Changes in leaf water potential and CAM inSempervivum montanum andSedum album in response to water availability in the field

Summary The short term effects of irrigation on diurnal changes in leaf and titratable acidity were examined both inSempervivum montanum and inSedum album, a facultative CAM plant, in the Spanish Pyrenees. InSemperivivum, leaf responded rapidly to irrigation and, in both the control and irrigated plants, increased during the day and decreased during the night and early morning. By contrast, leaf inSedum responded more slowly to irrigation and showed a decrease during the day and an increase in the period between evening and early morning. Under the conditions of the short-term experiments, changes in acid metabolism were not observed in either species following irrigation. The results suggest that transpirational water loss together with redistribution of water within the plant are more important than the osmotic concentration of malic acid in determining leaf in both species and that daytime water loss is greater inSedum than inSempervivum.The effect of long-term water stress on leaf and acid levels was also assessed in both species over a 3-week period. Both leaf and acidification inSempervivum decreased over this time period but could, at least partially, be reversed by irrigation. InSedum, leaf also declined but a more gradual reduction in acidification occurred than inSempervivum. Irrigation inSedum at least partially reversed the decline in leaf but produced a complex pattern of acid metabolism. Nocturnal acidification in the irrigated plants was lower than in the non-irrigated control when preceded by a cool day but showed complete recovery following a hot day. It is suggested inSedum album that C₃ photosynthesis during the preceding light period, as determined by light intensity and leaf temperature, may be important in determining the extent of nocturnal acidification under field conditions.     

The control by atmospheric factors and water stress of midday stomatal closure in Arbutus unedo growing in a natural macchia

Summary Midday closure of stomata of well-watered ( between-10 and-25 bar) or moderately stressed ( between-25 and-35 bar) Arbutus unedo plants occurs when midday leaf temperatures increase above 30°C and vapor pressure difference between leaf air spaces and the external air increases above approximately 30 mbar/bar. Moderate water stress decreases maximum conductance and may result in greater sensitivity to high leaf temperature and vapor pressure dificit, which results in earlier closure and later reopening of stomata. Severe water stress ( of-50 bar) changes the form of the daily pattern observed for leaf conductance. A single morning peak in conductance occurs followed by decrease in conductance over the remainder of the day. Morning fog in Portugal during the dry season may facilitate stomatal opening and may allow improvement of carbon balances of the leaves for short periods, but contributes little to improvement of plant water balances over the longer term.     

Changes in abscisic acid and its glucose ester in Phaseolus vulgaris L. during chilling and water stress

Levels of free and conjugated abscisic acid (ABA) were determined in leaves and roots of intact bean (Phaseolus vulgaris L., cv. Mondragone) seedlings under chilling (3C) and drought as well as during recovery from stress. Abscisic acid-glucose ester (ABAGE) was the only conjugate releasing free ABA after alkaline hydrolysis of the crude aqueous extracts. During the first 20–30 h chilled plants rapidly dehydrated and wilted without any change in ABA and ABAGE levels. Subsequently, leaf and root ABA levels increased and plants regained turgor. ABAGE concentration showed a slight increase in leaves but not in roots. Upon recovery from chilling a transient, but significant, rise in leaf ABA content was observed, while no appreciable change in ABAGE was found. Drought triggered ABA accumulation in leaves and roots, while a rise in ABAGE content was detected only in leaf tissues. Recovery from stress caused a drop in ABA levels without a correspondent increase in ABAGE concentration. We conclude that ABAGE is not a source of free ABA during either chilling or water stress and that only a small proportion of the ABA produced under stress is metabolised to ABAGE during recovery.Abbreviations ABA = abscisic acid - ABAGE = abscisic acid-glucose ester - DW = dry weight - FW = fresh weight - RIA = radioimmunoassay - RWC = relative water content - w = water potential - o = osmotic potential - p = turgor potential     

Seasonal Changes in Photosynthesis and Stomatal Conductance of Five Plant Species from a Semiarid Ecosystem

