Seasonal patterns of leaf water relations in four co-occurring forest tree species: Parameters from pressure-volume curves

Summary Leaf water relationships were studied in four widespread forest tree species (Ilex opaca Ait., Cornus florida L., Acer rubrum L., and Liriodendron tulipifera L.). The individuals studied all occurred on the same site and were selected to represent a range of growth forms and water relationships in some of the principal tree species of the region. The water relations of the species were analyzed using the concept of the water potential-water content relationship. The pressure-volume method was used to measure this relationship using leaf material sampled from naturally occurring plants in the field. Water potential components (turgor, osmotic, and matric) were obtained by analysis of the pressure-volume curves.Initial osmotic potentials (the value of the osmotic component at full turgidity) were highest (least negative) at the start of the growing season. They decreased (becoming progressively more negative) as the season progressed through a drought period. Following a period of precipitation at the end of the drought period, initial osmotic potentials increased toward the values measured earlier in the season.Seasonal osmotic adjustments were sufficient in all species to allow maintenance of leaf turgor through the season, with one exception: Acer appeared to undergo some midday turgor loss during the height of the July drought period.In addition to environmental influences, tissue stage of development played a role; young Ilex leaves had higher early season initial osmotic potentials than overwintering leaves from the same tree.The seasonal pattern of initial osmotic potential in Liriodendron and the observed pattern of leaf mortality suggested a possible role of osmotic potentials in the resistance of those leaves to drought conditions. The fraction of total leaf water which is available to affect osmotic potentials, called the osmotic water fraction in this study, was greatest in young tissue early in the season and declined as the season progressed.The results of this study showed that the water potential-water content relationship represents a dynamic mechanism by which plant internal water relations may vary in response to a changing external water-availability regime. The measured water relationships confirmed the relative positions of the species along a water-availability gradient, with Cornus at the wettest end and Ilex at the driest end of the gradient. Acer and Liriodendron were intermediate in their water relations. The spread of these species along a water-availability gradient on the same site suggested that coexistence is partially based on differential water use patterns.     

Growth Rate and Water Relations of Citrus Leaf Flushes

Growth rates of seasonal leaf flushes of ‘Valencia’orange [Citrus sinensis (L.) Osbeck] were measured and waterrelations characteristics of young (new) and over-wintered (old)citrus leaves were compared. New flush leaves had lower specificleaf weights and lower midday leaf water potentials than comparablyexposed old leaves. Spring and summer flush new leaves had higherosmotic potentials than old leaves. These differences becamenon-significant as the new leaves matured. During summer conditions,water-stressed new leaves reached zero turgor and stomatal conductancealso began to decrease in them at higher leaf water potentialsthan in old leaves. Old leaves were capable of maintaining openstomata at lower leaf water potentials. Opened flowers and newflush leaves lost more water, on a dry weight basis, than flowerbuds, fruit or mature leaves. The results illustrate differencesin leaf water potential and stomatal conductance which can beattributed to the maintenance of leaf turgor by decreases inleaf osmotic potentials as leaves mature. These changes in citrusleaf water relations are especially important since water stressresulting from high water loss rates of new tissues could reduceflowering and fruit set. Citrus sinensis (L.) Osbeck, orange, Citrus paradisi Macf., grapefruit, growth rate, leaf water relations, osmotic potential, water potential, stomatal conductance     

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.     

Grasstree (Xanthorrhoea preissii ) leaf growth in relation to season and water availability

Abstract Water stress usually arrests growth of even the most deep‐rooted species during summer drought in Mediterranean‐type climates. However, scant evidence suggests that grasstrees may represent an unusual exception. We used weather data and plant water potential to investigate the relationship between leaf growth and season in the grasstree, Xanthorrhoea preissii Endl. (Xanthorrhoeaceae). Leaf production in two contrasting habitats revealed continuous annual growth, oscillating between maximum rates (2.5–3.2 leaves/d) in late‐spring to autumn, to a minimum rate of 0.5 leaf/d during winter but never stopping. While the rate of leaf production during the fast‐growth season was positively correlated with temperature above 17–18°C, leaf elongation commenced substantially earlier in the year (from 12°C). Leaf water potentials cycled annually, with predawn readings commonly measured as zero during winter–spring and as low as ?1.26 MPa during summer, but never indicating stress by exceeding the turgor loss point. Leaf death was synchronized with summer drought. The fast (summer) growth period was characterized by rapidly fluctuating leaf production, particularly in banksia woodland, where plant growth reliably responded quickly to >18 mm of rainfall. Within 24 h of 59 mm of simulated rainfall, grasstrees in banksia woodland showed a marked increase in water potential, and leaf production reached 7.5 times the controls, confirming their capacity to respond to temporary spasmodic summer rains. Rainfall was the best climatic variable for predicting woodland grasstree leaf production during summer, whereas leaf production of forest grasstrees was most closely correlated with daylength. This plastic response of grasstrees between seasonal weather extremes is relatively rare among other mediterranean floras, and has implications for a recently proposed technique for ageing grasstrees.     

