High Atmospheric Evaporative Demand Research Articles

Foliage development and resource allocation determine the growth responses of silver birch (Betula pendula) to elevated environmental humidity.

Scenarios for future climate predict an increase in precipitation amounts and frequency of rain events, resulting in higher air humidity and soil moisture at high latitudes, including in northern Europe. We analysed the effects of artificially elevated environmental humidity (air relative humidity and soil moisture) on leaf gas exchange, water relations, growth and phenology of silver birch (Betula pendula) trees growing at the free air humidity manipulation experimental site situated in the hemiboreal vegetation zone, in eastern Estonia, with no occurring water deficit to the trees. The environmental humidity manipulation did not significantly affect the water relations traits but did affect some leaf gas exchange parameters, growth and phenology of the trees. Elevated air humidity (H) did not influence photosynthetic capacity and stomatal conductance, while the trees exhibited higher stomatal sensitivity to leaf-to-air vapour pressure difference compared with the trees at ambient conditions (C) or at elevated soil moisture (I). H trees demonstrated reduced height growth and foliage biomass, increased allocation to stem radial growth and prolonged leaf retention in autumn compared with the C trees. Increased air humidity supports longer leaf retention and growth period, but this does not translate into increased growth parameters at the tree level. The changes in tree growth in response to increasing atmospheric humidity could plausibly be explained by (i) retardation of foliage development and (ii) changes in resource allocation, causing a shift in the ratio of photosynthetic to non-photosynthetic tissues in favour of the latter. Under high atmospheric evaporative demand, higher stomatal sensitivity in H trees induces faster stomatal closure, which may result in carbon starvation. A future rise in atmospheric humidity at high latitudes may lead to reduced tree growth and forest productivity, in contrast to the predicted future of forests.

Response of evapotranspiration to the 2022 unprecedented extreme drought in the Yangtze River Basin

AbstractThe response of evapotranspiration (ET) to drought is essential for understanding water availability under drought conditions, which has received increasing scientific attention but remains highly uncertain. An unprecedented extreme drought sweeping the whole Yangtze River Basin during the summer and autumn of 2022 provides a novel opportunity to explore the ET response to drought within large river basins. Here, we apply multiple ET products to investigate the spatiotemporal variation of ET during the 2022 extreme drought and analyse the response of ET to drought in the Yangtze River Basin. The results show that this drought, characterized by widespread substantial precipitation deficit and high level of atmospheric evaporative demand (AED), leads to evident abnormal ET. Over the whole basin, ET exhibits a notable increase in the initial 2 months of this drought, which can significantly reduce water availability. However, despite continued high AED, the increase in ET fails to remain in the following months due to limited moisture supply. This suggests that ET response to drought varies as drought progresses. Among other drought events from 2003 to 2022, the response of ET is similar to that in the 2022 extreme drought. Specifically, for most sub‐basins, ET typically shows positive anomalies in the early stage of drought, followed by negative anomalies in the late stage. Generally, our findings improve the understanding of the dynamic response of ET to drought at the basin scale, which is crucial for basin water security.

Examining the outstanding Euro-Mediterranean drought of 2021–2022 and its historical context

Examining the outstanding Euro-Mediterranean drought of 2021–2022 and its historical context

What caused the unseasonal extreme dust storm in Uzbekistan during November 2021?

An unseasonal dust storm hit large parts of Central Asia on 4–5 November 2021, setting records for the column aerosol burden and fine particulate concentration in Tashkent, Uzbekistan. The dust event originated from an agropastoral region in southern Kazakhstan, where the soil erodibility was enhanced by a prolonged agricultural drought resulting from La Niña-related precipitation deficit and persistent high atmospheric evaporative demand. The dust outbreak was triggered by sustained postfrontal northerly winds during an extreme cold air outbreak. The cold air and dust outbreaks were preceded by a chain of processes consisting of recurrent synoptic-scale transient Rossby wave packets over the North Pacific and North Atlantic, upper-level wave breaking and blocking over Greenland, followed by high-latitude blocking over Northern Europe and West Siberia, and the equatorward shift of a tropopause polar vortex and cold pool into southern Kazakhstan. Our study suggests that the historic dust storm in Uzbekistan was a compound weather event driven by cold extreme, high winds, and drought precondition.

