Rapid leaf xylem acclimation diminishes the chances of embolism in grapevines.

Under most conditions tight stomatal regulation in grapevines (Vitis vinifera) avoids xylem embolism. The current study evaluated grapevine responses to challenging scenarios that might lead to leaf embolism and consequential leaf damage. We hypothesized that embolism would occur if the vines experienced low xylem water potential (Ψx) shortly after bud break or later in the season under a combination of extreme drought and heat. We subjected vines to two potentially dangerous environments: (i) withholding irrigation from a vineyard grown in a heatwave-prone environment, and (ii) subjecting potted vines to terminal drought 1 month after bud break. In the field experiment, a heatwave at the beginning of August resulted in leaf temperatures over 45 °C. However, effective stomatal response maintained the xylem water potential (Ψx) well above the embolism threshold, and no leaf desiccation was observed. In the pot experiment, leaves of well-watered vines in May were relatively vulnerable to embolism with 50% embolism (P50) at -1.8 MPa. However, when exposed to drought, these leaves acclimated their leaf P50 by 0.65 MPa in less than a week and before reaching embolism values. When dried to embolizing Ψx, the leaf damage proportion matched (percentage-wise) the leaf embolism level. Our findings indicate that embolism and leaf damage are usually avoided by the grapevines' efficient stomatal regulation and rapid acclimation of their xylem vulnerability.

Assessment of the Nutritional and Medicinal Potential of Tubers from Hairy Stork's-Bill (Erodium crassifolium L 'Hér), a Wild Plant Species Inhabiting Arid Southeast Mediterranean Regions.

Emerging needs for diversifying human diet and to explore novel therapeutic procedures have led to increasing attempts to retrieve traditional nourishments and recruit beneficial wild plant species. Species of the genus Erodium (Geraniaceae) harbor medicinal indications and substances known from folklore and scientific research. Hairy stork's bill (Erodium crassifolium L'Hér), is a small hemicryptophyte that inhabits arid southeast Mediterranean regions. E. crassifolium is among the very few Geraniaceae species known to produce tubers. Traditional knowledge holds that the tubers are edible and used by Bedouin tribes. However, no scientific information was found regarding nutrition or medicinal properties of these tubers. The objectives of our project are to unravel potential nutritional and medicinal benefits of the tubers, conduct initial steps towards domestication and develop agricultural practices enhancing E. crassifolium tuber yield and quality. Tubers show high water content (90%), low caloric value (23 Kcal 100-1 g) and considerable contents of minerals and vitamins. In addition, the tubers contain significant amounts of catechins and epigallocatechin, polyphenolic compounds known for their antioxidative, anti-inflammatory and antiproliferative activities. Furthermore, in vitro experiments demonstrated significant anti-inflammatory effects on human cell cultures. E. crassifolium is highly responsive to environmental changes; fertigation (700 mm) increased tuber yield by 10-fold, compared to simulated wild conditions (50-200 mm). These results indicate a significant potential of E. crassifolium becoming a valuable crop species. Therefore, there is a need for continued efforts in domestication, including ecotype selection, breeding, development of suitable agricultural practices and further exploration of its medicinal benefits.

Differential drought resistance strategies of co-existing woodland species enduring the long rainless Eastern Mediterranean summer.