Tissue water relations of four co-occurring chaparral shrubs

Summary Chaparral shrubs of California have a suite of morphological and physiological adaptations to withstand the prolonged summer droughts of a mediterranean climate. Not all species of chaparral have the same rooting depth and there is some evidence that those with shallow roots have tissue that is most tolerant to water stress. We tested this notion by comparing the tissue water relations of four co-occurring chaparral shrubs: Quercus durata, Heteromeles arbutifolia, Adenostoma fasciculatum, and Rhamnus californica. We used a pressure-volume technique and a dew-point hygrometer to metsure seasonal changes in osmotic potential when plant tissue was fully hydrated and osmotic potential at predawn, midday, and the turgor loss point. We also calculated seasonal changes in the minimum daily turgor potential, saturated weight/dry weight ratio of leaf tissue, and the bulk modulus of elasticity. We had information on the seasonal water use patterns and apparent rooting depths of these same four shrubs from a previous study (Davis and Mooney 1986). All evidence indicated that Rhamnus had shallow roots and Quercus deep roots. Our results indicated that the tissue water relations of our four co-occurring chaparral shrubs were not alike. Even though Rhamnus had shallow roots, it had the least xerophytic tissue. Seasonal osmotic potential and saturated weight/dry weight ratios were relatively high and bulk modulus of elasticity and minimum daily turgor potentials were low. Furthermore, even though Quercus had deep roots and experienced no seasonal water stress at our study site, its tissue water relations indicated relatively high tolerance to water stress. We conclude that seasonal drought tolerance of stem and leaf tissue of co-occurring chaparral shrubs does not necessarily correspond to rooting depth, to soil moisture resources available to the shrub, or to the degree of seasonal water stress experienced by the shrub.     

Water relations of compound leaves and phyllodes in Acacia koa var. latifolia

Abstract Moisture release characteristics and field measurements of physiological parameters (conductance and water potential) and environmental parameters (ambient temperature, water vapour saturation deficit and photosynthetic photon flux density) were measured for phyllodes and compound leaves of Acacia koa over a 2 month period at Hawaii Volcanoes National Park, Hawaii, to determine what differences in water relations might occur between leaf types. The phyllodes were found to contain more water at full turgor, use less water in turgor control and have stomatal conductances more closely associated with bulk leaf water status and environmental variables. These results suggest that the phyllodes are more drought adapted, whereas the compound leaves probably promote more rapid early growth during periods of high moisture availability.     

DIFFERENCES IN LEAF STRUCTURE,CHLOROPHYLL, AND NUTRIENTS FOR THE UNDERSTORY TREE ASIMINA TRILOBA

To investigate differences in leaf structure, chlorophyll and nutrients on terminal branches of the understory tree Asimina triloba, the first (proximal) and the last (distal) leaves to develop in the spring were compared. Proximal leaf expansion was completed before the overstory canopy was fully closed but distal leaf expansion occurred during and after the development of the overstory canopy. Fully expanded proximal leaves were 76% smaller in area, were 18% thicker and had 36% more stomates per m of leaf area when compared to distal leaves. In addition, maximum stomatal conductance to water vapor was greater (150 vs. 120 mmol m^−-2s^−-1) and the minimum PPFD required for maximum conductance was higher (200 vs. 150 μmol m^−-2s^−-1) for the proximal leaves. Chlorophyll content was also greater for proximal leaves, but nitrogen and phosphorus contents were lower throughout the entire summer. Seasonal measurements indicated an increase in chlorophyll a content and reductions in nitrogen content throughout the summer growth period for leaves from both positions. The results suggest that distal and proximal leaves differed physiologically and that the measured differences were related to the changing irradiance environment during leaf development. The time of leaf expansion, as indicated by leaf position on the branch, may be an important consideration when examining the water and photosynthetic relations of understory trees.     