Unenriched xylem water contribution during cellulose synthesis influenced by atmospheric demand governs the intra-annual tree-ring δ18 O signature.

The oxygen isotope composition (δ18 O) of tree-ring cellulose is used to evaluate tree physiological responses to climate, but their interpretation is still limited due to the complexity of the isotope fractionation pathways. We assessed the relative contribution of seasonal needle and xylem water δ18 O variations to the intra-annual tree-ring cellulose δ18 O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ18 O measurements of soil water, needle water, and twig xylem water with intra-annual δ18 O measurements of tree-ring cellulose, xylogenesis analysis, and mechanistic and structural equationmodeling. Intra-annual cellulose δ18 O values resembled source water δ18 O mean levels better than needle water δ18 O. Large parts of the rings were formed under high proportional exchange with unenriched xylem water (pex ). Maximum pex values were achieved in August and imprinted on sections at 50-75% of the ring. High pex values were associated with periods of high atmospheric evaporative demand (VPD). While VPD governed needle water δ18 O variability, we estimated a limited Péclet effect at both sites. Due to a variable pex , source water has a strong influence over large parts of the intra-annual tree-ring cellulose δ18 O variations, potentially masking signals coming from needle-level processes.

Atmospheric evaporative demand and water deficit on the ecophysiology of Rubber seedlings

The search for genetic materials resistant to adverse weather conditions has been a major focus in studies on species of economic interest. The objective of the present study was to assess the growth and photosynthesis of rubber seedlings clones under two conditions of atmospheric evaporative demand, characterized by fluctuations in temperature (TEMP) and vapor pressure deficit (VPD), associated to two water regimens. Hevea brasiliensis Muell. Arg (RRIM 600 and FX 3864) clones were assessed in two microclimates, at low (TEMP 21.2 ºC and VPD 0.29 Kpa) and high (TEMP 26.9 ºC and VPD 1.49 Kpa) atmospheric evaporative demand, under two water regimens: water deficit and well-watered. Water deficit 50% water availability was sufficient to reduce the net CO2 assimilation rate, leaf area and total chlorophyll of the clones studied that impacted growth in both microclimates. The effects of water deficit on growth and net carbon assimilation rate were intensified under high atmospheric evaporative demand. However, when comparing the two clones studied, RRIM 600 showed greater growth and photosynthesis without water restriction. The FX 3864 clone, despite the high CO2 assimilation values under high atmospheric demand and without water restriction, showed a reduced growth. The results of this study form an important basis for the selection of genotypes with the potential to develop in adverse climatic conditions. In this sense, the RRIM 600 genotype is recommended as a promising material that would best adapt under adverse climatic conditions.

Cotton stomatal closure under varying temperature and vapor pressure deficit, correlation with the hydraulic conductance trait

BackgroundCotton (Gossypium hirsutum L.) is often grown in locations characterized by high atmospheric evaporative demand. It has been hypothesized that plants which resist hydraulic flow under this condition will limit water use and conserve soil water. Therefore, in a series of controlled environment experiments ten cotton cultivars were exposed to two different temperature and vapor pressure deficit (VPD) conditions (i.e., 38 °C, > 3 kPa and 32 °C, 1∼1.5 kPa) as well as a progressive soil drying. Then, individual differences in shoot hydraulic conductance (Kshoot) was measured using a hydraulic conductance flow meter (HCFM). Physiological parameters were reported included leaf area, dry leaf weight, stomatal conductance (gs), and water use efficiency coefficient (WUEk).ResultsDifferences were observed in Kshoot among cultivars under the 38 °C, > 3 kPa but not the 32 °C, 1∼1.5 kPa environment. Under the 38 °C, > 3 kPa environment, correlations were found between Kshoot, stomatal conductance (gs), VPD breakpoint, WUEk, total leaf area, dry leaf weight, fraction transpirable soil water (FTSW) threshold, and slope of TR decline after FTSW threshold.ConclusionResults show that the ability of some cotton cultivars to restrict water loss under high evaporative demand through early stomatal closure is associated with the cultivars’ Kshoot. The Kshoot is influential in the limitation of TR trait under high temperature and VPD.