In anticipation of a drier climate and to project future changes in forest dynamics, it is imperative to understand species-specific differences in drought resistance. The objectives of this study were to form a comprehensive understanding of the drought resistance strategies adopted by Eastern Mediterranean woodland species, and to elaborate specific ecophysiological traits that can explain the observed variation in survival among these species. We examined leaf water potential (𝛹), gas exchange and stem hydraulics during 2-3 years in mature individuals of the key woody species Phillyrea latifolia L., Pistacia lentiscus L. and Quercus calliprinos Webb that co-exist in a dry woodland experiencing ~ 6 rainless summer months. As compared with the other two similarly functioning species, Phillyrea displayed considerably lower 𝛹 (minimum 𝛹 of -8.7 MPa in Phillyrea vs -4.2 MPa in Pistacia and Quercus), lower 𝛹 at stomatal closure and lower leaf turgor loss point (𝛹TLP ), but reduced hydraulic vulnerability and wider safety margins. Notably, Phillyrea allowed 𝛹 to drop below 𝛹TLP under severe drought, whereas the other two species maintained positive turgor. These results indicate that Phillyrea adopted a more anisohydric drought resistance strategy, while Pistacia and Quercus exhibited a more isohydric strategy and probably relied on deeper water reserves. Unlike the two relatively isohydric species, Phillyrea reached complete stomatal closure at the end of the dry summer. Despite assessing a large number of physiological traits, none of them could be directly related to tree mortality. Higher mortality was observed for Quercus than for the other two species, which may result from higher water consumption due to its 2.5-10 times larger crown volume. The observed patterns suggest that similar levels of drought resistance in terms of survival can be achieved via different drought resistance strategies. Conversely, similar resistance strategies in terms of isohydricity can lead to different levels of vulnerability to extreme drought.

Root Development of Bell Pepper (Capsicum annuum L.) as Affected by Water Salinity and Sink Strength.

Fruits are the dominant sinks for assimilates. At optimal conditions, assimilates supply can meet the demand of fruits and those of the vegetative organs; however, extreme circumstances such as strong sink strength or an environmental stress may disturb this fine balance. While most studies focus on aboveground parameters, information regarding root growth dynamics under variable sink strength are scarce. The objective of this study was to evaluate the effect of sink strength (represented by fruit load) and salinity on bell-pepper root development. Three levels of fruit load were combined with two salinity levels in plants grown in an aeroponic system. Root growth was determined both by root capacitance and destructive measurements. Salinity and sink strength significantly affected root, shoot and fruit growth dynamics. Root growth was less affected by fruit load. Salinity stress was negatively associated with shoot growth, but after an acclimation period, salinity enhanced root development. Additionally, this study shows for the first time that root capacitance is a valid approach for non-destructive measurement of root development in aeroponic systems. The good correlation measured by us (r2 0.86) opens new opportunities for continuous root growth monitoring in aeroponic systems in the future.

Tree growth and water-use in hyper-arid Acacia occurs during the hottest and driest season.

Drought-induced tree mortality has been recently increasing and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe's deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel (annual precipitation 20-70 mm, restricted to October-May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 °C and vapor pressure deficit of up to 9 kPa. Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.

Reductions in root hydraulic conductivity in response to clay soil and treated waste water are related to PIPs down-regulation in Citrus.

Citrus hydraulic physiology and PIP transcript levels were characterized in heavy (clay) and light (sandy loam) soils with and without treated waste water (TWW) irrigation after a summer irrigation season and at the end of a winter rainy season recovery period. Consistent reductions in clay soils compared to sandy loam were found for fresh water (FW) and TWW irrigation, respectively, in root water uptake, as well as in hydraulic conductivity of whole plant (Ks plant), stem (Ks stem) and root (Ks root). Transcript levels of most PIPs down-regulated following TWW irrigation in both soils, but relative gene expression of three PIPs was significantly higher in summer for sandy soil and FW than for clay soil and TWW; their mRNA levels was significantly correlated to Ks root. A pot experiment, which compared short term influences of saline and TWW found that both treatments, compared to FW, reduced root water uptake and PIPs mRNA levels by 2-fold after 20 days, and the decreases continued with time until the end of the experiment. These latter data indicated that salinity had an important influence. Our results suggest that plant hydraulic adjustment to soil texture and water quality occurs rapidly, i.e. within days, and is modulated by PIPs expression.

Indirect Evidence for Genetic Differentiation in Vulnerability to Embolism in Pinus halepensis.

