Water potential — water saturation deficit relationship during dehydration and resaturation of leaves

The relationship between the water potential (Ψ_w) and the water saturation deficit (Δ W _sat) in kale and maize leaf tissue was measured during dehydration and resaturation either of leavesin situ or of cut leaves. The curves relating Ψ_w toΔW _sat were similar in all variants, but at the same values ofΔ W _sat corresponding values of Ψ_w were always lower in leavesin situ than in cut leaves and during dehydration than during resaturation.     

Water loss regulation to soil drought associated with xylem vulnerability to cavitation in temperate ring-porous and diffuse-porous tree seedlings

Key message

Sustainable stomatal opening despite xylem cavitation occurs in ring-porous species and stomatal closure prior to cavitation in diffuse-porous species during soil drought.

Abstract

To elucidate the relationship between water loss regulation and vulnerability to cavitation associated with xylem structure, stomatal conductance (g _s), defoliation, vulnerability curves, and vessel features were measured on seedlings of ring-porous Zelkova serrata and Melia azedarach, and diffuse-porous Betula platyphylla var. japonica, Cerasus jamasakura and Carpinus tschonoskii. Under prolonged drought conditions, the percentage loss of hydraulic conductivity (PLC) increased and g _s decreased gradually with decreasing predawn (Ψ_pd) or xylem water potential (Ψ_xylem) in Z. serrata. During the gentle increase of PLC in M. azedarach, g _s increased in the early stages of dehydration while leaves were partly shed. A sharp reduction in g _s was observed before the onset of an increase in the PLC for drying plants of the three diffuse-porous species, suggesting cavitation avoidance by stomatal regulation. In the ring-porous species, xylem-specific hydraulic conductivity (K _s) was higher, whereas the vessel multiple fractions, the ratio of the number of grouped vessels to total vessels, was lower than that in the diffuse-porous species, suggesting that many were distributed as solitary vessels. This may explain the gradual increase in the PLC with decreasing Ψ_xylem because isolated vessels provide less opportunity for air seeding. Different water loss regulation to soil drought was identified among the species, with potential mechanisms being sustainable gas exchange at the expense of xylem dysfunction or partial leaf shedding, and the avoidance of xylem cavitation by strict stomatal regulation. These were linked to vulnerability to cavitation that appears to be governed by xylem structural properties.     

Water potential in soil and Atriplex nummularia (phytoremediator halophyte) under drought and salt stresses

Atriplex nummularia is a halophyte widely employed to recover saline soils and was used as a model to evaluate the water potentials in the soil-plant system under drought and salt stresses. Potted plants grown under 70 and 37% of field capacity irrigated with solutions of NaCl and of a mixture of NaCl, KCl, MgCl₂ and CaCl₂ reproducing six electrical conductivity (EC): 0, 5, 10, 20, 30, and 40 dS m^?1. After 100 days, total water (Ψ_{w, plant}) and osmotic (Ψ_{o, plant}) potentials at predawn and midday and Ψ_{o, soil}, matric potential (Ψ_{m, soil}) and Ψ_{w, soil} were determined. The type of ion in the irrigation water did not influence the soil potential, but was altered by EC. The soil Ψ_o component was the largest contributor to Ψ_{w, soil}. Atriplex is surviving ECs close to 40 dS m^?1 due to the decrease in the Ψ_w. The plants reached a Ψ_w of approximately ?8 MPa. The water potentials determined for different moisture levels, EC levels and salt types showed huge importance for the management of this species in semiarid regions and can be used to recover salt affected soils.     

Does pre-dawn water potential reflect conditions of equilibrium in plant and soil water status?

Variation in base water potential (Ψ_b, a daily maximum level of plant water potential, which is presumed to correspond to the equilibrium between soil and plant water potentials) was examined in shoots of Picea abies and Vaccinium myrtillus with respect to soil (available water storage, water potential, temperature) and atmospheric (temperature, relative humidity, vapour pressure deficit) conditions. The available soil water storage (W_tr) accounted for 77% of the dynamics of Ψ_b, while the influence of atmospheric factors became evident under high evaporative demand. Ψ_b was not always observable immediately before dawn, but on 30% of observation days, the recovery continued up to an hour or two after dawn. Full equilibrium between soil and plant water potentials in P. abies in northern conditions is rather improbable by dawn in summer-time, because of the shortness of the dark period and probable night-time transpiration in the case of high atmospheric vapour pressure deficit.     