Environmental determination of spring wheat yield in a climatic transition zone under global warming.

Understanding environmental determination of crop yield plays a critical role in agricultural. management in resource-limited areas. The climatic transition zone was a naturally ideal place to study. the relations between environmental factors and crop yield, due to its large annual variability of climatic factors and high speed of temperature increase under global warming. Our objectives were to identify the most critical environmental factor in determining spring wheat yield and analyze the convergence and divergence of water-yield relations for spring wheat in a typical climatic transition zone (semi-arid area). The study was conducted at two locations, Dingxi and Pengyang in Northwest China, with a long-term experiment (1987-2018) and two short-term irrigation experiments. Meanwhile, data of water use and spring wheat yield was collected from a series of previously published literature in the study area. The highest spring wheat yield was obtained under year pattern with higher soil water content at sowing (SWCS) and lower atmospheric dryness condition (ADC, the difference between reference evapotranspiration and precipitation during spring wheat growing season). SWCS was more important than precipitation during the growing season (PGS) in determining spring wheat yield in the study area. The relations between available water supply, water use, and spring wheat yield were convergence. However, SWCS had an impact on the relationship between yield and PGS and SWCS-yield relation was affected by ADC. We concluded that precipitation in 7months before sowing was the dominant factor determining spring wheat yield in the climatic transition zone under global warming whereas the impact of high atmospheric evaporative demand resulted from the increasing temperature on crop yields and SWCS-yield relation must be taken into account for the analysis of environmental determination of spring wheat yield.

A multiple-trait analysis of ecohydrological acclimatisation in a dryland phreatophytic shrub

Water is the main limiting factor for groundwater-dependent ecosystems (GDEs) in drylands. Predicted climate change (precipitation reductions and temperature increases) and anthropogenic activities such as groundwater drawdown jeopardise the functioning of these ecosystems, presenting new challenges for their management. We developed a trait-based analysis to examine the spatiotemporal variability in the ecophysiology of Ziziphus lotus, a long-lived phreatophyte that dominates one of the few terrestrial GDEs of semiarid regions in Europe. We assessed morpho-functional traits and stem water potential along a naturally occurring gradient of depth-to-groundwater (DTGW, 2–25 m) in a coastal aquifer, and throughout the species-growing season. Increasing DTGW and salinity negatively affected photosynthetic and transpiration rates, increasing plant water stress (lower predawn and midday water potential), and positively affected Huber value (sapwood cross-sectional area per leaf area), reducing leaf area and likely, plant hydraulic demand. However, the species showed greater salt-tolerance at shallow depths. Despite groundwater characteristics, higher atmospheric evaporative demand in the study area, which occurred in summer, fostered higher transpiration rates and water stress, and promoted carbon assimilation and water loss more intensively at shallow water tables. This multiple-trait analysis allowed us to identify plant ecophysiological thresholds related to the increase in salinity, but mostly in DTGW (13 m), and in the evaporative demand during the growing season. These findings highlight the existence of tipping points in the functioning of a long-lived phreatophyte in drylands and can contribute to the sustainable management of GDEs in southern Europe, paving the way for further studies on phreatophytic species.

How does mistletoe infection affect seasonal physiological responses of hosts with different leaf phenology?

