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.     

Phloem water relations and translocation

Satisfactory measurements of phloem water potential of trees can be obtained with the Richards and Ogata psychrometer and the vapor equilibration techniques, although corrections for loss of dry weight and for heating by respiration are required for the vapor equilibrium values. The psychrometer technique is the more satisfactory of the 2 because it requires less time for equilibration, less tissue, and less handling of tissue. Phloem water potential of a yellow-poplar tree followed a diurnal pattern quite similar to that of leaves, except that the values were higher (less negative) and changed less than in the leaves.

The psychrometer technique permits a different approach to the study of translocation in trees. Measurements of water potential of phloem discs followed by freezing of samples and determination of osmotic potential allows estimation of turgor pressure in various parts of trees as the difference between osmotic potential and total water potential. This technique was used in evaluating gradients in water potential, osmotic potential, and turgor pressure in red maple trees. The expected gradients in osmotic potential were observed in the phloem, osmotic potential of the cell sap increasing (sap becoming more dilute) down the trunk. However, values of water potential were such that a gradient in turgor pressure apparently did not exist at a time when rate of translocation was expected to be high. These results do not support the mass flow theory of translocation favored by many workers.

     

Comparison of pressure chamber and psychrometer estimates of leaf water potential in rice

Summary The need to compare pressure-chamber estimates of leaf water potential with a psychrometric method has been established for several crop species. We investigated this relationship for rice (Oryza sativa L.) as well as the need to protect leaves from water loss during sampling and measuring period in the pressure chamber. Two rice cultivars grown in containers on a clay-loam soil were stressed to varying degrees by withholding water. Fully expanded leaves were sampled for estimation of leaf water potential by the dew point hygrometer and pressure-chamber techniques. The same leaf was used in both methods allowing direct comparison. Additionally, two alternative methods of leaf handling for measurement by the pressure chamber technique were compared. Protection of leaf samples against water loss during excision, transport and handling was found to be more important at higher leaf water potentials (>−1.0 MPa). The two cultivars used appeared to differ in their response to protection of the leaf sample. These results serve to further caution pressure chamber users on extrapolating comparisons between the two measurement methods and between tissue handling techniques even within a crop species.     

Comparison between pressure-volume and dewpoint-hygrometry techniques for determining the water relations characteristics of grass and legume leaves

Summary The water relations characteristics of three grass species (Panicum maximum var. trichoglume, Cenchrus ciliaris, Heteropogon contortus), and a legume (Macroptilium atropurpureum) grown in the field were measured using both a modified pressure/volume technique with pressure bomb measurements on single leaves and a dewpoint hygrometry technique applied to fresh and to frozen and thawed leaf discs.The two techniques agreed well in the estimates of osmotic potential at full turgor and the water potential at zero turgor. However, for parameters such as the relative water content at zero turgor, bound water and bulk modulus of elasticity there was a poor correlation between the estimates from the two methods. The pressure/volume technique gave less variable results and is more convenient for field use than the hygrometry technique. The determination of the modulus of elasticity from various functions relating pressure potential to relative water content is discussed.     

The pressure-jump technique shows maize leaf growth to be enhanced by increases in turgor only when water status is not too high

This study on expansive growth of the first leaf of maize has two goals: one is to determine how the sensitivity of growth to changes in water status varies with the initial water status of the leaf, and the other is to adapt the pressure-jump technique of Okamoto et al. (1989 , Plant and Cell Physiology 30, 979–985), developed for studying growth of excised stem segments, for use on whole seedlings. Initial water status was varied by using: transpiring vs. non-transpiring conditions, seedlings differing in emerged leaf length and hence transpiring area, and root medium without mannitol vs. medium with added mannitol (to –0·3 MPa). The results show that growth changed with changes in plant water status when the water status was low, but was unaffected when water status was very high. A stepwise change in hydrostatic pressure on the root medium was quickly and fully transmitted to the base of the leaf. The increase in leaf elongation due to a pressure step of 0·025 MPa was negligible under conditions of high plant water status and became substantial under conditions of low water status. In adapting the pressure-jump method to the whole seedling, there was some loss of resolution, and the yield threshold Y of the Lockhart equation could not be estimated directly. Nonetheless, the data were suitable for the calculation of volumetric extensibility m and the estimation of growth effective turgor (turgor above Y ). Extensibility was shown to increase 3- to 4-fold when leaf water status was reduced from the maximum to the point where elongation rate was halved, while growth effective turgor was calculated to diminish even more markedly.     