In order to determine whether stomatal closure alone regulates photosynthesis during drought under natural conditions, seasonal changes in leaf gas exchange were studied in plants of five species differing in life form and carbon fixation pathway growing in a thorn scrub in Venezuela. The species were: Ipomoea carnea, Jatropha gossypifolia, (C3 deciduous shrubs), Alternanthera crucis (C4 deciduous herb), and Prosopis juliflora and Capparis odoratissima (evergreen phreatophytic trees). Xylem water potential () of all species followed very roughly the precipitation pattern, being more closely governed by soil water content in I. carnea and A. crucis. Maximum rate of photosynthesis, Pmax, decreased with in I. carnea, J. gossypifolia, and A. crucis. In I. carnea and J. gossypifolia stomatal closure was responsible for a 90 % decline in net photosynthetic rate (PN) as decreased from -0.3 to -2.0 MPa, since stomatal conductance (gs) was sensitive to water stress, and stomatal limitation on PN increased with drought. In A. crucis, PN decreased by 90 % at a much lower (-9.3 MPa), and gs was relatively less sensitive to . In P. juliflora and C. odoratissima, Pmax, gs, and intercellular CO2 concentration (Ci) were independent of soil water content. In the C3 shrubs stomatal closure was apparently the main constraint on photosynthesis during drought, Ci declining with in I. carnea. In the C4 herb, Ci was constant along the range of values, which suggested a coordinated decrease in both gs and mesophyll capacity. In P. juliflora Ci showed a slow decrease with which may have been due to seasonal leaf developmental changes, rather than to soil water availability.     

The effect of humidity and light on cellular water relations and diffusion conductance of leaves ofTradescantia virginiana L.

Turgor (_p) and osmotic potential (_s) in epidermal and mesophyll cells, in-situ xylem water potential (-xyl) and gas exchange were measured during changes of air humidity and light in leaves ofTradescantia virginiana L., Turgor of single cells was determined using the pressure probe. Sap of individual cells was collected with the probe for measuring the freezing-point depression in a nanoliter osmometer. Turgor pressure was by 0.2 to 0.4 MPa larger in mesophyll cells than in epidermal cells. A water-potential gradient, which was dependent on the rate of transpiration, was found between epidermis and mesophyll and between tip and base of the test leaf. Step changes of humidity or light resulted in changes of epidermal and mesophyll turgor (_p-epi, _p-mes) and could be correlated with the transpiration rate. Osmotic potential was not affected by a step change of humidity or light. For the humidity-step experiments, stomatal conductance (g) increased with increasing epidermal turgor.g/_p-epi appeared to be constant over a wide range of epidermal turgor pressures. In light-step experiments this type of response was not found and stomatal conductance could increase while epidermal turgor decreased.Symbols E transpiration - g leaf conductance - w leaf/air vapour concentration difference - -epi water potential of epidermal cells - -mes water potential of mesophyll cells - -xyl water potential of xylem - _p-epi turgor pressure of epidermal cells - _p-mes turgor pressure of mesophyll cells - _s-epi osmotic potential of epidermal cells - _s-mes osmotic potential of mesophyll cells     

The role of abscisic acid and water relations in drought responses of subterranean clover

The role of water relations and abscisic acid (ABA) in the responsesto drought were studied in a mediterranean forage crop, Trifoliumsubterraneum L. under field conditions. Soil and plant waterstatus, leaf gas exchange parameters, and xylem sap ABA contentwere determined at different times during a long-term soil dryingepisode in irrigated and droughted plants. The diurnal time-coursesof these parameters were also measured at the end of a droughtperiod. In response to soil drying stomatal conductance (g) was reducedearly to 50% that of irrigated plants before any substantialchange in water potential was detected. A close logarithmicregression between photosynthesis rate (A) and g was present.For the first weeks of drought the decline in A was less pronouncedthan in g, thus increasing water use efficiency. Stomatal conductanceduring diurnal time-courses showed no consistent relationshipswith respect to etther ABA or leaf water potential. Throughoutthe experimental period dependence of g on leaf water statuswas evident from the tight correlation (r²=0.88, P<0.01)achieved between stomatal conductance and midday water potential,but the correlation was also high when comparing g with respectto ABA content in xylem sap (r=0.83, P<0.001). However, thestomata from drought acclimated plants were apparently moresensitive to xylem ABA content. For similar xylem ABA concentrationsstomatal conductance was significantly higher in irrigated thanin waterstressed plants. Key words: Drought, stomatal conductance, water potential, abscisic acid     

Diurnal change in the relationship between stomatal conductance and abscisic acid in the xylem sap of field-grown peach trees