• Infected and non-infected branches showed distinct stomata responses depending on host phenology. • Differences in regulation of water loss between branches were more apparent in the wet season. • Pre-dawn leaf water potentials were consistently more negative for infected branches. • Leaf mobile nutrient concentrations were lower in infected branches. • Host species adjust at the individual level to mistletoe physiological stress throughout the year. Plants should have the ability to perceive physiological changes within their branches when infected by mistletoes, adjusting the use of resources between infected and uninfected branches which can be crucial for their survival in the long-term. Here we investigated how branches infected by the mistletoe Passovia ovata (Pohl ex DC.) Tiegh. and uninfected branches within the same individual tree respond to seasonal environmental changes across two hosts of contrasting leaf phenology (the evergreen Miconia albicans (SW.) Triana and the deciduous Byrsonima verbascifolia (L.) DC.). We measured key leaf traits (instantaneous gas exchange rates, diurnal courses of stomatal conductance, leaf water potential, specific leaf area and leaf macronutrient concentrations) during the peak of the wet and dry season in a seasonal savanna of central Brazil. Pre-dawn leaf water potentials were consistently more negative for infected branches of both hosts, suggesting that overnight water refilling of infected branches was more limited. However, infected and uninfected branches exhibited similar leaf water potentials at midday, suggesting that they undergo similar imbalances in water supply and demand during periods of high atmospheric evaporative demand. Infected and non-infected branches of the evergreen mistletoe showed tighter regulation of water loss, whereas infected branches of the deciduous host were less constrained in regulating leaf transpiration. We also found differences for nutrient concentrations: N, P and K were lower, while Ca was higher in leaves of infected branches. Physiological changes induced by mistletoe infection affected host performance, and were reflected in water and nutrient use differences between infected and uninfected branches. Our findings show that infection responses by mistletoes can be detected between branches within individual trees, and that host species with distinct patterns of leaf phenology are capable to adjust, at the individual level, to cope with mistletoe's imposed physiological stress throughout the year.

Effect of root-zone vertical soil moisture heterogeneity on water transport safety in soil-plant-atmosphere continuum in Robinia pseudoacacia

Abstract Soil moisture in root zone is highly heterogeneous in space, while its effect on water transport safety in soil-plant-atmosphere continuum (SPAC) remains poorly understood. In this study, we conducted vertical spilt-root experiments in R. pseudoacacia using loamy clay and sandy loam soils in greenhouse, and measured the dynamics of midday transpiration rate, predawn and midday leaf water potential with the lower root zone remaining drought and the upper root zone undergoing the drought-rewatered-drought process. The plant supply-demand hydraulic model was calibrated with the measured data, indicating that the model could efficiently simulate SPAC water transport in R. pseudoacacia under the condition of vertical soil moisture heterogeneity. On this basis, we set various combinations of soil moisture in the upper and lower root zones under different soil types and atmospheric evaporative demands in the model, and simulated the variations of indicators describing water transport safety in SPAC, including actual transpiration rate (E), the critical leaf transpiration rate at hydraulic failure (Ecrit), hydraulic safety margin (HSM), and percentage loss of soil-plant hydraulic conductance (PLK). The numerical simulations suggested that the water transport safety in SPAC varied substantially with vertical soil moisture heterogeneity, and the responses were impacted by soil types and atmospheric evaporative demand. With decreasing soil moisture in the upper root zone (SMCup), Ecrit, E and HSM remained steady at first and then decreased rapidly when SMCup below a threshold, while PLK exhibited an opposite trend. With decreasing soil moisture in the lower root-zone (SMCdown), the curves of Ecrit, E and HSM presented a descending trend, while the curve of PLK went up. Water transport safety in SPAC declined with decreasing SWCup and SWCdown and became more sensitive to SWCup with a lower SWCdown. SWCdown had greater impact on water transport safety in SPAC under coarser-textured soil with higher atmospheric evaporative demand. The results were supplemental to the traditional analysis of soil water availability to plants under homogeneous condition, and would be helpful for analyzing functional difference of water storage in different soil depths as well as for optimizing water resource management.

Fog mitigates the consequences of a profligate water use strategy in a Macaronesian cloud forest tree species

How plants thrive in a cloud immersed environment where leaves are intermittently wet and the environmental conditions that drive transpiration and growth are limited, raises a relevant question in fog affected ecosystems. In order to provide insight into how cloud immersion and fog interception may affect Macaronesian laurisilva forests, micrometeorological variables, artificial fog water collection, throughfall, soil water content and the altitude of the trade wind inversion layer, together with the hourly sap flow, Qt, of a dominant tree species, Myrica faya, were measured at an exposed site of the Anaga Massif Rural Park Biosphere Reserve (Tenerife, Canary Islands) over a period of one year. Foggy conditions led to a 45.1% reduction in global radiation and a more than a 10-fold decrease in sap flow, throughout all day hours. M. faya showed a weak control of the transpiration rate and a profligate water use strategy, such that a substantial night-time sap flow, Qtn, was observed under high nocturnal atmospheric evaporative demand, representing 23.3% of the total daily Qt, even though fog was more frequent at night. Fog water interception resulted in canopy wetting and dripping for at least 55.0% of the time, and an associated downward xylematic sap transport in the most apical branches, i.e. in foliar water uptake. This represented 4.0% of the upward sap flow and was observed in 26.7% of the hourly Qt records. Nocturnal transpiration was also enhanced by the entry of previous foliar moisture. This general plant and climatic phenomenology was related at the mesoscale with the trade wind inversion height in the subtropical Macaronesia area.