Psychrometric Field Measurement of Water Potential Changes following Leaf Excision

In situ measurement of sudden leaf water potential changes has not been performed under field conditions. A laboratory investigation involving the measurement of leaf water potential prior to and 2 to 200 minutes after excision of citrus leaves (Citrus jambhiri) showed good linear correlation (r = 0.99) between in situ leaf psychrometer and Scholander pressure chamber measurements. Following this, a field investigation was conducted which involved psychrometric measurement prior to petiole excision and 1 minute after excision. Simultaneous pressure chamber measurements were performed on neighboring leaves prior to the time of excision and then on the psychrometer leaf about 2 minutes after excision. These data indicate that within the first 2 minutes after excision, psychrometer and pressure chamber measurements were linearly correlated (r = 0.97). Under high evaporative demand conditions, the rate of water potential decrease was between 250 and 700 kilopascals in the first minute after excision. These results show that the thermocouple psychrometer can be used as a dynamic and nondestructive field technique for monitoring leaf water potential.     

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.     

A critical comparison of the pressure-probe and pressure-chamber techniques for estimating leaf-ceil turgor pressure in Kalanchoë daigremontiana

The aim of the present study was to test the accuracy of the pressure-chamber technique as a method for estimating leaf-cell turgor pressures. To this end, pressure-probe measurements of cell turgor pressure (P^cell) were made on mesophyll cells of intact, attached leaves of Kalanchoë daigremontiana. Immediately following these measurements, leaves were excised and placed in a pressure chamber for the determination of balance pressure (P^bal). Cell-sap osmotic pressure (?^cell) and xylem-sap osmotic pressure (?^xyl) were also measured, and an average cell turgor pressure calculated as P^cell=?^cell–?^xyl–P^bal. The apparent value of P^bal was positively correlated with the rate of increase of chamber pressure, and there was also a time-dependent increase associated with water loss. On expressing sap from the xylem, ?^xyl fell to a plateau value that was positively correlated with ?^cell. Correcting for these effects yielded estimates of P^bal and ?^xyl at the time of leaf excision. On average, the values of P^cell obtained with the two techniques agreed to within ±002 MPa (errors are approximate 95% confidence limits). If ?^xyl were ignored, however, the calculated turgor pressures would exceed the measured values by an average of 0.074 ± 0.012MPa, or 48% at the mean measured pressure of 0.155 MPa. We conclude that the pressure-chamber technique allows a good estimate to be made of turgor pressure in mesophyll cells of K. daigremontiana, provided that ?^xyl is included in the determination. The 1:1 relationship between the measured and calculated turgor pressures also implies that the weighted-average reflection coefficient for the mesophyll cell membranes is close to unity.     

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.

     

Acclimation of leaf growth to low water potentials in sunflower

Abstract Leaf growth is one of the most sensitive of plant processes to water deficits and is frequently inhibited in field crops. Plants were acclimated for 2 weeks under a moderate soil water deficit to determine whether the sensitivity of leaf growth could be altered by sustained exposure to low water potentials. Leaf growth under these conditions was less than in the controls because expansion occurred more slowly and for less of the day than in control leaves. However, acclimated leaves were able to grow at leaf water potentials (Ψ₁) low enough to inhibit growth completely in control plants. This ability was associated with osmotic adjustment and maintenance of turgor in the acclimated leaves. Upon rewatering, the growth of acclimated leaves increased but was less than the growth of controls, despite higher concentrations of cell solute and greater turgor in the acclimated leaves than in controls. Therefore, factors other than turgor and osmotic adjustment limited the growth of acclimated leaves at high ψ₁ Four potentially controlling factors were investigated and the results showed that acclimated leaves were less extensible and required more turgor to initiate growth than control leaves. The slow growth of acclimated leaves was not due to a decrease in the water potential gradient for water uptake, although changes in the apparent hydraulic conductivity for water transport could have occurred. It was concluded that leaf growth acclimated to low ψ₁, by adjusting osmotically, and the concomitant maintenance of turgor permitted growth where none otherwise would occur. However, changes in the extensibility of the tissue and the turgor necessary to initiate growth caused generally slow growth in the acclimated leaves.     