Seasonal variation in the tissue water relations of Picea glauca

Seasonal variation in water relations of 3-yearold white spruce (Picea glauca (Moench) Voss) shoots, monitored with pressure-volume curves over 28 months, was closely related to shoot phenology and was sensitive to environmental fluctuations during both summer growth and winter dormancy. Turgor maintenance capacity was lowest during rapid shoot elongation from late May to early July; this was indicated by the lowest total turgor pressures, the highest (least negative) osmotic potentials at full turgor and the turgor loss point, the smallest differences between osmotic potentials at full turgor and the turgor loss point, the highest relative water contents at turgor loss and a linear decline in cell elasticity with decreasing turgor pressure. This suggests that the high susceptibility of white spruce seedlings to growth check after transplanting is largely attributable to the poor turgor maintenance capacity of this species in early summer.     

Solute accumulation in leaves and roots of woody plants subjected to water stress

Summary Young seedlings of English Oak, Quercus robur L., and Silver Birch, Betula verrucosa Ehrl., were subjected to a number of consecutive periods during which water was withheld. During one 14-day period leaf-and soil-water potentials and leaf- and root-solute potentials of two groups of plants were sampled at noon of each day. One group of plants was watered every day while water was withheld from the other group. Solute accumulation in roots and leaves of oak seedlings subjected to water stress resulted in maintenance of turgor and high leaf conductance as the soil dried. In birch seedlings turgor was only maintained by stomatal closure at high soil water potential.Fourteen consecutive water stress cycles greatly reduced the growth of birch seedlings but had little effect on oak seedlings other than to alter root morphology. Water stress treatment resulted in the production of long thin roots in this plant. Stomatal behaviour in oak and birch seedlings during the 14-week stress period was consistent with observed changes in leaf water and solute potentials. Daily solute accumulation in oak leaves was presumably responsible for the maintenance of plant growth as water potentials fell.     

Aspects of tissue water relations and seasonal changes of leaf water potential components of evergreen and deciduous species coexisting in tropical dry forests

Summary This study compared the tissue water relations and seasonal changes in leaf water potential components of an evergreen tree,Morisonia americana, and two evergreen shrubs,Capparis verrucosa andC. aristiquetae, with two deciduous trees,Humboltiella arborea andLonchocarpus dipteroneurus, and the deciduous vineMansoa verrucifera. All these species coexist in a tropical dry forest in Venezuela. Leaves of the evergreen species are sclerophyllous, while those of the deciduous species are mesophytic. Leaf area to leaf weight ratios of fully mature leaves were about 75 and 170 cm² g^–1 in evergreen and deciduous species, respectively. Seasonal fluctuations of leaf water content per unit of dry weight, water potential, and turgor pressure were smaller in evergreen than in deciduous species. The analysis of tissue water relations using pressurevolume curves showed that evergreen species could develop a higher leaf turgor and lose turgor at lower leaf water potentials than deciduous species. This was related to a lower osmotic potential at full turgor in evergreen (-3.0 MPa)_than in deciduous (-2.0 MPa) species, rather than to the elastic properties of leaf tissue. The volumetric modulus of elasticity was 14 MPa in evergreen compared with 7–10 MPa in deciduous species. Thus, leaf characteristics are important in determining the drought resistance of evergreen species of this tropical dry forest.     

Environmental control of canopy dynamics and photosynthetic rate in the evergreen tussock grass Stipa tenacissima