Climate change is increasing mean temperatures and in the eastern Mediterranean is expected to decrease annual precipitation. The resulting increase in aridity may be too rapid for adaptation of tree species unless their gene pool already possesses variation in drought resistance. Vulnerability to embolism, estimated by the pressure inducing 50% loss of xylem hydraulic conductivity (P 50), is strongly associated with drought stress resistance in trees. Yet, previous studies on various tree species reported low intraspecific genetic variation for this trait, and therefore limited adaptive capacities to increasing aridity. Here we quantified differences in hydraulic efficiency (xylem hydraulic conductance) and safety (resistance to embolism) in four contrasting provenances of Pinus halepensis (Aleppo pine) in a provenance trial, which is indirect evidence for genetic differences. Results obtained with three techniques (bench dehydration, centrifugation and X-ray micro-CT) evidenced significant differentiation with similar ranking between provenances. Inter-provenance variation in P 50 correlated with pit anatomical properties (torus overlap and pit aperture size). These results suggest that adaptation of P. halepensis to xeric habitats has been accompanied by modifications of bordered pit function driven by variation in pit aperture. This study thus provides evidence that appropriate exploitation of provenance differences will allow continued forestry with P. halepensis in future climates of the Eastern Mediterranean.

Impact of treated wastewater on growth, respiration and hydraulic conductivity of citrus root systems in light and heavy soils.

Roots interact with soil properties and irrigation water quality leading to changes in root growth, structure and function. We studied these interactions in an orchard and in lysimeters with clay and sandy loam soils. Minirhizotron imaging and manual sampling showed that root growth was three times lower in the clay relative to sandy loam soil. Treated wastewater (TWW) led to a large reduction in root growth with clay (45-55%) but not with sandy loam soil (<20%). Treated wastewater increased salt uptake, membrane leakage and proline content, and decreased root viability, carbohydrate content and osmotic potentials in the fine roots, especially in clay. These results provide evidence that TWW challenges and damages the root system. The phenology and physiology of root orders were studied in lysimeters. Soil type influenced diameter, specific root area, tissue density and cortex area similarly in all root orders, while TWW influenced these only in clay soil. Respiration rates were similar in both soils, and root hydraulic conductivity was severely reduced in clay soil. Treated wastewater increased respiration rate and reduced hydraulic conductivity of all root orders in clay but only of the lower root orders in sandy loam soil. Loss of hydraulic conductivity increased with root order in clay and clay irrigated with TWW. Respiration and hydraulic properties of all root orders were significantly affected by sodium-amended TWW in sandy loam soil. These changes in root order morphology, anatomy, physiology and hydraulic properties indicate rapid and major modifications of root systems in response to differences in soil type and water quality.

Lower leaf gas-exchange and higher photorespiration of treated wastewater irrigated Citrus trees is modulated by soil type and climate.

Water quality, soil and climate can interact to limit photosynthesis and to increase photooxidative damage in sensitive plants. This research compared diffusive and non-diffusive limitations to photosynthesis as well as photorespiration of leaves of grapefruit trees in heavy clay and sandy soils having a previous history of treated wastewater (TWW) irrigation for >10 years, with different water qualities [fresh water (FW) vs TWW and sodium amended treated wastewater (TWW + Na)] in two arid climates (summer vs winter) and in orchard and lysimeter experiments. TWW irrigation increased salts (Na(+) and Cl(-) ), membrane leakage, proline and soluble sugar content, and decreased osmotic potentials in leaves of all experiments. Reduced leaf growth and higher stomatal and non-stomatal (i.e. mesophyll) limitations were found in summer and on clay soil for TWW and TWW + Na treatments in comparison to winter, sandy soil and FW irrigation, respectively. Stomatal closure, lower chlorophyll content and altered Rubisco activity are probable causes of higher limitations. On the other hand, non-photochemical quenching, an alternative energy dissipation pathway, was only influenced by water quality, independent of soil type and season. Furthermore, light and CO2 response curves were investigated for other possible causes of higher non-stomatal limitation. A higher proportion of non-cyclic electrons were directed to the O2 dependent pathway, and a higher proportion of electrons were diverted to photorespiration in summer than in winter. In conclusion, both diffusive and non-diffusive limitations contribute to the lower photosynthetic performance of leaves following TWW irrigation, and the response depends on soil type and environmental factors.

Simulating nectarine tree transpiration and dynamic water storage from responses of leaf conductance to light and sap flow to stem water potential and vapor pressure deficit.