Xylem cavitation vulnerability influences tree species’ habitat preferences in miombo woodlands

Although precipitation plays a central role in structuring Africa’s miombo woodlands, remarkably little is known about plant-water relations in this seasonally dry tropical forest. Therefore, in this study, we investigated xylem vulnerability to cavitation for nine principal tree species of miombo woodlands, which differ in habitat preference and leaf phenology. We measured cavitation vulnerability (Ψ₅₀), stem-area specific hydraulic conductivity (K _S), leaf specific conductivity (K _L), seasonal variation in predawn water potential (Ψ_PD) and xylem anatomical properties [mean vessel diameter, mean hydraulic diameter, mean hydraulic diameter accounting for 95 % flow, and maximum vessel length (V _L)]. Results show that tree species with a narrow habitat range (mesic specialists) were more vulnerable to cavitation than species with a wide habitat range (generalists). Ψ₅₀ for mesic specialists ranged between ?1.5 and ?2.2 MPa and that for generalists between ?2.5 and ?3.6 MPa. While mesic specialists exhibited the lowest seasonal variation in Ψ_PD, generalists displayed significant seasonal variations in Ψ_PD suggesting that the two miombo habitat groups differ in their rooting depth. We observed a strong trade-off between K _S and Ψ₅₀ suggesting that tree hydraulic architecture is one of the decisive factors setting ecological boundaries for principal miombo species. While vessel diameters correlated weakly (P > 0.05) with Ψ₅₀, V _L was positively and significantly correlated with Ψ₅₀. Ψ_PD was significantly correlated with Ψ₅₀ further reinforcing the conclusion that tree hydraulic architecture plays a significant role in species’ habitat preference in miombo woodlands.     

Effects of elevated CO2, drought and temperature on the water relations and gas exchange of groundnut (Arachis hypogaea) stands grown in controlled environment glasshouses

Stands of groundnut (Arachis hypogaea L. cv. Kadiri‐3) were grown in controlled environment glasshouses at mean atmospheric CO₂ concentrations of 375 or 700 μmol mol^?1 and daily mean air temperatures of 28 or 32°C on irrigated or drying soil profiles. Leaf water (Ψ_l) and solute potential (Ψ_s), relative water content (RWC), stomatal conductance (g_l) and net photosynthesis (P_n) were measured at midday for the youngest mature leaf throughout the growing season. Elevated CO₂ and temperature had no detectable effect on the water relations of irrigated plants, but higher values of RWC, Ψ_l and Ψ_s were maintained for longer under elevated CO₂ during progressive drought. Turgor potential (Ψ_p) reached zero when Ψ_l declined to ?1.6 to ?1.8 MPa in all treatments; turgor was lost sooner when droughted plants were grown under ambient CO₂. A 4°C increase in mean air temperature had no effect on Ψ_s in droughted plants, but elicited a small increase in Ψ_l; midday g_l values were lower under elevated than under ambient CO₂, and Ψ_l and g_l declined below ?1.5 MPa and 0.25 cm s^?1, respectively, as the soil dried. Despite the low g_l values recorded for droughted plants late in the season, P_n was maintained under elevated CO₂, but declined to zero 3 weeks before final harvest under ambient CO₂. Concurrent reductions in g_l and increases in water use efficiency under elevated CO₂ prolonged photosynthetic activity during drought and increased pod yields relative to plants grown under ambient CO₂. The implications of future increases in atmospheric CO₂ for the productivity of indeterminate C₃ crops grown in rainfed subsistence agricultural systems in the semi‐arid tropics are discussed.     

