Leaf hydraulic architecture and water relations of three ferns from contrasting light habitats

Leaf gas exchange and water relation characteristics of field quinoa ( Chenopodium quinoa Willd.) during soil drying

Trends in leaf water, osmotic, and turgor potentials of cotton (Gossypium hirsutum L., cv. ‘Dunn 56C’) and sorghum (Sorghum bicolor L. Moench, cv. ‘RS671’) were monitored on both a seasonal and diurnal basis. The effects of differential soil water availability on leaf water potential components were examined in order to ascertain the differences in the water relations of these two species.Decreasing availability of soil water was responsible for decreased morning and afternoon leaf water potentials in sorghum and cotton. The magnitude of degression in leaf water potential was greater in cotton than in sorghum at equivalent soil water potentials. Decreasesin osmotic potentials maintained positive turgor in both species when sufficient soil water was available. However, as water stress increased, turgor potentials became zero due to the failure of osmotic potentials to decrease more than water potentials.Diurnal changes in water potential components were distinctly different for each species. Leaf water potentials of sorghum came into approximate equilibrium with soil water potential in the early morning, whereas leaf water potentials of cotton did not. Concomitant changes in leaf osmotic potentials on a diurnal basis resulted in specific trends of increasing turgor in sorghum, while anomalous fluctuations were evident in cotton leaf turgor potentials.The relationship between leaf water potential and relative water content was determined for each species. The change in relative water content per unit change in leaf water potential was greater in cotton than in sorghum.Drought tolerance of these two species appears to be dependent on species speciIic relationships in leat water potential components.

Responses of leaf growth and leaf water relations to seasonal drought were monitored during two successive years in three cultivars (Galhosa, Espargal and Mulata) of Ceratonia siliqua L. growing in the field in southern Portugal. Leaf water relations of fully expanded leaves were characterized by pressureâ volume analysis, and morphometric measurements of petiolar xylem were made. The three cultivars differed with respect to onset of leaf initiation. In Galhosa, there was a sharp peak of leaf initiation in June that was immediately followed by a period when leaf expansion rates were highest. The onset of leaf growth occurred earlier in Espargal and Mulata than in Galhosa, and both cultivars continued producing new leaves throughout the summer period. The diurnal pattern of water relations in recently expanded leaves indicated that, during midsummer, Galhosa was the only cultivar in which leaf water potential did not fall below the turgor loss point and leaf relative water content remained above 90%. The occurrence of osmotic adjustment in recently expanded leaves of Galhosa was not demonstrated conclusively. However, during the dry season following leaf formation, a seasonal decrease in osmotic potential sufficient to maintain turgor was detected in 1-year-old leaves of Galhosa but not in 1-year-old leaves of the other cultivars. Among cultivars, Galhosa petioles had the widest xylem conduits, which may partly explain why midday leaf water potential in Galhosa never decreased below -2.0 MPa even at the end of the summer drought.

SummaryFor many field-grown crops, including sugar beet, there is little information on the seasonal changes in leaf water potential and its components as the soil dries. Therefore, seasonal changes in leaf water, osmotic and turgor potentials of sugar beet were measured in two seasons, in crops that experienced differing degrees of soil moisture stress. In 1983 potentials of crops exposed to early and late droughts were compared with those of irrigated crops, and in 1984 measurements were made in a non-irrigated crop. In the irrigated crop the midday leaf water potential changed little during the season, except in response to fluctuating evaporative demand. In the drought and non-irrigated treatments there was a sharp fall in leaf water potential as soon as the soil water potential decreased. The size of the midday leaf water potential was primarily determined by soil dryness. However, the leaf water potential did not decrease below about — 1·5 MPa in either year. The leaf osmotic potential declined at the same time as the leaf water potential, but the extent to which this happened differed in the two years. Only in the 1984 non-irrigated crop did the osmotic potential continue to decrease as the soil dried, suggesting that osmotic adjustment had taken place in 1984 but not in 1983. Thus higher turgor was maintained in the 1984 crop than in the 1983 drought-affected crops. Some turgors were recorded as apparently negative in 1983.Since the leaf water potential declined to a minimum of about — 1·5 MPa, the soil water potential minima were also about — 1·5 MPa. However, deeper soil was not dried to this extent, suggesting that the extra resistance for water uptake from deep soil was limiting or the rooting density was too low.The pattern of recovery of leaf water potential overnight suggested that the rhizosphere resistance to water movement was small, even as the soil dried. However, measurement of stem water potentials in 1984 indicated that a significant resistance to water flow existed within the aerial part of sugar beet plants. This shows that the use of the water potential in leaves as an estimate of that in stems or roots can be misleading.