To evaluate whether abscisic acid (ABA) in the xylem sap playsan important role in controlling stomatal aperture of field-grownPrunus persica trees under drought conditions, stomatal conductance(g) and xylem ABA concentrations were monitored both in irrigatedand non-irrigated trees, on two consecutive summer days (threetimes a day). Stomata1 conductance of non-irrigated trees hada morning maximum and declined afterwards. The changes in gduring the day, rather than resulting from variations in theconcentrations of ABA in the xylem sap or the delivery rateof this compound to the leaves, were associated with changesin the relationship between g and xylem ABA. The stomata ofwater-stressed trees opened during the first hours of the day,despite the occurrence of a high concentration of ABA in thexylem sap. However, stomatal responsiveness to ABA in the xylemwas enhanced throughout the day. As a result, a tight inverserelationship between g and the logarithm of xylem ABA concentrationwas found both at midday and in the afternoon. A similar relationshipbetween g and ABA was found when exogenous ABA was fed to leavesdetached from well-watered trees. These results indicate thatABA derived from the xylem may account for the differences ing observed between field-grown peach trees growing with differentsoil water availabilities. Several possible explanations forthe apparent low stomatal sensitivity to xylem ABA in the morning,are discussed, such as high leaf water potential, low temperatureand high cytokinin activity. Key words: Prunus persica L., stomata, xylem ABA, water deficits, root-to-shoot communication     

Abscisic acid, temperature and salinity interactions on growth and some mineral elements in Carthamus plants

Growth and contents of sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), chloride (Cl), phosphorus (P) and sulphur (S) in shoot and root tissues of Carthamus tinctorius plants were measured at combinations of four nutrient solution osmotic potentials (_s=0, -0.3, -0.6 and -0.9 MPa) induced by NaCl and CaCl treatments, three constant temperatures (T) ranging from 15 to 35°C and four abscisic acid (ABA) concentrations (0,10,50 and 100 mg L^–1). Unstressed and stressed plants grown in optimal temperature conditions (25°C) maintained higher growth rates (dry mass production) than plants grown under low and high temperatures (15 and 35°C respectively). Shoot and root growth (dry mass production) were largely inhibited by salinity but the magnitude of growth inhibition was temperature dependent. Safflower plants respond to salinity stress by increases in Ca, Cl and to a lesser extent Na in their shoots and roots and by a decrease in the ratio of fresh to dry weight. The ratio of K/Na was decreased progressively on salinization. With stressed plants, ABA application reduced the toxicity of salt treatment, improved K uptake under salinity, effectively increased K/Na ratio and helped the plants to avoid Na toxicity and sometimes enhanced growth. The effect of ABA on the growth was more pronounced at optimum temperature (25°C). The association between the internal mineral element concentrations was largely affected by ABA application and temperature change but a wide fluctuation in response was noticed. The effects of single factors (_s, T and ABA) on the growth and mineral contents were statistically significant. Also, bifactorial (_s× T, _s × ABA and T × ABA) and three factorial (_s × T × ABA) interactions significantly affected the parameters. Further statistical treatment of the data (coefficient of determination ²) led to four important findings: (1) Salinity (_s) was dominant in affecting Ca and Cl contents in both shoot and root as well as root Na content. (2) Temperature (T) had a dominant effect on growth, shoot K, Mg, P, S and root P, and S contents (3) The share of _s × T × ABA interaction was dominant for root Na and Mg contents. (4) The single factors and their interactions had a dual role in their subsidiary effects.Abbreviations ABA abscisic acid - _s osmotic potential - ² coefficient of determination - F.wt fresh weight - d.m. dry matter - T temperature - MPa mega pascal - SAR sodium adsorption ratio - P phosphorus - S sulphur     

Stomatal response to leaf water potential in almond trees under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L. cv. Garrigues) were grown in the field under drip irrigated and non irrigated conditions. Leaf water potential () and leaf conductance (g₁) were determined at three different times of the growing season (spring, summer and autumn). The relationships between and g₁ in both treatments showed a continuous decrease of g₁ as decreased in spring and summer. Data from the autumn presented a threshold value of (approx. –2.7 MPa in dry treatment, and approx. –1.4 MPa in wet treatment) below which leaf conductance remained constant.     