Population differentiation inSpartina patens: Water potential components and bulk modulus of elasticity

Pressure-volume technique was utilized to evaluate salinity response among three populations ofSpartina patens (Ait.) Muhl. from Louisiana Gulf coast marshes. Plants were subjected to salinities of 85 and 425 mol m^?3 for 77 d in a greenhouse. Ψ_w and Ψ_π decreased in all populations in response to increases in salinity. There were 32% decrease in Ψ_sat, 42% decrease in Ψ_tlp in response to salinity changes from 85 to 425 mol m^?3 in the Ferblanc population. Similarly, there were 35% and 41% decrease in Ψ_sat in the Clovelly and Lake Tambour populations, respectively. All populations showed the ability to adapt to the increased salinity as was evidenced by osmotic adjustment. However, the Lake Tambour population appeared to have superior ability to adapt to high salinity through having a significantly lower osmotic potential at saturation (Ψ_sat), osmotic potential at turgor loss point (Ψ_tlp), and maximum turgor potential (Ψ_P(max)) compared to other populations. Ferblanc and Clovelly populations revealed the ability to adapt to saline environments to a lesser extent as compared to the Lake Tambour population. Results indicate that there is a potential for selection of superior strains ofSpartina patens for use in marsh restoration projects aiming at prevention of wetland loss in certain coastal areas.     

A Simple model of the leaf daily water potential dynamics of some forest tree species

A simple model was developed to characterize the daily water potential dynamics (Ψ_x of sun and shade leaves of three forest tree species (Quercus cerris, Acer campestre andCarpinus betulus) under anticyclonic weather types. Input data used for this-model were the vapour pressure deficit (d) and the soil moisture content (w.). The model is usable for the calculation of the actual Ψ_x-values with a probable error 0.18 –0.28 MPa and limits the maximum and minimum Ψ_x-values which may occur with the particular tree species. The model makes it possible to establish for each species the regime, determined byd andw, at which the water potential of the leaves reacts most sensitively to the changes of the environmental parameters.     

Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis

The extent to which stomatal conductance (g_s) was capable of responding to reduced hydraulic conductance (k)and preventing cavitation-inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. From 1 to 18 d after shoot injection we measured midday transpiration rate (E), g_s, and xylem pressure (Ψ_p-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2–3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and g_s. The k measurement was expressed per unit leaf area (k_l, leaf-specific conductance). Leaves measured within 2 d of shoot injection showed reduced g_s and E relative to non-injected controls, and both parameters were strongly correlated with k_l At this time, there was no difference in leaf Ψ_p-xylem between injected shoots and controls, and leaf Ψ_p-xylem was not significantly different from the highest cavitation-inducing pressure (Ψ_p-cav) in the branch xylem (-1.43 ± 0.029 MPa, n=8). Leaves measured 7–18 d after shoots were injected exhibited a partial return of g_s and E values to the control range. This was associated with a decrease in leaf Ψ_p-xylem below Ψ_p-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.     

Germination sensitivities to water potential among co‐existing C3 and C4 grasses of cool semi‐arid prairie grasslands

An untested theory states that C₄ grass seeds could germinate under lower water potentials (Ψ) than C₃ grass seeds. We used hydrotime modelling to study seed water relations of C₄ and C₃ Canadian prairie grasses to address Ψ divergent sensitivities and germination strategies along a risk‐spreading continuum of responses to limited water. C₄ grasses were Bouteloua gracilis, Calamovilfa longifolia and Schizachyrium scoparium; C₃ grasses were Bromus carinatus, Elymus trachycaulus, Festuca hallii and Koeleria macrantha. Hydrotime parameters were obtained after incubation of non‐dormant seeds under different Ψ PEG 6000 solutions. A t‐test between C₃ and C₄ grasses did not find statistical differences in population mean base Ψ (Ψ_b(50)). We found idiosyncratic responses of C₄ grasses along the risk‐spreading continuum. B. gracilis showed a risk‐taker strategy of a species able to quickly germinate in a dry soil due to its low Ψ_b(50) and hydrotime (θ_H). The high Ψ_b(50) of S. scoparium indicates it follows the risk‐averse strategy so it can only germinate in wet soils. C. longifolia showed an intermediate strategy: the lowest Ψ_b(50) yet the highest θ_H. K. macrantha, a C₃ grass which thrives in dry habitats, had the highest Ψ_b(50), suggesting a risk‐averse strategy for a C₃ species. Other C₃ species showed intermediate germination patterns in response to Ψ relative to C₄ species. Our results indicate that grasses display germination sensitivities to Ψ across the risk‐spreading continuum of responses. Thus seed water relations may be poor predictors to explain differential recruitment and distribution of C₃ and C₄ grasses in the Canadian prairies.     