SUMMARYFifteen cultivars of orchardgrass (Dactylis glomerata L.) were grown in the field at Hiroshima University, Japan, to investigate seasonal changes in leaf water relations and cell membrane stability (CMS) measured by the polyethylene glycol (PEG) test. Leaf water potential and osmotic potential were measured from August 1988 to August 1989. Solute concentration in leaf cell sap was also estimated.Cell membrane stability increased, leaf water potential and osmotic potential decreased and turgor potential increased with decreasing environmental temperatures during autumn and winter. The significant increases observed in CMS may enable plants to tolerate freezing temperatures during winter. Decrease in leaf water potential may be a result of water-deficit effects due to soil freezing at low temperatures and the decrease in osmotic potential may help plants to maintain turgor and tolerate freezing conditions. Plants maintained higher turgor as the osmotic potential decreased to values as low as – 3·98 MPa during winter; the maintenance of turgor helps to maintain water uptake under water deficit conditions at low temperatures.Sugar and K were the major osmotic contributors in orchardgrass leaves. Sugar and Ca concentrations increased and Mg and P concentrations decreased at cold temperatures. K concentration increased in six cultivars and decreased in nine others at cold temperatures. Sugar concentration in cell sap was negatively correlated with osmotic potential. It was concluded that seasonal changes in CMS may be mainly associated with the osmotic potential of the leaf tissues.

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.

Lupins (Lupinus angustifolius and L. cosentinii) growing in 321 containers in a glasshouse were exposed to drought by withholding water. Leaf water potential (Ψ1), and leaf osmotic potential (Ψs) were measured daily as soil water became depleted. Leaf water relations were further assessed by a pressure-volume technique and by measuring Ψs and relative water content of leaves after rehydration. Analysis by pressure-volume or cryoscopic techniques showed that leaf osmotic potential at saturation (Ψs100) decreased from -0.6 MPa in well watered to -0.9 MPa in severely droughted leaves, and leaf water potential at zero turgor (Ψzt) decreased from about -0.7 to -1.1 MPa in well watered and droughted plants, respectively. Relative water content at zero turgor (RWCzt) was high (88%) and tended to be decreased by drought. The ratio of turgid leaf weight to dry weight was not influenced by drought and was high at about 8.0. The bulk elastic modulus (ɛ) was approximately halved by drought when related to leaf turgor potential (Ψp) and probably mediated turgor maintenance during drought. The latter was found to be negatively influenced by rate of drought. Supplying the plants with high levels of K salts did not promote adjustment or turgor maintenance.

Climate change exposes vegetation to unusual levels of drought, risking a decline in productivity and an increase in mortality. It still remains unclear how trees and forests respond to such unusual drought, particularly Southeast Asian tropical rain forests. To understand leaf ecophysiological responses of tropical rain forest trees to soil drying, a rainfall exclusion experiment was conducted on mature canopy trees of Dryobalanops aromatica Gaertn.f. (Dipterocarpaceae) for 4 months in an aseasonal tropical rain forest in Sarawak, Malaysia. The rainfall was intercepted by using a soft vinyl chloride sheet. We compared the three control and three treatment trees with respect to leaf water use at the top of the crown, including stomatal conductance (gsmax), photosynthesis (Amax), leaf water potential (predawn: Ψpre; midday: Ψmid), leaf water potential at turgor loss point (πtlp), osmotic potential at full turgor (π100) and a bulk modulus of elasticity (ε). Measurements were taken using tree-tower and canopy-crane systems. During the experiment, the treatment trees suffered drought stress without evidence of canopy dieback in comparison with the control trees; e.g., Ψpre and Ψmid decreased with soil drying. Minimum values of Ψmid in the treatment trees decreased during the experiment, and were lower than πtlp in the control trees. However, the treatment trees also decreased their πtlp by osmotic adjustment, and the values were lower than the minimum values of their Ψmid. In addition, the treatment trees maintained gs and Amax especially in the morning, though at midday, values decreased to half those of the control trees. Decreasing leaf water potential by osmotic adjustment to maintain gs and Amax under soil drying in treatment trees was considered to represent anisohydric behavior. These results suggest that D. aromatica may have high leaf adaptability to drought by regulating leaf water consumption and maintaining turgor pressure to improve its leaf water relations.

Citrus blight or young tree decline, is a wilt-like disease of unknown etiology which is characterized by restricted water movement and an upset in normal zinc distribution patterns. Diurnal leaf and fruit water potentials and leaf stomatal conductances of sweet orange Citrus sinensis (L.) Osbeck leaves on trees in various stages of decline were characterized to determine the progression of this disorder. All blight affected trees, regardless of severity of tree condition, had similar diurnal water relations. Blight affected trees have fewer and smaller leaves, less leaf area per tree, lower stomatal conductances, and lower diurnal transpiration rates than healthy trees. These differences did not result in any apparent changes in specific leaf weight, leaf osmotic potentials or in the critical leaf water deficits at which leaf turgor was lost. At equivalent transpirational fluxes, leaf water potential was much lower in blight affected trees than in healthy trees. Therefore, the water stress symptoms associated with blight are related to increased resistances in the water transport system and are not a result of lost stomatal function or changes in water relations characteristics of leaves that remain on blight affected trees.