For isohydric trees mid-day water uptake is stable and depends on soil water status, reflected in pre-dawn leaf water potential (Ψpd) and mid-day stem water potential (Ψmd), tree hydraulic conductance and a more-or-less constant leaf water potential (Ψl) for much of the day, maintained by the stomata. Stabilization of Ψl can be represented by a linear relationship between canopy resistance (Rc) and vapor pressure deficit (D), and the slope (BD) is proportional to the steady-state water uptake. By analyzing sap flow (SF), meteorological and Ψmd measurements during a series of wetting and drying (D/W) cycles in a nectarine orchard, we found that for the range of Ψmd relevant for irrigated orchards the slope of the relationship of Rc to D, BD is a linear function of Ψmd. Rc was simulated using the above relationships, and its changes in the morning and evening were simulated using a rectangular hyperbolic relationship between leaf conductance and photosynthetic irradiance, fitted to leaf-level measurements. The latter was integrated with one-leaf, two-leaf and integrative radiation models, and the latter gave the best results. Simulated Rc was used in the Penman-Monteith equation to simulate tree transpiration, which was validated by comparing with SF from a separate data set. The model gave accurate estimates of diurnal and daily total tree transpiration for the range of Ψmds used in regular and deficit irrigation. Diurnal changes in tree water content were determined from the difference between simulated transpiration and measured SF. Changes in water content caused a time lag of 90-105 min between transpiration and SF for Ψmd between -0.8 and -1.55 MPa, and water depletion reached 3 l h(-1) before noon. Estimated mean diurnal changes in water content were 5.5 l day(-1) tree(-1) at Ψmd of -0.9 MPa and increased to 12.5 l day(-1) tree(-1) at -1.45 MPa, equivalent to 6.5 and 16.5% of daily tree water use, respectively. Sixteen percent of the dynamic water volume was in the leaves. Inversion of the model shows that Ψmd can be predicted from D and Rc, which may have some importance for irrigation management to maintain target values of Ψmd. That relationship will be explored in future research.

Inactive xylem can explain differences in calibration factors for thermal dissipation probe sap flow measurements.

Thermal dissipation probes (TDPs) were calibrated in three diffuse porous fruit trees and one ornamental species in the field by comparison with heat pulse probes (nectarine and persimmon), in a greenhouse on lysimeters (apple and persimmon) and in the laboratory by pushing water through cut branches (apple, Peltophorum and nectarine). Two operational methods were used: continuous (constant thermal dissipation, CTD) and discontinuous, or transient, heating (transient thermal dissipation, TTD). Correction for the radial distribution of sap flux density was with an analytical function derived from a linear decrease in flux density with depth, as measured with a multi-depth 'Tmax' heat pulse system. When analyzed with previous calibration factors, the measured sap flow was <50% of actual value. The underestimations were consistent, and calibrations for each species in the field, greenhouse and laboratory gave approximately the same factors. Reasonable values of tree water use were obtained with the new calibration factors. Evidence is provided that even though the xylem was diffuse porous, the underestimations were caused by contact of the probes with inactive xylem along their length. The average portion of probe in contact with inactive xylem, measured in stained branches following laboratory calibrations, was 0.2-0.24. Using the measured fractions to correct temperature differentials between heated and unheated probes for CTD and TTD, based on Clearwater et al. (in Potential errors in measurement of nonuniform sap flow using heat dissipation probes. Tree Physiol 1999;19:681-687) almost completely compensated for the underestimations. Calibrations are given for each species both before and after corrections of temperature differentials, along with a multispecies calibration. These results should be an important step in reconciling many reports of different calibration factors for TDP probes.

The importance of iron supply during repetitive harvesting of Aster tripolium.