Changes in leaf hydraulic conductance correlate with leaf vein embolism in<Emphasis Type="Italic"> Cercis siliquastrum</Emphasis> L.

The impact of xylem cavitation and embolism on leaf (K _leaf) and stem (K _stem) hydraulic conductance was measured in current-year shoots of Cercis siliquastrum L. (Judas tree) using the vacuum chamber technique. K _stem decreased at leaf water potentials (Ψ_L) lower than ?1.0 MPa, while K _leaf started to decrease only at Ψ_L L K _leaf changes. Field measurements of leaf conductance to water vapour (g _L) and Ψ_L showed that stomata closed when Ψ_L decreased below the Ψ_L threshold inducing loss of hydraulic conductance in the leaf. The partitioning of hydraulic resistances within shoots and leaves was measured using the high-pressure flow meter method. The ratio of leaf to shoot hydraulic resistance was about 0.8, suggesting that stem cavitation had a limited impact on whole shoot hydraulic conductance. We suggest that stomatal aperture may be regulated by the cavitation-induced reduction of hydraulic conductance of the soil-to-leaf water pathway which, in turn, strongly depends on the hydraulic architecture of the plant and, in particular, on leaf hydraulics.     

Allocation to leaf area and sapwood area affects water relations of co-occurring savanna and forest trees

Water availability is a principal factor limiting the distribution of closed-canopy forest in the seasonal tropics, suggesting that forest tree species may not be well adapted to cope with seasonal drought. We studied 11 congeneric species pairs, each containing one forest and one savanna species, to test the hypothesis that forest trees have a lower capacity to maintain seasonal homeostasis in water relations relative to savanna species. To quantify this, we measured sap flow, leaf water potential (Ψ_L), stomatal conductance (g _s), wood density, and Huber value (sapwood area:leaf area) of the 22 study species. We found significant differences in the water relations of these two species types. Leaf area specific hydraulic conductance of the soil/root/leaf pathway (G _t) was greater for savanna species than forest species. The lower G _t of forest trees resulted in significantly lower Ψ_L and g _s in the late dry season relative to savanna trees. The differences in G _t can be explained by differences in biomass allocation of savanna and forest trees. Savanna species had higher Huber values relative to forest species, conferring greater transport capacity on a leaf area basis. Forest trees have a lower capacity to maintain homeostasis in Ψ_L due to greater allocation to leaf area relative to savanna species. Despite significant differences in water relations, relationships between traits such as wood density and minimum Ψ_L were indistinguishable for the two species groups, indicating that forest and savanna share a common axis of water-use strategies involving multiple traits.     

Osmotic and elastic adjustments in cold desert shrubs differing in rooting depth: coping with drought and subzero temperatures

Physiological adjustments to enhance tolerance or avoidance of summer drought and winter freezing were studied in shallow- to deep-rooted Patagonian cold desert shrubs. We measured leaf water potential (Ψ_L), osmotic potential, tissue elasticity, stem hydraulic characteristics, and stomatal conductance (g _S) across species throughout the year, and assessed tissue damage by subzero temperatures during winter. Species behavior was highly dependent on rooting depth. Substantial osmotic adjustment (up to 1.2?MPa) was observed in deep-rooted species exhibiting relatively small seasonal variations in Ψ_L and with access to a more stable water source, but having a large difference between predawn and midday Ψ_L. On the other hand, shallow-rooted species exposed to large seasonal changes in Ψ_L showed limited osmotic adjustment and incomplete stomatal closure, resulting in turgor loss during periods of drought. The bulk leaf tissue elastic modulus (ε) was lower in species with relatively shallow roots. Daily variation in g _S was larger in shallow-rooted species (more than 50?% of its maximum) and was negatively associated with the difference between Ψ_L at the turgor loss point and minimum Ψ_L (safety margin for turgor maintenance). All species increased ε by about 10?MPa during winter. Species with rigid tissue walls exhibited low leaf tissue damage at ?20?°C. Our results suggest that osmotic adjustment was the main water relationship adaptation to cope with drought during summer and spring, particularly in deep-rooted plants, and that adjustments in cell wall rigidity during the winter helped to enhance freezing tolerance.     