Seasonal changes in leaf demography and gas exchange physiology in the tall evergreen tussock grass Stipa tenacissima, one of the few dominant plant species in the driest vegetation of Europe, were monitored over a period of two years at a field site in semi-arid south-eastern Spain. Three age-classes of leaves – young, mature and senescent – were distinguished in the green canopy. Production of new leaves and extension growth of older leaves occurred exclusively from October–November to May–June. The rate of extension was significantly correlated with gravimetric soil water content. Leaf growth ceased after gravimetric soil water content fell below 0.015 g g^–1 at the beginning of the dry season which corresponded to pre-dawn leaf water potentials of -3.0 MPa. Leaf senescence and desiccation reduced green leaf area by 43–49% during the dry season. Diurnal changes in the net photosynthetic rate of all three cohorts of leaves were bimodal with an early morning maximum, a pronounced midday depression and a small recovery late in the afternoon. Maximum photosynthetic rates of 10–16 mol CO₂ m^–2 s^–1 were attained from November 1993 to early May 1994 in young and mature leaves. Photosynthetic rate declined strongly during the dry season and was at or below compensation in September 1994. Gas exchange variables of young and mature leaves were not significantly different, but photosynthetic rate and diffusive conductance to water vapour of senescing leaves were significantly lower than in the two younger cohorts. Leaf nitrogen content of mature leaves varied seasonally between 2.9 and 5.2 g m^–2 (based on projected area of folded leaves), but was poorly correlated with maxima of the photosynthetic rate. There was a stronger linear relationship between the daily maxima of leaf conductance and pre-dawn leaf water potential than with atmospheric water vapour saturation deficit. Seasonal and between-year variation in daily carbon assimilation were caused mainly by differences in climatic conditions and canopy size whereas the effect of age structure of canopies was negligible. Since water is the most important limiting factor for growth and reproduction of S. tenacissima, any future rise in mean temperature, which might increase evapotranspiration, or decrease in rainfall, may considerably reduce the productivity of the grasslands, particularly at the drier end of their geographical distribution.     

Effects of nursery preconditioning through mycorrhizal inoculation and drought in Arbutus unedo L. plants

The influence of a water deficit treatment and mycorrhizal inoculation with Pisolithus tinctorius (Pers.) Coker and Couch on the water relations, gas exchange, and plant growth in Arbutus unedo L. plants was studied in order to evaluate the hardening process during the nursery period. The ability to withstand the adverse conditions after transplantation was also studied. Mycorrhizal and non-mycorrhizal seedlings of A. unedo were pot-grown for 4 months in a greenhouse (nursery period), during which time two irrigation treatments, well watered (100% water holding capacity, leaching 20% of the applied water) and deficit irrigation (50% of the well watered), were applied. Subsequently, the plants were transplanted to the field and well irrigated (transplanting period), after which and until the end of the experiment they received no water (establishment period). At the end of the nursery period, both water deficit and mycorrhizae were seen to have altered the plant morphology. Mycorrhizal plants had lower leaf area and improved leaf color parameters, while the water deficit increased root dry weight and the root/shoot ratio. Mycorrhizal plants had higher leaf water potential values than non-inoculated plants. Mycorrhizae increased stomatal conductance and photosynthesis values, especially in stressed plants. Drought led to an osmotic adjustment and a decrease in the leaf water potential values at turgor loss point in the mycorrhizal plants. Cell wall rigidity, measured as increased bulk modulus of elasticity, was decreased by the mycorrhizae effect. After transplanting, no differences were found in the water relations or gas exchange values between treatments. During the establishment period, the plants that had been exposed to both drought and mycorrhizae showed a better water status (higher leaf water and turgor potential values) and higher gas exchange values. In conclusion, water deficit and mycorrhizal inoculation of A. unedo plants in nursery produced changes in tissue water relations, gas exchange, and growth, related with the acclimation process in the seedlings, which could provide better resistance to drought and stress conditions following planting.     

Leaf age and salinity influence water relations of pepper leaves

Plant growth is reduced under saline conditions even when turgor in mature leaves is maintained by osmotic adjustment. The objective of this study was to determine if young leaves from salt-affected plants were also osmotically adjusted. Pepper plants (Capsicum annuum L. cv. California Wonder) were grown in several levels of solution osmotic potential and various components of the plants' water relations were measured to determine if young, rapidly growing leaves could accumulate solutes rapidly enough to maintain turgor for normal cell enlargement. Psychrometric measurements indicated that osmotic adjustment is similar for both young and mature leaves although osmotic potential is slightly lower for young leaves. Total water potential is also lower for young leaves, particularly at dawn for the saline treatments. The result is reduced turgor under saline conditions at dawn for young but not mature leaves. This reduced turgor at dawn, and presumably low night value, is possibly a cause of reduced growth under saline conditions. No differences in leaf turgor occur at midday. Porometer measurements indicated that young leaves at a given salinity level have a higher stomatal conductance than mature leaves, regardless of the time of day. The result of stomatal closure is a linear reduction of transpiration.     

Does turgor limit growth in tall trees?