CO2 Assimilation and Water Relations of Almond Tree (Prunus amygdalus Batsch) Cultivars Grown Under Field Conditions

Gas exchanges and leaf water potential (_w) of six-years-old trees of fourteen Prunus amygdalus cultivars, grafted on GF-677, were studied in May, when fruits were in active growing period, and in October, after harvesting. The trees were grown in the field under rain fed conditions. Predawn _w showed lower water availability in October compared with May. The lowest _w values at midday in May increased gradually afterwards, while in October they decreased progressively until night, suggesting a higher difficulty to compensate the water lost by transpiration. However, relative water content (RWC) measured in the morning was similar in both periods, most likely due to some rainfall that occurred in September and first days of October that could be enough to re-hydrate canopy without significantly increasing soil water availability. The highest net photosynthetic rate (P _N) was found in both periods early in the morning (08:00–11:00). Reductions in P _N from May to October occurred in most cultivars except in José Dias and Ferrastar. In all cultivars a decrease in stomatal conductance (g _s) was observed. Photosynthetic capacity (P _max) did not significantly change from spring to autumn in nine cultivars, revealing a high resistance of photosynthetic machinery of this species to environmental stresses, namely high temperature and drought. Osmotic adjustment was observed in some cultivars, which showed reductions of ca. 23 % (Duro d' Estrada, José Dias) and 15 % (Tuono) in leaf osmotic potential (). Such decreases were accompanied by soluble sugars accumulation. The Portuguese cultivar José Dias had a higher photosynthetic performance than the remaining genotypes.     

Root and stem xylem embolism,stomatal conductance,and leaf turgor in Acer grandidentatum populations along a soil moisture gradient

The objective of this study was to determine how adjustment in stomatal conductance (g ^s) and turgor loss point (_tlp) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993–1994), the slope site had substantially lower xylem pressures (_px) and g ^s than the riparian site, particularly during the drought year of 1994. The _tlp was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday _px and the critical pressure triggering catastrophic xylem embolism (_pxCT). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from _pxCT averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g _s=9±2 versus 79±15 mmol m^-2 s^-1 at riparian site) and made any further osmotic adjustment of _tlp non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.     

Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na₂SO₄ were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (_w), leaf osmotic potential (_o) and leaf turgor potential (_p) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na₂SO₄-treated plants. The reduction in stomatal conductance was higher for NaCl-treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na⁺, Cl^– and SO₄ ^2–). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl^–, SO₄ ^2– and/or Na⁺. However, turgor of NaCl-treated plants was maintained at low EC (3 and 4 dS m^–1) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na⁺ in the plant tissues compared with the Na₂SO₄ treatment, could be the cause of the smaller decrease in growth.     

Mechanisms contributing to seasonal homeostasis of minimum leaf water potential and predawn disequilibrium between soil and plant water potential in Neotropical savanna trees

Seasonal regulation of leaf water potential (_L) was studied in eight dominant woody savanna species growing in Brazilian savanna (Cerrado) sites that experience a 5-month dry season. Despite marked seasonal variation in precipitation and air saturation deficit (D), seasonal differences in midday minimum _L were small in all of the study species. Water use and water status were regulated by a combination of plant physiological and architectural traits. Despite a nearly 3-fold increase in mean D between the wet and dry season, a sharp decline in stomatal conductance with increasing D constrained seasonal variation in minimum _L by limiting transpiration per unit leaf area (E). The leaf surface area per unit of sapwood area (LA/SA), a plant architectural index of potential constraints on water supply in relation to transpirational demand, was about 1.5–8 times greater in the wet season compared to the dry season for most of the species. The changes in LA/SA from the wet to the dry season resulted from a reduction in total leaf surface area per plant, which maintained or increased total leaf-specific hydraulic conductance (G_t) during the dry season. The isohydric behavior of Cerrado tree species with respect to minimum _L throughout the year thus was the result of strong stomatal control of evaporative losses, a decrease in total leaf surface area per tree during the dry season, an increase in total leaf-specific hydraulic conductance, and a tight coordination between gas and liquid phase conductance. In contrast with the seasonal isohydric behavior of minimum _L, predawn _L in all species was substantially lower during the dry season compared to the wet season. During the dry season, predawn _L was more negative than bulk soil estimated by extrapolating plots of E versus _L to E=0. Predawn disequilibrium between plant and soil was attributable largely to nocturnal transpiration, which ranged from 15 to 22% of the daily total. High nocturnal water loss may also have prevented internal water storage compartments from being completely refilled at night before the onset of transpiration early in the day.     

Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher , g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and , but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO₂ concentration relationships were consistent with the explanation that increased allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low- limitations on non-stomatal components of A. Our working hypothesis is that galls increase and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.     

Water relations of crok-oak (Quercus suber L.) under natural conditions

Daily and annual courses of leaf transpiration, stomatal conductance and shoot water potential of four Quercus suber individuals were compared in a semi-natural stand in southwest Portugal, from spring 1989 to early summer 1990.The trees investigated showed annual patterns typical of evergreen sclerophyllous species but varied in their range of stomatal operation. This appeared to be related to differences in hydraulic conductivity in the root-to-leaf pathway.Maximum stomatal conductance and transpiration rates occurred from March to June.Water stress was found to be moderate and winter cold stress due to low air and soil temperatures appeared to have an influence on plant water balance through their effects on flow resistances.Abbreviations g_sw stomatal conductance - g_max maximum stomatal conductance - PAR photosynthetically active radiation - RH relative humidity of the air - T leaf transpiration - Ta air temperature - TL leaf temperature - T_max maximum leaf transpiration - W air-to-leaf vapor pressure difference - shoot water potential - PD predawn shoot water potential - MIN minimum shoot water potential     

Compartmental distribution and redistribution of abscisic acid in intact leaves

Using a computer model written for whole leaves (Slovik et al. 1992, Planta 187, 14–25) we present in this paper calculations of abscisic acid (ABA) redistribution among different leaf tissues and their compartments in relation to stomatal regulation under drought stress. The model calculations are based on experimental data and biophysical laws. They yield the following results and postulates: (i) Under stress, compartmental pH-shifts come about as a consequence of the inhibition of the pH component of proton-motive forces at the plasmalemma. There is a decrease of net proton fluxes by about 8.6 nmol · s^–1 · m^–2. (ii) Using stress-induced pH-shifts we demonstrate how stress intensities can be quantified on a molecular basis. (iii) As the weak acid ABA is the only phytohormone which behaves in vivo and in vitro ideally according to the Henderson-Hasselbalch equation, pH-shifts induce a complicated redistribution amongst compartments in the model leaf. (iv) The final accumulation of ABA in guard-cell walls is intensive: up to 16.1-fold compared with only up to 3.4-fold in the guard-cell cytosol. We propose that the binding site of the guard-cell ABA receptor faces the apoplasm. (v) A twoto three-fold ABA accumulation in guard-cell walls is sufficient to induce closure of stomata. (vi) The minimum time lag until stomata start to close is 1–5 min; it depends on the stress intensity and on the guard-cell sensitivity to ABA: the more moderate the stress is, the later stomata start to close or they do not close at all. (vii) In the short term, there is almost no influence of the velocity of pH-shifts on the velocity of the ABA redistribution, (viii) Six hours after the termination of stress there is still an ABA concentration 1.4-fold the initial level in the guard-cell cytosol (delay of ABA relaxation, aftereffect), (ix) The observed induction of net ABA synthesis after onset of stress may be explained by a decrease in cytosolic ABA degradation. About 1 h after onset of stress the model leaf would start to synthesise ABA (and its conjugates) automatically, (x) This ABA net synthesis serves to inform roots via an increased ABA concentration in the phloem sap. The stress-induced ABA redistribution is per se not sufficient to feed the ploem sap with ABA. (xi) The primary target membrane of stress is the plasmalemma, not thylakoids. (xii) The effective stress sensor, which induces the proposed signal chain finally leading to stomatal closure, is located in epidermal cells. Mesophyll cells are not capable of creating a significant ABA signal to guard cells if the epidermal plasmalemma conductance to undissociated molecular species of ABA (HABA) is indeed higher than the plasmalemma conductance of the mesophyll (plasmodesmata open), (xiii) All model conclusions which can be compared with independent experimental data quantitatively fit to them. We conclude that the basic experimental data of the model are consistent. A stress-induced ABA redistribution in the leaf lamina elicits stomatal closure.Abbreviations ABA abscisic acid - CON vacuolar ABA conjugates We are grateful to Prof. U. Heber (Lehrstuhl Botanik I, University of Würzburg, FRG) for stimulating discussions. This work has been performed within the research program of the Sonderforschungsbereich 251 (TP 3 and 4) of the University of Würzburg. It has been also supported by the Fonds der Chemischen Industrie.     