Effects of vegetation on the spatial and temporal variation of microclimate in the urbanized Salt Lake Valley

Land cover characteristics can produce changes in spatial and temporal variability in potential temperature (θ) at scales that are relevant to city inhabitants. Using data from 60 sensors positioned in five urban parks and surrounding residential neighborhoods in the semi-arid Salt Lake Valley, we evaluated the effect of turfgrass cover versus tree shading on spatial and temporal variation in microclimate in a semi-arid urban area. Our results indicated that the replacement of vegetation by impervious surfaces plays a role in microclimatic homogenization. Tree shading was associated with higher spatial variability of θ during the day, while turfgrass cover was associated with higher spatial variability of θ at night. Contrary to our expectation of tree shading governing daytime θ, we also found turfgrass cover to have an efficient cooling effect. This suggests a prominent role of turfgrass evapotranspiration enhanced by high atmospheric evaporative demand, high soil moisture availability, and increased radiation loads that are characteristic of semi-arid urban landscapes. Given that turfgrass cover was also associated with reduced θ at night, mixed tree and grass landscapes may be effective in alleviating the urban heat island effect in semi-arid urban settings.

Effects of grafting and gradual rootstock substitution on gas exchanges of orange seedlings under high atmospheric evaporative demand

Abstract Interstocked seedlings are produced by making two successive grafts. In one of the methods used, interstocked seedlings can be produced by introducing a rootstock by inarching and a scion by budding in a previously formed seminiferous seedling. The hypothesis of the present research is that double grafting may reduce shoot hydraulic conductivity and reduce water supply to the scion, affecting leaf gas exchanges and photochemical efficiency. Thus, in the present study the effect was assessed of sequential grafting (inarching and budding) during the production process of interstocked ‘Pera’ sweet orange plants on their net photosynthesis and growth capacity. The grafts were made on ‘Seleta’ (S) sweet orange or Rangpur lime (R) nucellar seedlings. First the seedlings were inarched with Poncirus trifoliata var. monstrosa ‘Flying Dragon’ (FD) and in the second stage, they were budding with ‘Pera’ orange (P), to form the scion. A completely randomized design was used, with three treatments, seven replications and one plant per plot. The treatments consisted of the following combinations between scion, interstock and rootstock: P/S/FD; P/R/FD and P/R, and the latter combination was used as control. At 390 days after sowing, the seedlings had two functional root systems and their height and diameter were measured. The net CO2 assimilation rate (A), stomatal conductance (gs), transpiration (E), internal CO2 concentration in mesophyll (Ci), maximum quantum efficiency of Photosystem II (Fv/Fm), photosynthetic index (Pi), leaf temperature (°C) at 8:00 and 13:00 and the SPAD reading were determined on the leaves. The seedlings did not differ in its height or in diameter of their inarch. Among the seedlings with double rootstocks, higher diameter was observed in the ‘Seleta’ sweet orange stem. The connections in the plant, caused by inarching and budding, and the gradual rootstock substitution, made these plants more sensitive to high atmospheric evaporative demand during nursery stage, and reduced photosynthetic carbon assimilation.

Controlling stomatal aperture in semi-arid regions—The dilemma of saving water or being cool?