The gravitational component of water potential contributes a standing 0.01 MPa m^?1 to the xylem tension gradient in plants. In tall trees, this contribution can significantly reduce the water potential near the tree tops. The turgor of cells in buds and leaves is expected to decrease in direct proportion with leaf water potential along a height gradient unless osmotic adjustment occurs. The pressure–volume technique was used to characterize height‐dependent variation in leaf tissue water relations and shoot growth characteristics in young and old Douglas‐fir trees to determine the extent to which growth limitation with increasing height may be linked to the influence of the gravitational water potential gradient on leaf turgor. Values of leaf water potential (Ψ_l), bulk osmotic potential at full and zero turgor, and other key tissue water relations characteristics were estimated on foliage obtained at 13.5 m near the tops of young (approximately 25‐year‐old) trees and at 34.7, 44.2 and 55.6 m in the crowns of old‐growth (approximately 450‐year‐old) trees during portions of three consecutive growing seasons. The sampling periods coincided with bud swelling, expansion and maturation of new foliage. Vertical gradients of Ψ_l and pressure–volume analyses indicated that turgor decreased with increasing height, particularly during the late spring when vegetative buds began to swell. Vertical trends in branch elongation, leaf dimensions and leaf mass per area were consistent with increasing turgor limitation on shoot growth with increasing height. During the late spring (May), no osmotic adjustment to compensate for the gravitational gradient of Ψ_l was observed. By July, osmotic adjustment had occurred, but it was not sufficient to fully compensate for the vertical gradient of Ψ_l. In tall trees, the gravitational component of Ψ_l is superimposed on phenologically driven changes in leaf water relations characteristics, imposing potential constraints on turgor that may be indistinguishable from those associated with soil water deficits.     

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.     

WATER RELATIONS AND GROWTH CHARACTERISTICS OF PROSOPIS GLANDULOSA VAR. TORREYANA IN A SIMULATED PHREATOPHYTIC ENVIRONMENT

Recent studies of Prosopis glandulosa have demonstrated a unique system of a deeply rooted species with significant water stress tolerance. Several growth and developmental characteristics have been correlated with water stress and nitrogen availability during field studies. Here we present a lab experiment in which a phreatophytic regime is simulated and the availability of nitrogen and water are varied. Increased ground water salinity caused lower plant water potentials and greater osmotic adjustment without significant increases in leaf Na⁺ concentrations. Leaf conductance was higher in the higher salinity treatments. Low water potential was also associated with reduced leaf size, reduced leaf area per plant and increased root to shoot ratio. Specific leaf weight and the transpiration ratio were unaffected by the low water potentials induced by increased salinity. Increasing nitrogen availability caused increased growth rates but did not influence water use efficiency. Net assimilation rates increased with increasing nitrogen availability but relative growth rates were more dependent on overall plant size than treatment conditions. The responses of P. glandulosa to the simulated phreatophytic environment were similar to those predicted by field measurements.     

Osmotic relations of the drought-tolerant shrub Artemisia tridentata in response to water stress

Turgor maintenance, solute content and recovery from water stress were examined in the drought-tolerant shrub Artemisia tridentata. Predawn water potentials of shrubs receiving supplemental water remained above ?2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to ?5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustment (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentration of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments.     

Leaf water relations characteristics of differently potassium fertilized and watered field grown barley plants