Abscisic acid and indole-3-acetic acid contents in orange trees infected by Xylella fastidiosa and submitted to cycles of water stress

Pêra sweet orange plants (Citrus sinensis L. Osbeck) grafted on Rangpur lime rootstock (1 year-old) (Citrus limonia Osbeck) were inoculated with Xylella fastidiosa, a xylem-limited bacterium pathogen, which causes Citrus Variegated Chlorosis (CVC). Since it was known that water deficiency in the field enhances CVC-effects on the plant, the trees were submitted to three cycles of water stress during a one year period (March and October, 1998; and April, 1999) and divided in four treatments: healthy plants (HP); water-stressed healthy plants (WSHP); diseased plants (DP) and water-stressed diseased plants (WSDP). Stomatal conductance (g _s) of water-stressed diseased plants decreased in the first and second cycles of water deficiency, as the stress was increasing. The low stomatal conductance verified may be due to the high concentrations of abscisic acid (ABA) found in these plants. In the third cycle, values of g _s in diseased plants were, usually, lower than in the healthy ones. In healthy plants, g _s was reduced when these plants were submitted to water deficiency, independently of the cycle. The drop in leaf water potential in healthy plants was faster after irrigation was withheld, because healthy plants transpired more and, therefore, the water content of the substrate decreased more quickly. When the irrigation of WSDP was withheld in the third cycle, it was not possible to detect increases in ABA contents, suggesting that other factors could be acting to diminish the stomatal conductance in these plants. The presence of Xylella fastidiosa did not induce an increase in indole-3-acetic acid content in the leaves. After three cycles of water deficiency, the concentrations of indole-3-acetic acid in WSHP and WSDP were lower than those concentrations in the irrigated controls on the day water stress was more severe.     

Drought acclimation of two deciduous tree species of different layers in a temperate forest canopy

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (), bulk-leaf abscisic acid concentration ([ABA]_leaf), abscisic acid concentration in xylem sap ([ABA]_xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g_s), and the threshold dose per leaf area of the exogenous ABA (d_a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d_w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the , w, L, g_s, d_a and d_w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g_s, d_a, d_w and v in T. cordata, and on , w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer—T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA]_leaf had not been significantly changed in these trees, and [ABA]_xyl had increased during the drought period only in P. tremula. The relations between [ABA]_leaf, [ABA]_xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.     

Influence of Phosphorus Application on Water Relations, Biochemical Parameters and Gum Content in Cluster Bean Under Water Deficit

Relative water content (RWC), leaf water potential (_w) and osmotic potential (_s), contents of chlorophyll (Chl) a, Chl b, soluble sugars, and seed quality (gum content) were used to evaluate the role of phosphorus in alleviation of the deleterious effect of water deficit in clusterbean (Cyamopsis tetragonoloba L. Taub). Under water stress, _w, _s, and Chl and gum contents decreased and soluble sugar contents increased. Phosphorus application increased Chl and sugar contents in control plants and ameliorated negative effects of water stress.     

Daily fluctuations in water potential and associated ionic changes in Atriplex canescens

Summary Water potential () measurements of Atriplex canescens at the base of the Red Desert near Tipton in Wyoming, revealed a range between-15.5 to-45.1 bars. Minimum values coincided with the lowest air and soil temperatures, maximum with the greatest atmospheric evaporative demand. Change in exceeded 12 bars h^-1 during periods of rapidly moving storm systems. Changes in appeared to be independent of plant size, age, sex, and the spatial location of plants. Chemical analyses revealed that xylem sap was up to three times more concentrated at high than at low . It was observed that the flow rate of sap was greater at lower than at higher and that the increase in water movement accounted for the dilution of the baseline concentration of sap solutes. Together, Ca, Mg, K and Na contributed 58% of the mean osmolality of the xylem sap; the dominant ions, however, were K and Cl. We suggest that the ability of the species to respond rapidly to changing atmospheric conditions affords it a distinct advantage in a harsh environment.