Aster tripolium L. is a salt marsh halophyte that has recently gained interest as a cash crop vegetable. Leaf yield and quality were investigated in plants grown with salinity in experiments with Perlite in pots and in plots on dune sand. Plants were repetitively harvested in a 14-day cycle. A. tripolium irrigated with 50mM NaCl exhibited the highest yield when grown in pots, whereas in the plot experiment no significant differences in biomass accumulation occurred up to 80mM NaCl in the irrigation water. Chemical leaf composition changed with salinity, exhibiting higher levels of electrical conductivity, total soluble solutes and the non-enzymatic antioxidant compounds ascorbic acid and polyphenols compared with control plants grown without NaCl supplementation. Using the repetitive harvest regime, leaf chlorosis occurred, a symptom shared by deficiencies in either nitrogen or iron. Comparative applications of five iron chelate formulations in plants grown with 50mM NaCl in pots revealed improved leaf colour and chlorophyll content for only two of the applied Fe-chelates. Concomitantly with leaf colour restoration, the activity of nitrate reductase, the first enzyme during nitrate assimilation, which requires heme-iron for its proper function, increased 3-fold as a result of the iron treatment in the plot experiment. Importantly, the enhancement of nitrate reductase activity was associated with a considerable decrease in the leaf nitrate concentration. Therefore, we concluded that iron deficiency, in addition to leaf chlorosis, reduces A. tripolium leaf quality as a vegetable by increasing the leaf nitrate content. Furthermore, nitrate reductase (NR) activity levels in A. tripolium leaves may act as an indicator of iron deficiency that manifests itself as reduced nitrate content owing to the higher NR activity upon proper iron nutrition. These results demonstrate the importance of salinity level and the application of an appropriate iron-chelating formulation to generate marketable yields of Aster tripolium leafy vegetable when grown commercially on dune sand.

Differential ecophysiological response of a major Mediterranean pine species across a climatic gradient.

The rate of migration and in situ genetic variation in forest trees may not be sufficient to compete with the current rapid rate of climate change. Ecophysiological adjustments of key traits, however, could complement these processes and allow sustained survival and growth across a wide range of climatic conditions. This was tested in Pinus halepensis Miller by examining seven physiological and phenological parameters in five provenances growing in three common garden plots along a climatic transect from meso-Mediterranean (MM) to thermo-Mediterranean (TM) and semi-arid (SA) climates. Differential responses to variations in ambient climatic conditions were observed in three key traits: (i) growing season length decreased with drying in all provenances examined (from 165 under TM climate to 100 days under SA climate, on average); (ii) water use efficiency (WUE) increased with drying, but to a different extent in different provenances, and on average from 80, to 95, to 110 µmol CO(2) mol(-1) H(2)O under MM, TM and SA climates, respectively; (iii) xylem native embolism was stable across climates, but varied markedly among different provenances (percent loss of conductivity, was below 5% in two provenances and above 35% in others). The results indicated that changes in growing season length and WUE were important contributors to tree growth across climates, whereas xylem native embolism negatively correlated with tree survival. The results indicated that irrespective of slow processes (e.g., migration, genetic adaptation), the capacity for ecophysiological adjustments combined with existing variations among provenances could help sustain P. halepensis, a major Mediterranean tree species, under relatively extreme warming and drying climatic trends.

Estimating sap flux densities in date palm trees using the heat dissipation method and weighing lysimeters.

In a world of diminishing water reservoirs and a rising demand for food, the practice and development of water stress indicators and sensors are in rapid progress. The heat dissipation method, originally established by Granier, is herein applied and modified to enable sap flow measurements in date palm trees in the southern Arava desert of Israel. A long and tough sensor was constructed to withstand insertion into the date palm's hard exterior stem. This stem is wide and fibrous, surrounded by an even tougher external non-conducting layer of dead leaf bases. Furthermore, being a monocot species, water flow does not necessarily occur through the outer part of the palm's stem, as in most trees. Therefore, it is highly important to investigate the variations of the sap flux densities and determine the preferable location for sap flow sensing within the stem. Once installed into fully grown date palm trees stationed on weighing lysimeters, sap flow as measured by the modified sensors was compared with the actual transpiration. Sap flow was found to be well correlated with transpiration, especially when using a recent calibration equation rather than the original Granier equation. Furthermore, inducing the axial variability of the sap flux densities was found to be highly important for accurate assessments of transpiration by sap flow measurements. The sensors indicated no transpiration at night, a high increase of transpiration from 06:00 to 09:00, maximum transpiration at 12:00, followed by a moderate reduction until 08:00; when transpiration ceased. These results were reinforced by the lysimeters' output. Reduced sap flux densities were detected at the stem's mantle when compared with its center. These results were reinforced by mechanistic measurements of the stem's specific hydraulic conductivity. Variance on the vertical axis was also observed, indicating an accelerated flow towards the upper parts of the tree and raising a hypothesis concerning dehydrating mechanisms of the date palm tree. Finally, the sensors indicated reduction in flow almost immediately after irrigation of field-grown trees was withheld, at a time when no climatic or phenological conditions could have led to reduction in transpiration.