Seasonal leaf water relations characteristics were studied in fully irrigated spring barley (Hordeum distichum L. cv. Gunnar) fertilized at low (50 kg K ha⁻¹) or high (200 kg K ha⁻¹) levels of potassium applied as KCl. The investigation was undertaken from about 14 days before anthesis until the milk ripe stage in leaves of different position and age. Additionally, the effects of severe water stress on leaf water relations were studied in the middle of the grain filling period in spring barley (cv. Alis). The leaf water relations characteristics were determined by the pressure volume (PV) technique. Water relations of fully irrigated plants were compared in leaf No 7 with the water relations of slowly droughted plants (cv. Alis). Leaf osmotic potential at full turgor (ψ _π ¹⁰⁰ ) decreased 0.1 to 0.3 MPa in droughted leaves indicating a limited osmotic adjustment due to solute accumulation. The leaf osmotic potential at zero turgor (ψ _π ⁰ ) was about −2.2 MPa in fully irrigated plants and −2.6 MPa in droughted plants. The relative water content at zero turgor (R⁰) decreased 0.1 unit in severely droughted leaves. The ratio of turgid leaf weight to dry weight (TW/DW) tended to be increased by drought. The tissue modulus of elasticity (ε) decreased in droughted plants and together with osmotic adjustment mediated turgor maintenance during drought. A similar response to drought was found in low and high K plants except that the R⁰ and ε values tended to be higher in the high K plants. Conclusively, during drought limited osmotic adjustment and increase in elasticity of the leaf tissue mediated turgor maintenance. These effects were only slightly modified by high potassium application. The seasonal analysis in fully irrigated plants (cv. Gunnar) showed that within about 14 days from leaf emergence ψ _π ¹⁰⁰ decreased from about −0.9 to −1.6 MPa in leaf No 7 (counting the first leaf to emerge as number one) and from about −1.1 to −1.9 MPa in leaf No 8 (the flag leaf) due to solute accumulation. A similar decrease took place in ψ _π ⁰ except that the level of ψ _π ⁰ was displaced to a lower level of about 0.2 to 0.3 MPa. Both ψ _π ¹⁰⁰ and ψ _π ⁰ tended to be 0.05 to 0.10 MPa lower in high K than in low K plants. R⁰ was about 0.8 to 0.9 and was independent of leaf position and age, but tended to be highest in high K plants. The TW/DW ratio decreased from about 5.5 in leaf No 6 to 4.5 in leaf No 7 and 3.8 in leaf No 8. The TW/DW ratio was 4 to 10% higher in high K than in low K plants indicating larger leaf cell size in the former. The apoplastic water content (V_a) at full turgor constituted about 15% in leaf No 7. ε was maximum at full turgor and varied from about 11 to 34 MPa. ε tended to be higher in high K plants. Conclusively, in fully watered plants an ontogenetically determined accumulation of solutes (probably organic as discussed) occurred in the leaves independent of K application. The main effect of high K application on water relations was an increase in leaf water content and a slight decrease in leaf ψ_π. The effect of K status on growth and drought resistance is discussed.     

Turgor and osmotic relations of the desert shrub Larrea tridentata

Abstract Leaf water relations characteristics of creosote bush, Larrea tridentata, were studied in view of previous reports that its leaves commonly experience zero or negative turgor under dry conditions. Leaf turgor loss point () was determined by a pressure-volume method for samples subjected to a hydration procedure and for untreated samples. Hydration caused to increase by as much as 3 M Pa. Hydration of samples also caused changes in other leaf water relations characteristics such as symplastic solute content, tissue elasticity and symplasmic water fraction, but total leaf solute content was unchanged. Comparison of our field plant water potential data with values of obtained by the two methods resulted in predictions of turgor loss during part or all of a diurnal cycle based on hydrated samples, and turgor maintenance (at least 0.3 MPa) based on untreated samples. Pooled data for obtained from both partially hydrated and untreated samples showed that L. tridentata maintains fairly constant levels of turgor over a wide range of leaf water potential. Dilution of cell contents by apoplastic water introduced significant errors in psychrometric determinations of osmotic potential in both frozen and thawed leaf tissue and expressed cell sap. Use of these values of osmotic potential resulted in predictions of zero turgor at all plant water potentials measured in the field.     

Environmental and genetic control of phenotypic adaptation to drought in Potentilla glandulosa Lindl.

Summary Two populations of Potentilla glandulosa (Rosaceae) have strongly contrasting abilities to seasonally modify their phenotypic sensitivity to drought by low temperature-induced changes in leaf morphology. One population is native to an inland continental climate with unpredictable droughts occurring at any time during the year. The second population is native to a coastal mediterranean climate with a highly predictable annual cycle of winter rain and summer drought. In response to low temperatures in the autumn, the inland plants produce a compact rosette of small leaves and shed their large summer leaves, thereby reducing the total plant leaf area. The inland plants begin growth in the spring in the compact rosette phenotype and are much less sensitive to drought, maintaining higher values of stomatal conductance and leaf water potential, than when in the largeleaved summer phenotype. The coastal plants do not exhibit the low temperature-induced change in leaf morphology and are relatively sensitive to drought when grown at high or low temperatures. The F₁ hybrids of a coastal x inland cross exhibit an intermediate response to low temperature and drought. In the F₂ generation the inland parental class of individuals fully responding to low temperature segregates in a 1:63 ratio.