Abstract: . A process-based methodology is applied to estimate land-surface evaporation from multi-satellite information. GLEAM (Global Land-surface Evaporation: the Amsterdam Methodology) combines a wide range of remotely-sensed observations to derive daily actual evaporation and its different components. Soil water stress conditions are defined from a root-zone profile of soil moisture and used to estimate transpiration based on a Priestley and Taylor equation. The methodology also derives evaporationfrom bare soil and snow sublimation. Tall vegetation rainfall interception is independently estimated by means of the Gash analytical model. Here, GLEAM is applied daily, at global scale and a quarter degree resolution. Triple collocation is used to calculate the error structure of the evaporation estimates and test the relative merits of two different precipitation inputs. The spatial distribution of evaporation – and its different components – is analysed to understand the relative importance of each component over different ecosystems. Annual land evaporation is estimated as 67.9 × 103 km3, 80% corresponding to transpiration, 11% to interception loss, 7% to bare soil evaporation and 2% snow sublimation. Results show that rainfall interception plays an important role in the partition of precipitation into evaporation and water available for runoff at a continental scale. This study gives insights into the relative importance of precipitation and net radiation in driving evaporation, and how the seasonal influence of these controls varies over different regions. Precipitation is recognised as an important factor driving evaporation, not only in areas that have limited soil water availability, but also in areas of high rainfall interception and low available energy.

Abstract: Brassinolides (BRs) are naturally occurring substances, which modulate plant growth and development events and have been known to improve the crop tolerance to abiotic stresses. In this study, possible role of exogenously applied brassinolide (BR) in alleviating the detrimental effects of drought in maize was evaluated in a rain-protected wire-house. Maize was subjected to drought at the start of tasseling for 6 days by withholding water application followed by foliar spray of BR (0.1 mg l−1) to assess the changes in growth, gas exchange, chlorophyll contents, protein, relative leaf water contents (RLWC), proline, malonialdehyde (MDA) and enzymatic antioxidants. Drought substantially reduced the maize growth in terms of plant height, leaf area and plant biomass. Moreover, substantial decrease in gas exchange attributes (net photosynthetic rate (A), transpiration rate (E), stomatal conductance (gs), water use efficiency (WUE), instantaneous water use efficiency (WUEi) and intercellular CO2 (Ci) was also recorded. However, exogenous application of BR remarkably improved the gas exchange attributes, plant height, leaf area, cobs per plant, seedling dry weight both under drought and well-watered conditions. BR-induced promotion in growth and physiological and metabolic activities were mediated through increased protein synthesis enabling maintenance of tissue water potential and activities of antioxidant enzymes lowering the lipid peroxidation under drought.

Abstract: Water, the most important component of life, is rapidly becoming a critically short commodity for humans and crop production. Limited water supply is one of the major abiotic factors that adversely affect agricultural crop production worldwide. Drought stress influences the normal physiology and growth of plants in many ways. It results in an increase of solute concentration outside the roots compared to the internal environment of the root and causes reverse osmosis. As a result, the cell membrane shrinks from the cell wall and may eventually lead to death of the cell. Water stress tends to shrink away from the interface with water-absorbing roots, creating a gap in the soil-plant-air continuum. As the plant continues to lose water via transpiration, water is drawn from root cells resulting in shrinkage of cell membranes and results in decreased integrity of the cell membrane and the living cell may be destroyed. Drought stress inhibits photosynthesis in plants by closing stomata and damaging the chlorophyll contents and photosynthetic apparatus. It disturbs the balance between the production of reactive oxygen species (ROS) and the antioxidant defence, causing accumulation of ROS which induces oxidative stress to proteins, membrane lipids and other cellular component. Mineral elements have numerous functions in plants including maintaining charge balance, electron carriers, structural components, enzyme activation, and providing osmoticum for turgor and growth .In this paper, an overview of some macronutrients (nitrogen, phosphorus, potassium, calcium and magnesium), micronutrients (Zinc, Boron, Copper) and silicon has been discussed in detail as how these nutrients play their role in decreasing the adverse effects of drought in crop plant.

Abstract: Carbon and water flow between plants and the atmosphere is regulated by the opening and closing of minute stomatal pores in surfaces of leaves. By changing the aperture of stomata, plants regulate water loss and photosynthetic carbon gain in response to many environmental stimuli, but stomatal movements cannot yet be reliably predicted. We found that the complexity that characterizes stomatal control in seed plants is absent in early-diverging vascular plant lineages. Lycophyte and fern stomata are shown to lack key responses to abscisic acid and epidermal cell turgor, making their behavior highly predictable. These results indicate that a fundamental transition from passive to active metabolic control of plant water balance occurred after the divergence of ferns about 360 million years ago.