Dynamic Relation between Expansion and Cellular Turgor in Growing Grape (Vitis vinifera L.) Leaves

Measurements of the growth and water relations of expanding grape (Vitis vinifera L.) leaves have been used to determine the relationship between leaf expansion rate and leaf cell turgor. Direct measurement of turgor on the small (approximately 15 micrometer diameter) epidermal cells over the midvein of expanding grape leaves was made possible by improvements in the pressure probe technique. Leaf expansion rate and leaf water status were perturbed by environmentally induced changes in plant transpiration. After establishing a steady state growth rate, a step decrease in plant transpiration resulted in a rapid and large increase in leaf cell turgor (0.25 megapascal in 5 minutes), and leaf expansion rate. Subsequently, leaf expansion rate returned to the original steady state rate with no change in cell turgor. These results indicate that the expansion rate of leaves may not be strongly related to the turgor of the leaf cells, and that substantial control of leaf expansion rate, despite changes in turgor, may be part of normal plant function. It is suggested that a strictly physical interpretation of the parameters most commonly used to describe the relationship between turgor and growth in plant cells (cell wall extensibility and yield threshold) may be inappropriate when considering the process of plant cell expansion.     

Photosynthetic activity and water relations of sprouts ofPasania edulis

In order to investigate the factors causing fast growth of sprouts ofPasania edulis, photosynthetic activity and water relation characteristics of lower (mature) leaves and upper (expanding) leaves of the sprouts were compared with those of seedlings and adult trees ofP. edulis. Apparent quantum yield was generally low. Maximum photosynthetic rate was highest in the lower leaves of sprouts. Stomatal frequency was higher in sprout leaves than in seedling leaves. Osmotic potential at the water saturation point and water potential at the turgor loss point, in leaves, were higher in sprouts than in seedlings and adult trees. Symplasmic water content per unit leaf area was higher in sprouts than in seedlings. These water relation parameters in leaves indicated that sprout leaves are superior in maintaining cell turgor against water loss, but are not tolerant to water stress. In field measurements, sprout leaves showed higher stomatal conductance and transpiration rates. These results indicated that sprout leaves fully realized their high potential productivity even under field conditions. The leaf specific conductance, from the soil to the leaf, was higher in sprouts than in seedlings. Large and deep root systems of the original stumps of the sprouts may be attributed to the high leaf specific conductance.     

Pressure probe and isopiestic psychrometer measure similar turgor

Turgor measured with a miniature pressure probe was compared to that measured with an isopiestic thermocouple psychrometer in mature regions of soybean (Glycine max [L.] Merr.) stems. The probe measured turgor directly in cells of intact stems whereas the psychrometer measured the water potential and osmotic potential of excised stem segments and turgor was calculated by difference. When care was taken to prevent dehydration when working with the pressure probe, and diffusive resistance and dilution errors with the psychrometer, both methods gave similar values of turgor whether the plants were dehydrating or rehydrating. This finding, together with the previously demonstrated similarity in turgor measured with the isopiestic psychrometer and a pressure chamber, indicates that the pressure probe provides accurate measurements of turgor despite the need to penetrate the cell. On the other hand, it suggests that as long as precautions are taken to obtain accurate values for the water potential and osmotic potential, turgor can be determined by isopiestic psychrometry in tissues not accessible to the pressure probe for physical reasons.     

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.     

Effect of Cuticular Abrasion on Thermocouple Psychrometric In Situ Measurement of Leaf Water Potential

Cuticular resistance to water vapour diffusion between the substomatalcavity and the sensing psychrometer junction is a problem uniqueto leaf hygrometry. This resistance is not encountered in soilor solution hygrometry. The cuticular resistance may introduceerror in the measurement of leaf water potential. Using in situleaf hygrometers, we studied the effect of abrading the cuticleof Citrus jambhiri Lushington leaves, to reduce the diffusiveresistance. Field measurements of psychrometer water potentialwere compared with Scholander pressure chamber values for adjacentleaves. Different treatments were compared by sealing pairsof psychrometers on either side of the midrib. The time forwater vapour equilibration between the leaf and the psychrometerchamber was greater than 5 h for no abrasion. For abraded leaves,the true water potential value was obtained within an hour.After equilibration, psychrometer values compared favourablywith pressure chamber values for adjacent leaves (r > 0.97).Measured water potential for unabraded leaves did not correlatewell with corresponding pressure chamber measurements. Scanning electron micrographs indicated that the damage causedby abrading leaves for 60 s using carborundum powder (60 µmdiameter) was surface localized, with numerous scratchings ofthe leaf cuticle. The coarse abrasion treatment (aluminium oxide,75 µm diameter) resulted in fewer but larger cavitiesin the epidermis, which may explain the observed variabilityin the corresponding psychrometric measurements. Key words: Leaf water potential, Cuticular resistance, Leaf abrasion, Thermocouple psychrometer     

