in maintaining some level of photosynthesis.     

Function and structure of leaves contributing to increasing water storage with height in the tallest <Emphasis Type="Italic">Cryptomeria japonica</Emphasis> trees of Japan

Key message

In Cryptomeria japonica , transfusion tissue in leaves may have functions of water storage and supply, which could compensate for hydraulic constraints with increasing height.

Abstract

The tallest trees of Cryptomeria japonica occur in climatic regions similar to the world’s tallest trees. We hypothesized that tall C. japonica trees would have evolved adaptive mechanisms to overcome height growth limitation. Here, we focused on foliar water storage, a mechanism recently discovered in Sequoia sempervirens. In C. japonica, leaf water potential at turgor loss did not change with height or light availability, while leaf hydraulic capacitance and succulence (water content per leaf surface area) increased, suggesting hydraulic compensation. Plasticity of leaf morphology could contribute to avoiding negative effects of height on photosynthesis. We also focused on the structure and function of transfusion tissue in leaves and its role in water storage and supply. Cross-sectional area of transfusion tissue increased with height, whereas that of xylem was constant. We confirmed that water flowed from vascular bundle to mesophyll via the transfusion tissue. Cryo-scanning electron microscopy images of leaf cross sections showed that transfusion cells were flattened, but not fully dehydrated when leaf water potential decreased in situ and by experimental dehydration, and cell deformation was more marked for treetop leaves than for lower-crown leaves. The shape of transfusion cells recovered at predawn as well as after experimental rehydration. As in S. sempervirens, transfusion tissue of C. japonica may function as a hydraulic buffer, absorbing and releasing water according to leaf water status. Anatomical and hydraulic properties contributing to foliar water storage may be an adaptive mechanism acquired by tall Cupressaceae trees to overcome the hydraulic constraints on physiological function with increasing height.

     

Rapid estimates of relative water content

Relative water content may be accurately estimated using the ratio of tissue fresh weight to tissue turgid weight, termed here relative tissue weight. That relative water content and relative tissue weight are linearly related is demonstrated algebraically. The mean value of r² for grapevine (Vitis vinifera L. cv. Shiraz) leaf tissue over eight separate sampling occasions was 0.993. Similarly high values were obtained for maize (Zea mays cv. Cornell M-3) (0.998) and apple (Malus sylvestris cv. Northern Spy) (0.997) using a range of leaf ages. The proposal by Downey and Miller (1971. Rapid measurements of relative turgidity in maize (Zea mays L.). New Phytol. 70: 555-560) that relative water content in maize may be estimated from water uptake was also investigated for grapevine leaves; this was found to be a less reliable estimate than that obtained with relative tissue weight. With either method, there is a need for calibration, although this could be achieved for relative tissue weight at least with only a few subsamples.     

On the Resistance to Transpiration of the Sites of Evaporation within the Leaf

The rates of transpiration from the upper and lower surfaces of leaves of Gossypium hirsutum, Xanthium strumarium, and Zea mays were compared with the rates at which helium diffused across those leaves. There was no evidence for effects of CO₂ concentration or rate of evaporation on the resistance to water loss from the evaporating surface (“resistance of the mesophyll wall to transpiration”) and no evidence for any significant wall resistance in turgid tissues. The possible existence of a wall resistance was also tested in leaves of Commelina communis and Tulipa gesneriana whose epidermis could be easily peeled. Only when an epidermis was removed from a leaf, evaporation from the mesophyll tissue declined. We conclude that under conditions relevant to studies of stomatal behavior, the water vapor pressure at the sites of evaporation is equal to the saturation vapor pressure.     

Influence of elevated carbon dioxide on water relations of soybeans

Soybean (Glycine max L. Merrill cv `Bragg') plants were grown in pots at six elevated atmospheric CO₂ concentrations and two watering regimes in open top field chambers to characterize leaf xylem potential, stomatal resistance and conductance, transpiration, and carbohydrate contents of the leaves in response to CO₂ enrichment and water stress conditions. Groups of plants at each CO₂ concentration were subjected to water stress by withholding irrigation for 4 days during the pod-filling stage.

Under well watered conditions, the stomatal conductance of the plants decreased with increasing CO₂ concentration. Therefore, although leaf area per plant was greater in the high CO₂ treatments, the rate of water loss per plant decreased with CO₂ enrichment. After 4 days without irrigation, plants in lower CO₂ treatments showed greater leaf tissue damage, lower leaf water potential, and higher stomatal resistance than high CO₂ plants. Stomatal closure occurred at lower leaf water potentials for the low CO₂ grown plants than the high CO₂ grown plants. Significantly greater starch concentrations were found in leaves of high CO₂ plants, and the reductions in leaf starch and increases in soluble sugars due to water stress were greater for low CO₂ plants. The results showed that even though greater growth was observed at high atmospheric CO₂ concentrations, lower rates of water use delayed and, thereby, prevented the onset of severe water stress under conditions of low moisture availability.