Irrigation effects on whole-plant sap flow and leaf-level water relations were characterised throughout a growing season in an experimental olive (Olea europaea L.) orchard. Atmospheric evaporative demand and soil moisture conditions for irrigated and non-irrigated olive trees were also monitored. Whole-plant water use in field-grown irrigated and rain fed olive trees was determined using a xylem sap flow method (compensation heat-pulse velocity). Foliage gas exchange and water potentials were determined throughout the experimental period. Physiological parameters responded diurnally and seasonally to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the Penman–Monteith equation in the field. Summer drought caused decreasing leaf gas exchange and water potentials, and a progressive increase in hydraulic conductance (stronger in non-irrigated than irrigated trees), probably attributable to modifications in hydraulic properties at the soil-root interface. Negligible hysteresis, attributable to low plant capacitance, was observed in the relationship between leaf water potential and sap flow. A proportional decrease in maximum daily leaf conductance with increasing vapour pressure deficit was observed, while mean daytime canopy stomatal conductance decreased with the season. As a result, plant water use was limited and excessive drought stress prevented. Non-irrigated olive trees recovered after the summer drought, showing a physiological behaviour similar to that of irrigated trees. In addition to physiological and environmental factors, there are endogenous keys (chemical signals) influencing leaf level parameters. Olive trees are confirmed to be economical and sparing users of soil water, with an efficient xylem sap transport, maintenance of significant gas exchange and transpiration, even during drought stress.

In the previous report, it was considered that the slight variation of leaf water content on an areal basis (leaf water content) gave much influence upon stomatal aperture). The present study was conducted to establish other measurement methods of water status in the rice plant, i.e., leaf water potential and water saturation deficit (WSD), and to discuss which of them was satisfactory for measuring water status in the rice plant. The results obtained are as follows: Three hours for equilibration of water potential between gas phase and rice plant leaves in psychrometer chamber were required for leaf water potential measurement (Fig. 1) and full turgidity was attained in four hours after immersing leaves in water for WSD measurement (Fig. 2). Diurnal changes of water status in leaf blades were measured by leaf water potential, leaf water content and WSD. The results were that water in leaf blades was contained sufficiently in the early morning and it decreased in the daytime and again increased towards the evening (Fig. 3 and 4). Close correlation was found between leaf water potential and leaf water content, and between leaf water potential and WSD at each leaf position on the stem, and the height of those regression lines was different from each other depending on leaf position on the stem (Fig. 5). In the early morning when there was no water stress in the rice plant, leaf water potential was constant being about -2 bars through all leaf position investigated. On the contrary, leaf water content and WSD were different from each other depending on leaf position on the stem, i.e., the maximal values of leaf water content and WSD were obtained for 11th and 12th leaves, 10th and 11th leaves, respectively. The values of leaf water content and WSD gradually decreased both to upper and to lower leaves as they were away from the leaves with maximum values (Fig. 6). At the same degree of wilting of leaves or when apparent photosynthetic rate was reduced to zero due to water deficit in leaves, leaf water potential was almost constant through all leaf position investigated, but leaf water content and WSD were different from each other depending on leaf position on the stem (Table 1 and 2). From the results mentioned above and that the standard deviation of measured values of leaf water potential at the same water status was much less than that of leaf water content and WSD, it is considered that leaf water potential expresses leaf water status physiologically and it is more suitable for measurement of water status in the rice plant of which stomatal aperture is much influenced by the slight variation of water status. It is assumed that stomatal aperture changes with leaf water potential and that the stomata of the rice plant begin to close due to the slight decrease of leaf water potential.