Stomatal regulation of leaf gas exchange with the atmosphere is a key process in plant adaptation to the environment, particularly in semi-arid regions with high atmospheric evaporative demand. Development of stomata, integrating internal signaling and environmental cues sets the limit for maximum diffusive capacity of stomata, through size and density and is under a complex genetic control, thus providing multiple levels of regulation. Operational stomatal conductance to water vapor and CO2 results from feed-back and/or feed-forward mechanisms and is the end-result of a plethora of signals originated in leaves and/or in roots at each moment. CO2 assimilation versus water vapor loss, proposed to be the subject of optimal regulation, is species dependent and defines the water use efficiency (WUE). WUE has been a topic of intense research involving areas from genetics to physiology. In crop plants, especially in semi-arid regions, the question that arises is how the compromise of reducing transpiration to save water will impact on plant performance through leaf temperature. Indeed, plant transpiration by providing evaporative cooling, is a major component of the leaf energy balance. In this paper we discuss the dilemma of 'saving water or being cool' bringing about recent findings from molecular genetics, to development and physiology of stomata. The question of 'how relevant is screening for high/low WUE in crops for semi-arid regions, where drought and heat co-occur' is discussed.

Quantifying potential water savings from clearing invasive alien Eucalyptus camaldulensis using in situ and high resolution remote sensing data in the Berg River Catchment, Western Cape, South Africa

Abstract Eucalyptus camaldulensis (Red River Gum) is an aggressive alien invasive species in South Africa where it forms dense self-established stands mainly along river channels and on flood plains throughout the country. The actual water use by invasive E. camaldulensis has not yet been compared with that of regenerating native vegetation after the invasions have been cleared. This study addresses this knowledge gap by determining how much more water E. camaldulensis invasions use relative to native vegetation, to provide an indication of the impacts of clearing on streamflow under Mediterranean climatic conditions. The study was conducted at two adjacent sites in the Berg River catchment, Western Cape Province in South Africa. One site was densely invaded by E. camaldulensis while the other was cleared of the invasions in 2010. Scattered shrubby Kiggelaria Africana (Wild Peach), grasses and herbaceous shrubs were the dominant native species at the cleared site. E. camaldulensis transpiration was quantified using heat pulse velocity sap flow gauges while evapotranspiration (ET) at the cleared site was measured using an energy balance system comprising a boundary layer scintillometer, weather station and additional energy balance sensors. Daily peak transpiration by a large E. camaldulensis tree ∼50 cm diameter at breast height was up to 260 l in hot and dry weather. Despite the fact that the trees grew next to the river channel and had access to river water, substantial transpiration reduction occurred with increasing soil water deficit in the upper soil horizons under conditions of high atmospheric evaporative demand. This suggests that rain or floodwater stored in the shallow soil layers is an important source of water for the invasive eucalypts in this catchment, but water availability may be limited. Stand level transpiration (∼833 mm/y) by E. camaldulensis was similar to the annual ET (∼865 mm/y) at the cleared site. Additional ET estimates from two remote sensing models of ET, one using the Surface Energy Balance Algorithm for Land (SEBAL) and the other using a dual source Penman–Monteith model, showed that up to 2.0 ± 0.3 ML of water can potentially be saved per year for each hectare of the eucalypt invasions cleared.

Coping with low light under high atmospheric dryness: shade acclimation in a Mediterranean conifer (Abies pinsapo Boiss.)

Plant species living in the understory increase carbon (C) allocation toward leaf production for maximizing light capture at the expense of roots and stems, with negative consequences for the whole-plant hydraulic conductance. Moreover, under some conditions, the high atmospheric evaporative demand occurring in Mediterranean areas may be not well buffered by the canopy, which might be the case for relict conifer Abies pinsapo Boiss. growing in the forest understory. We hypothesized that acclimation to combined understory shade and high atmospheric dryness can be achieved through the adjustment of water losses to cope with the restriction in water transport. The results reveal high structural plasticity in A. pinsapo that allows light harvesting of this species to maximize light capture in the forest understory, and maintain a positive C balance under low light conditions. However, growth in the understory resulted in reduced leaf-specific conductivity, up to approximately four to five times, implying decreased plant capacity to supply water to the leaves. In order to cope with the high atmospheric evaporative demand in the understory, there is an adjustment of the stomatal conductance to the hydraulic conductivity by means of a reduction in the stomatal density in understory individuals, which is due to the almost complete lack of stomata in the adaxial side of the needles. To the extent of our knowledge, such a drastic phenotypic response found in a conifer when growing under shaded conditions had not been previously reported.