Declining hydraulic efficiency as transpiring leaves desiccate: two types of response

The conductance of transpiring leaves to liquid water (Kleaf) was measured across a range of steady-state leaf water potentials (Psileaf). Manipulating the transpiration rate in excised leaves enabled us to vary Psileaf in the range -0.1 MPa to less than -1.5 MPa while using a flowmeter to monitor the transpiration stream. Employing this technique to measure how desiccation affects Kleaf in 19 species, including lycophytes, ferns, gymnosperms and angiosperms, we found two characteristic responses. Three of the six angiosperm species sampled maintained a steady maximum Kleaf while Psileaf remained above -1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in Kleaf. In all other species measured, declining Psileaf led to a proportional decrease in Kleaf, such that midday Psileaf of unstressed plants in the field was sufficient to depress Kleaf by an average of 37%. It was found that maximum Kleaf was strongly correlated with maximum CO2 assimilation rate, while Kleaf = 0 occurred at a Psileaf slightly less negative than at leaf turgor loss. A strong linear correlation across species between Psileaf at turgor loss and Psileaf at Kleaf = 0 raises the possibility that declining Kleaf was related to declining cell turgor in the leaf prior to the onset of vein cavitation. The vulnerability of leaves rehydrating after desiccation was compared with vulnerability of leaves during steady-state evaporation, and differences between methods suggest that in many cases vein cavitation occurs only as Kleaf approaches zero.     

The Relationship Between Leaf Thickness and Plant Water Potential

Leaf thickness was continuously measured in a wide range ofenvironments using a new type of displacement transducer whichis easy to set-up and automatically compensates for the effectsof temperature. Simultaneous measurements were made of waterpotential using either a psychrometer attached to the leaf petioleor a leaf pressure chamber. Thickness of leaves was a sensitiveindicator of plant water status but calibrations against anindependent method were necessary in every plant for accurateestimates of water potential. The relationship between leafthickness changes and water potential, measured in detachedleaves, was usually curvilinear and was strongly influencedby leaf age, stress history and, in young leaves, by the effectsof leaf growth. Leaf thickness growth was absent in mature cabbageleaves. Key words: Leaf thickness, plant water potential, psychrometer     

In situ field measurement of leaf water potential using thermocouple psychrometers

Thermocouple psychrometers are the only instruments which can measure the in situ water potential of intact leaves, and which can possibly be used to monitor leaf water potential. Unfortunately, their usefulness is limited by a number of difficulties, among them fluctuating temperatures and temperature gradients within the psychrometer, sealing of the psychrometer chamber to the leaf, shading of the leaf by the psychrometer, and resistance to water vapor diffusion by the cuticle when the stomates are closed. Using Citrus jambhiri, we have tested several psychrometer design and operational modifications and showed that in situ psychrometric measurements compared favorably with simultaneous Scholander pressure chamber measurements on neighboring leaves when the latter were corrected for the osmotic potential.     

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.

     

Water Potential-Water Content Relationships In Apple Leaves

Three methods for determining the relationship between xylempressure potential as measured in a pressure chamber (an estimateof leaf water potential) and leaf relative water content werecompared for apple leaves. A range of leaf water contents wasobtained either by sampling leaves in the field at differenttimes of day and on days with differing evaporative demand,or by allowing evaporation from excised leaves in the laboratory,or by expressing sap by overpressurization in a pressure chamber.The first two methods gave very similar results, but the lasttended to give rather lower water potentials at any given watercontent. A possible explanation for these results and theirimplications for the estimation of osmotic potentials usingpressure-volume curves are discussed. Some osmotic adjustmentwas observed in trees droughted for 3 months, with estimatedosmotic potentials, both at full turgor and zero turgor, beingnearly 0.3 MPa lower than in irrigated controls.