Water relations of Caatinga trees in the dry season

The Caatinga is one of the world's richest dry forests. This forest occurs only in Brazil, but is the least studied and protected Brazilian ecosystem. There are few reports about drought tolerance mechanisms in Caatinga trees. This work evaluates water relations of six adult species in the middle of the dry season to further understand water relations in this ecosystem, which will be important for future reforestation and management projects. Based on results, the trees were classified into four groups: (I), Mimosa caesalpiniifolia had low leaf water potential (Ψ_w) at predawn and no significant decrease at midday. Stomatal conductance (g_s) analyses indicates that plants have reached its lowest Ψ_w; (II), Caesalpinia pyramidalis and Auxemma oncocalyx had low Ψ_w at predawn and significant decrease at midday. For these species the recuperation of water status at night may have been sufficient for maintaining stomata open during the day; (III), Caesalpinia ferrea and Calliandra spinosa had relatively high Ψ_w at predawn and a significant decrease at midday. These species might maintain their water status similar to individuals of group II, but they might also have deeper root systems; and (IV), Tabebuia caraiba with the highest Ψ_w at predawn and no significant decrease at midday, possibly indicating a combination of good stomatal control of water loss and a deeper root system. Moreover, except for individuals of group I, both in species with lower and higher Ψ_w at predawn it was not observed strong inhibition of g_s.     

Hydraulic limitation on maximum height of Pinus strobus trees in northern Minnesota,USA

Key message

Using comparisons within and between trees, the authors show evidence for hydraulic limitation of tree height in a humid-climate species that is far from the global maximum tree height.

Abstract

We measured water status and two indicators of drought stress as a function of height within the canopies of four tall (32–35 m) eastern white pines (Pinus strobus) at an old-growth site in northern Minnesota, USA. Pre-dawn and midday xylem pressure potential measured on terminal shoots (Ψ _shoot), needle length, and foliar carbon isotope discrimination (δ ¹³C) all showed within-canopy gradients consistent with increasing drought stress with height. Midday Ψ _shoot near tree tops was ?1.8 MPa, close to values associated with stomatal closure for other temperate conifers. Pre-dawn Ψ _shoot decreased with height at >2× the gradient in gravitational potential. δ ¹³C was strongly correlated with height and weakly correlated with light. Needles were 15–25 % shorter at canopy top compared to the bottom of the canopy. Midday Ψ _shoot and needle length showed significant differences in regression model coefficients from tree to tree. The patterns are consistent with hydraulic constraints on height growth of white pine at this site.     

Water Potential Modulates Extensibility of Rye Coleoptile Cell Walls

Extensibility of walls of frozen/thawed rye (Secale cereale) coleoptile segments as a function of the water potential of the incubation solution (Ψ₀) was analyzed employing the creep test method. Negative Ψ₀ exerts an inhibiting effect on extension of isolated walls. The lower the Ψ₀ of polyethylene glycol 6000 (PEG), the less the walls of frozen/thawed segments extended under load. This inhibiting effect of Ψ₀ on wall creep was reversible and independent of the preincubation temperature of the segments. An increase in Ψ₀ resulted in increased extension rate within 2–4 min, whereas a decrease in Ψ₀ resulted in gradually decreasing extension rate after 8–12 min. This finding implies that wall extension changes during growth induced by changes of Ψ₀ in vivo are not only due to changes of turgor pressure but also due to a direct influence by negative Ψ₀ on physical wall properties. The results are discussed with respect to the regulation of extension growth during conditions of water stress.     