Multiple abiotic stresses occurring with salinity stress in citrus

Abstract Citrus, one of the most important fruit crops in the world, is sensitive to many environmental stresses including salt stress. The negative effects of stresses often lead to poor tree growth and reductions in fruit yield and quality. Under natural conditions, citrus trees often experience multiple stresses at the same time so there are direct and indirect interactions between salinity and almost all physical abiotic stresses that include flooding, drought, nutrient deficiency, high irradiance, high temperature, and high atmospheric evaporative demand. In addition, salinity stress also has direct effects on roots predisposing trees to biotic environmental stresses including attack by root rot, nematodes and bacterial disease. The agronomical and physiological responses of citrus exposed to two or more stress factors, can differ depended on stress intensity or duration. Since citrus leaf Cl− accumulation has been linked to water use, for example, other environmental factors including high CO2 concentration, lowered temperature and high relativity humidity which decrease leaf transpiration, can improve the salt tolerance. Citrus rootstocks known to be tolerant to root rot and nematode pests, can become more susceptible to these biotic stresses when irrigated with high salinity water. Root pests can, in turn, affect the salt tolerance of citrus roots and may increase salt uptake. Moderate salinity stress, however, can reduce physiological activity and growth allowing citrus seedlings to survive cold stress and can even enhance flowering after the salinity stress is relieved. In this review, we discuss the currently available information about the effects of salinity in citrus trees from an agronomic and physiological point of view, and how these responses interact with other abiotic/physical and biotic environmental factors. Short-term potential benefits of moderate stresses including salinity, will also be discussed.

Hydraulic traits are associated with the distribution range of two closely related Mediterranean firs, Abies alba Mill. and Abies pinsapo Boiss.

Abies alba and Abies pinsapo are two closely related fir species that occur in the Iberian Peninsula under very different environmental conditions. Abies alba proliferates in the humid European mountains, including the Spanish Pyrenees. In contrast, A. pinsapo is a relict species that occurs in some restricted areas of the Mediterranean mountain ranges in Spain and Morocco, which experience intense summer drought periods. To cope with the high atmospheric evaporative demand during summer, A. pinsapo may either have a high resistance to xylem cavitation or develop a very efficient conducting system to reduce the soil-to-leaf water potential gradient. To investigate such hypotheses, we measured (i) the xylem vulnerability to cavitation for different populations, and (ii) several anatomical and hydraulic parameters indicating xylem sufficiency for -supplying water to the shoot in two contrasting populations of both species. Our results show that the resistance to cavitation was not different between species or populations. However, hydraulic conductivity (K(h)), specific hydraulic conductivity (K(s)), leaf-specific conductivity (LSC) and whole-shoot hydraulic conductance (K(shoot)) were higher in A. pinsapo, indicating a higher efficiency of water transport, which should contribute to maintaining its xylem tension below the threshold for rapidly increasing cavitation. The higher K(s) in A. pinsapo was largely a result of its wider tracheids, suggesting that this species may be much more vulnerable to freeze-thaw-induced cavitation than A. alba. This is consistent with the absence of A. pinsapo in northern mountain ranges with cooler winters. These physiological differences could partly explain the niche segregation and the geographical separation of these two firs.

Water relations of coast redwood planted in the semi‐arid climate of southern California

Trees planted in urban landscapes in southern California are often exposed to an unusual combination of high atmospheric evaporative demand and moist soil conditions caused by irrigation. The water relations of species transplanted into these conditions are uncertain. We investigated the water relations of coast redwood (Sequoia sempervirens) planted in the urbanized semi-arid Los Angeles Basin, where it often experiences leaf chlorosis and senescence. We measured the sap flux (J(O)) and hydraulic properties of irrigated trees at three sites in the Los Angeles region. We observed relatively strong stomatal regulation in response to atmospheric vapour pressure deficit (D; J(O) saturated at D < 1 kPa), and a linear response of J(O) to photosynthetically active radiation. Total tree water use by coast redwood was relatively low, with plot-level transpiration rates below 1 mm d(-1) . There was some evidence of xylem cavitation during the summer, which appeared to be reversed in fall and early winter. We conclude that water stress was not a direct factor in causing leaf chlorosis and senescence as has been proposed. Instead, the relatively strong stomatal control that is adaptive in the native habitat of coast redwood may lead to carbon limitation and other stresses in semi-arid, irrigated habitats.