Hydraulic adjustments underlying drought resistance of Pinus halepensis.

Drought-induced tree mortality has increased over the last decades in forests around the globe. Our objective was to investigate under controlled conditions the hydraulic adjustments underlying the observed ability of Pinus halepensis to survive seasonal drought under semi-arid conditions. One hundred 18-month saplings were exposed in the greenhouse to 10 different drought treatments, simulating combinations of intensities (fraction of water supply relative to control) and durations (period with no water supply) for 30 weeks. Stomata closed at a leaf water potential (Ψ(l)) of -2.8 MPa, suggesting isohydric stomatal regulation. In trees under extreme drought treatments, stomatal closure reduced CO(2) uptake to -1 µmol m(-2) s(-1), indicating the development of carbon starvation. A narrow hydraulic safety margin of 0.3 MPa (from stomatal closure to 50% loss of hydraulic conductivity) was observed, indicating a strategy of maximization of CO2 uptake in trees otherwise adapted to water stress. A differential effect of drought intensity and duration was observed, and was explained by a strong dependence of the water stress effect on the ratio of transpiration to evapotranspiration T/ET and the larger partitioning to transpiration associated with larger irrigation doses. Under intense or prolonged drought, the root system became the main target for biomass accumulation, taking up to 100% of the added biomass, while the stem tissue biomass decreased, associated with up to 60% reduction in xylem volume.

LeFRK2 is required for phloem and xylem differentiation and the transport of both sugar and water.

It has been suggested that LeFRK2, the major fructose-phosphorylating enzyme in tomato plants, may be required for stem xylem development. Yet, we do not know if this enzyme affects the development of individual vessels, whether it affects water conductance, or whether it affects phloem development and sugar transport. Here, we show that suppression of LeFRK2 results in a significant reduction in the size of vascular cells and slows fiber maturation. The vessels in stems of LeFRK2-antisense plants are narrower than in WT plants and have thinner secondary cell walls. Although the cambium produces rounded secondary vessels, these vessels become deformed during the early stages of xylem maturation. Water conductance is then reduced in stems, roots, and leaves, suggesting that LeFRK2 influences xylem development throughout the entire vascular system. Interestingly, the build-up of positive xylem pressure under static (no-flow) conditions was also decreased. Suppression of LeFRK2 reduced the length and width of the sieve elements, as well as callose deposition. To examine the effect of LeFRK2 suppression on phloem transport, we created triple-grafted plants in which a portion of the wild-type stem was replaced with an antisense interstcok, and compared the contents of the transported sugar, sucrose, in the different portions of these stems. Sucrose contents above and within the LeFRK2-antisense interstock were significantly higher than those below the graft. These results show that the antisense interstock restricted the downward movement of sucrose, suggesting that LeFRK2 is required for both phloem and xylem development.

Fertilization and blending alternatives for irrigation with desalinated water.

In arid-zone agriculture where available irrigation water is saline, desalination is becoming an attractive method for increasing yields and reducing negative environmental consequences. However, irrigation with desalinated water can be problematic if essential nutrients, including Ca, Mg, and S, removed during reverse osmosis, are not reintroduced. We evaluated two strategies for supplying these nutrients - direct fertilization and blending of desalinated with saline groundwater -experimentally in a greenhouse and in a model for a case study regarding pepper (Capsicum annuum L.) production. Reducing salinity from electrical conductivity (EC) 3.20 to EC 0.40 dS m(-1) by reverse-osmosis desalination increased maximum yields by almost 50% while allowing a reduction of applied irrigation water to half of that with the saline water, but the associated cost of fertilizing with Ca, Mg, and S minerals was high (around $0.50 m(-3)). Blending 30% saline water with 70% desalinated water brought Ca, Mg, and S minerals to satisfactory levels while producing water with salinity of EC = 1.35 dS m(-1). Comparison of relative pepper yields and analysis of simulated results showed that irrigation with blended water maintained yields greater than 90% compared to irrigation with fully desalinated water, but only as irrigation rates were increased by more than 50%. The environmental cost of the increase in irrigation-water salinity from EC 0.40 to EC 1.35 dS m(-1) in the blended water was shown to be substantial as it involved five times greater loading (into the soil) and leaching (beyond the root zone) of salts and other contaminants.