Water relation parameters of six Peltigera species correlate with their habitat preferences

Water relation parameters were measured in six congeneric lichen species with different requirements for water availability and with green algae (Peltigera aphthosa, Peltigera leucophlebia, Peltigera venosa) or cyanobacteria (Peltigera horizontalis, Peltigera praetextata, Peltigera rufescens) as main photobionts. Pressure–volume analysis was performed with a dewpoint hygrometer and integrated with anatomical analyses. The Peltigera species typical of arid environments were characterized by relatively lower osmotic potential (π₀) and turgor loss point (Ψ_TLP), and higher values of bulk modulus of elasticity (?). Both π₀ and Ψ_TLP were correlated with the size of medullary cells, while ? was negatively correlated with cell dimensions. The adaptive value of low Ψ_TLP might reside in the capability to maintain cell turgor for longer time intervals under dry conditions. High ? might allow xerophilous lichens to regain cell turgor more promptly even for small amounts of water uptake, thus enlarging the cumulative period of positive carbon balance in environments with fluctuating water availability. The influence of the photobiont type is also discussed.     

Leaf anatomy,water relations and crassulacean acid metabolism in the chlorenchyma and colourless internal water-storage tissue of Carpobrotus edulis and Senecio ?mandraliscae

Both Carpobrotus edulis and Senecio ?mandraliscae possess leaves with a peripheral chlorenchyma and colourless internal water-storage tissue. Water stress in C. edulis growing under semi-natural conditions resulted in the induction of weak Crassulacean acid metabolism (CAM) whereas well-watered plants of S. ?mandraliscae exhibited a similar degree of CAM. Titratable acidity in the separated water-storage tissue was substantially lower than in the chlorenchyma in both species but, nevertheless, increased during the night and decreased during the day either when sampled from the intact plant or from incubated tissue slices. Indeed, the increase in nocturnal titratable acidity produced by the water-storage tissue in situ accounted for approx. 30% of total acidification on a per-leaf basis. It appears that during the night the water-storage tissue in these species is able to fix CO₂ which is subsequently released during the day to enter the photosynthetic carbon-reduction cycle of the chlorenchyma. Diurnal rhythms of water potential (Ψ) and osmotic potential (Ψs) were measured in separated chlorenchyma and water-storage tissue by thermocouple psychrometry. Both parameters increased during the latter part of the daytime and initial nocturnal period and decreased during the rest of the night and into the post-dawn period. The chlorenchyma of water-stressed plants of C. edulis appeared to possess a marked negative turgor pressure (as determined from Ψ-Ψs) but this was caused by a severe underestimation in the measurement of the chlorenchyma Ψ. It is suggested that this artefact arose from release of colloidal polysaccharide mucilage, or possibly tannins, from broken tannin cells producing a lowering of water activity when measured using thermocouple psychrometry.     

The ecosystem wilting point defines drought response and recovery of a Quercus-Carya forest

Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem-scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (Ψ_EWP), a property that integrates the drought response of an ecosystem's plant community across the soil–plant–atmosphere continuum. Specifically, Ψ_EWP defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the Ψ_EWP of a Quercus-Carya forest from an “ecosystem pressure–volume (PV) curve,” which is analogous to the tissue-level technique. When community predawn leaf water potential (Ψ_pd) was above Ψ_EWP (=−2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψ_pd fell below Ψ_EWP, the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, Ψ_EWP is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall-induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψ_pd observations fell below Ψ_EWP, the forest is commonly only 2–4 weeks of intense drought away from reaching Ψ_EWP, and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom-up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of Ψ_EWP, and species in an ecosystem require compatible leaf-level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil.     

Water potential,water saturation deficit and their relationship in leaves of different insertion levels

The water potential (Ψ _w ) and the water saturation deficit (δW _sat) in leaves of different insertion levels of potted kale plants were simultaneously measured. In non-wilting plantsδW _sat gradually decreased andΨ _w slightly increased from the upper to the lower leaves. During the wilting of the plants induced by decreasing of soil moistureΨw practically decreased paralelly in all the leaves but the same decrease ofΨ _w was connected with the lowest increase ofδW _sat in upper leaves and the highest increase ofδW _sat in lower leaves. Not only the values ofΨ _w andδW _sat but also their relationship varied considerably with the leaf insertion levels.