We investigated seasonal patterns of water relations in current-year leaves of three evergreen broad-leaved trees (Ilex pedunculosa Miq., Ligustrum japonicum Thunb., and Eurya japonica Thunb.) with delayed greening in a warm-temperate forest in Japan. We used the pressure-volume method to: (1) assess the extent to which seasonal variation in leaf water relations is attributable to leaf development processes in delayed greening leaves versus seasonal variation in environmental variables; and (2) investigate variation in leaf water relations during the transition from the sapling to the adult tree stage. Leaf mass per unit leaf area was generally lowest just after completion of leaf expansion in May (late spring), and increased gradually throughout the year. Osmotic potential at full turgor (Psi(o) (ft)) and leaf water potential at the turgor loss point (Psi(w) (tlp)) were highest in May, and lowest in midwinter in all species. In response to decreasing air temperature, Psi(o) (ft) dropped at the rate of 0.037 MPa degrees C(-1). Dry-mass-based water content of leaves and the symplastic water fraction of total leaf water content gradually decreased throughout the year in all species. These results indicate that reductions in the symplastic water fraction during leaf development contributed to the passive concentration of solutes in cells and the resulting drop in winter Psi(o) (ft). The ratio of solutes to water volume increased in winter in current-year leaves of L. japonicum and E. japonica, indicating that osmotic adjustment (active accumulation of solutes) also contributed to the drop in winter in Psi(o) (ft). Bulk modulus of elasticity in cell walls fluctuated seasonally, but no general trend was found across species. Over the growing season, Psi(o) (ft) and Psi(w) (tlp) were lower in adult trees than in saplings especially in the case of I. pedunculosa, suggesting that adult-tree leaves are more drought and cold tolerant than sapling leaves. The ontogenetic increase in the stress resistance of I. pedunculosa may be related to its characteristic life form because I. pedunculosa grows taller than the other species studied.

The incidence and severity of global mangrove mortality due to drought is increasing. Yet, little is understood of the capacity of mangroves to show long-term acclimation of leaf water relations to severe drought. We tested for differences between mid-dry season leaf water relations in two cooccurring mangroves, Aegiceras corniculatum and Rhizophora stylosa before a severe drought (a heatwave combined with low rainfall) and after its relief by the wet season. Consistent with ecological stress memory, the legacy of severe drought enhanced salinity tolerance in the subsequent dry season through coordinated adjustments that reduced the leaf water potential at the turgor loss point and increased cell wall rigidity. These adjustments enabled maintenance of turgor and relative water content with increasing salinity. As most canopy growth occurs during the wet season, acclimation to the 'memory'of higher salinity in the previous dry season enables greater leaf function with minimal adjustments, as long-lived leaves progress from wet through dry seasons. However, declining turgor safety margins - the difference between soil water potential and leaf water potential at turgor loss - implied increasing limitation to water use with increasing salinity. Thus, plasticity in leaf water relations contributes fundamentally to mangrove function under varying salinity regimes.

Drought and salt stress is the major constrains to crop productivity. However, resistant genotypes improve their physiological mechanisms to cope with these stresses. In this study, we have investigated the influences of drought and salt stress on dry weight, leaf osmotic potential, leaf water potential, leaf temperature and stomatal conductance in sensitive and resistant melon genotypes. Four melon genotypes (sensitive, CU 40 and CU 252; resistant CU 196 and CU 159) were grown in a mixture of peat:perlite of 2:1 ratio in growth chamber. Salt and drought stresses were observed in 30 days old melon plants. In order to perform salinity stress, 200 mM NaCI was used. The drought stress was achieved by decreasing irrigation water gradually and finally irrigation was completely stopped. The plants were subjected to the salt and drought stresses for 12 days. At the end of the experiment; shoot dry weight, osmotic potential, leaf water potential and stomatal conductance were lower in salt and drought-sensitive genotypes (CU 40 and CU 252) than the resistant ones (CU 159 and CU 196). The leaf temperature was increased under stress conditions in melon genotypes. The results showed that resistant melon genotypes have more efficient stress protection mechanisms to survive under salinity and drought conditions.

Cultivated tomato Lycopersicon esculentum (L.) Mill. cv. P‐73 and its wild salt‐tolerant relative L. pennellii (Correll) D'Arcy accession PE‐47 growing on silica sand in a growth chamber were exposed to 0, 70, 140 and 210 mM NaCl nutrient solutions 35 days after sowing. The saline treatments were imposed for 4 days, after which the plants were rinsed with distilled water. Salinity in L. esculentum reduced leaf area and leaf and shoot dry weights. The reductions were more pronounced when sodium chloride was removed from the root medium. Reduction in leaf area and weight in L. pennellii was only observed after the recovery period. In both genotypes salinity induced a progressive reduction in leaf water potential and leaf conductance. During the recovery period leaf water potential (ψ1) and leaf conductance (g1) reached levels similar to those of control plants in wild and cultivated species, respectively. Leaf osmotic potential at full turgor (ψos) decreased in the salt treated plants of both genotypes, whereas the bulk modulus of elasticity was not affected by salinity. Leaf water potential at turgor loss point (ψtlp) and relative water content at turgor loss point (RWCtlp) appeared to be controlled by leaf osmotic potential at full turgor (ψos) and by bulk modulus of elasticity, respectively. At lowest salinity, the wild species carried out the osmotic adjustment based almost exclusively on Cl− and Na+, with a marked energy savings. Under highest salinity, this species accommodate the stress through a higher expenditure of energy due to the contribution of organic solutes to the osmotic adjustment. The domesticated species carried out the osmotic adjustment based always on an important contribution of organic solutes.