Isotope effects in the evaporation of water: a status report of the Craig-Gordon model.

The Craig-Gordon model (C-G model) [H. Craig, L.I. Gordon. Deuterium and oxygen 18 variations in the ocean and the marine atmosphere. In Stable Isotopes in Oceanographic Studies and Paleotemperatures, E. Tongiorgi (Ed.), pp. 9-130, Laboratorio di Geologia Nucleare, Pisa (1965).] has been synonymous with the isotope effects associated with the evaporation of water from surface waters, soils, and vegetations, which in turn constitutes a critical component of the global water cycle. On the occasion of the four decades of its successful applications to isotope geochemistry and hydrology, an attempt is made to: (a) examine its physical background within the framework of modern evaporation models, (b) evaluate our current knowledge of the environmental parameters of the C-G model, and (c) comment on a general strategy for the use of these parameters in field applications. Despite its simplistic representation of evaporation processes at the water-air interface, the C-G model appears to be adequate to provide the isotopic composition of the evaporation flux. This is largely due to its nature for representing isotopic compositions (a ratio of two fluxes of different isotopic water molecules) under the same environmental conditions. Among many environmental parameters that are included in the C-G model, accurate description and calculations are still problematic of the kinetic isotope effects that occur in a diffusion-dominated thin layer of air next to the water-air interface. In field applications, it is of importance to accurately evaluate several environmental parameters, particularly the relative humidity and isotopic compositions of the 'free-atmosphere', for a system under investigation over a given time-scale of interest (e.g., hourly to daily to seasonally). With a growing interest in the studies of water cycles of different spatial and temporal scales, including paleoclimate and water resource studies, the importance and utility of the C-G model is also likely to grow in the future.

Low air humidity increases leaf-specific hydraulic conductance of Arabidopsis thaliana (L.) Heynh (Brassicaceae).

The typical isohydric plant response to low relative humidity involves stomatal closure, followed by long-term responses like adjustment of shoot-to-root ratios. Little information is available on the early responses of the root system to exposure of shoots to low humidity, nor is it clear to what extent responses of Arabidopsis thaliana conform to the isohydric model. In this study, A. thaliana plants grown hydroponically at high humidity were exposed to two constant relative humidities, 17% and 77%, while the root system remained in aerated nutrient solution. Leaf conductance (g(s)), transpiration, water potential (Psi(l)), osmotic potential, and whole plant hydraulic conductance (K) were determined for the following time intervals: 0-10, 10-20, and 20-40 min, and 0-5, 5-10, and 24-29 h. At low relative humidity, no change in g(s) was detected. Psi(l) decreased by 0.28 MPa during the first 5 h and then remained stable. During the first hour, leaf-specific K averaged 1.6 x 10(-5) kg MPa(-1) m(-2) s(-1) at high humidity. At low humidity it increased >3-fold to 5.8 x 10(-5) kg MPa(-1) m(-2) s(-1). Similar significant differences in K were observed during all time periods. Low concentration mercury amendments in the hydroponic solution (5 microM and 10 microM HgCl(2)) had no discernible influence, but pre-exposure to 50 microM HgCl(2) reduced K differences between humidity treatments. As HgCl(2) is known to be a potent inhibitor of aquaporin function, this suggests that aquaporins may have played a role in the fast hydraulic response of plants transferred to low humidity. The rapid hydraulic response and the influence of mercury raise the possibility that an alternative response to atmospheric dryness is increased K modulated by aquaporins.