Abstract: Across plant species, leaves vary enormously in their size and their venation architecture, of which one major function is to replace water lost to transpiration. The leaf hydraulic conductance (K(leaf)) represents the capacity of the transport system to deliver water, allowing stomata to remain open for photosynthesis. Previous studies showed that K(leaf) relates to vein density (vein length per area). Additionally, venation architecture determines the sensitivity of K(leaf) to damage; severing the midrib caused K(leaf) and gas exchange to decline, with lesser impacts in leaves with higher major vein density that provided more numerous water flow pathways around the damaged vein. Because xylem embolism during dehydration also reduces K(leaf), we hypothesized that higher major vein density would also reduce hydraulic vulnerability. Smaller leaves, which generally have higher major vein density, would thus have lower hydraulic vulnerability. Tests using simulations with a spatially explicit model confirmed that smaller leaves with higher major vein density were more tolerant of major vein embolism. Additionally, for 10 species ranging strongly in drought tolerance, hydraulic vulnerability, determined as the leaf water potential at 50% and 80% loss of K(leaf), was lower with greater major vein density and smaller leaf size (|r| = 0.85-0.90; P < 0.01). These relationships were independent of other aspects of physiological and morphological drought tolerance. These findings point to a new functional role of venation architecture and small leaf size in drought tolerance, potentially contributing to well-known biogeographic trends in leaf size.

Abstract: Water-use efficiency (WUE) has been recognized as an important characteristic of ecosystem productivity, which links carbon (C) and water cycling. However, little is known about how WUE responds to climate change at different scales. Here, we investigated WUE at leaf, canopy, and ecosystem levels under increased precipitation and warming from 2005 to 2008 in a temperate steppe in Northern China. We measured gross ecosystem productivity (GEP), net ecosystem CO2 exchange (NEE), evapotranspiration (ET), evaporation (E), canopy transpiration (Tc), as well as leaf photosynthesis (Pmax) and transpiration (Tl) of a dominant species to calculate canopy WUE (WUEc 5GEP/T), ecosystem WUE (WUEgep 5GEP/ET or WUEnee 5NEE/ET) and leaf WUE (WUEl 5Pmax/Tl). The results showed that increased precipitation stimulated WUEc, WUEgep and WUEnee by 17.1%, 10.2% and 12.6%, respectively, but decreased WUEl by 27.4%. Climate warming reduced canopy and ecosystem WUE over the 4 years but did not affect leaf level WUE. Across the 4 years and the measured plots, canopy and ecosystem WUE linearly increased, but leaf level WUE of the dominant species linearly decreased with increasing precipitation. The differential responses of canopy/ecosystem WUE and leaf WUE to climate change suggest that caution should be taken when upscaling WUE from leaf to larger scales. Our findings will also facilitate mechanistic understanding of the C‐water relationships across different organism levels and in projecting the effects of climate warming and shifting precipitation regimes on productivity in arid and semiarid ecosystems.

Abstract: A goal of modern agriculture is to improve plant drought tolerance and production per amount of water used, referred to as water use efficiency (WUE). Although stomatal density has been linked to WUE, the causal molecular mechanisms have yet to be determined. Arabidopsis thaliana GT-2 LIKE 1 (GTL1) loss-of-function mutations result in increased water deficit tolerance and higher integrated WUE by reducing daytime transpiration without a demonstrable reduction in biomass accumulation. gtl1 plants had higher instantaneous WUE that was attributable to ~25% lower transpiration and stomatal conductance but equivalent CO(2) assimilation. Lower transpiration was associated with higher STOMATAL DENSITY AND DISTRIBUTION1 (SDD1) expression and an ~25% reduction in abaxial stomatal density. GTL1 expression occurred in abaxial epidermal cells where the protein was localized to the nucleus, and its expression was downregulated by water stress. Chromatin immunoprecipitation analysis indicated that GTL1 interacts with a region of the SDD1 promoter that contains a GT3 box. An electrophoretic mobility shift assay was used to determine that the GT3 box is necessary for the interaction between GTL1 and the SDD1 promoter. These results establish that GTL1 negatively regulates WUE by modulating stomatal density via transrepression of SDD1.

Abstract: Despite its importance for urban planning, landscape management, and water management, there are very few in situ estimates of urban-forest transpiration. Because urban forests contain an unusual and diverse mix of species from many regions worldwide, we hypothesized that species composition would be a more important driver of spatial variability in urban-forest transpiration than meteorological variables in the Los Angeles (California, USA) region. We used constant-heat sap-flow sensors to monitor urban tree water use for 15 species at six locations throughout the Los Angeles metropolitan area. For many of these species no previous data on sap flux, water use, or water relations were available in the literature. To scale sap-flux measurements to whole trees we conducted a literature survey of radial trends in sap flux across multiple species and found consistent relationships for angiosperms vs. gymnosperms. We applied this relationship to our measurements and estimated whole-tree and plot-level transpiration at our sites. The results supported very large species differences in transpiration, with estimates ranging from 3.2 +/- 2.3 kg x tree(-1) x d(-1) in unirrigated Pinus canariensis (Canary Island pine) to 176.9 +/- 75.2 kg x tree(-1) x d(-1) in Platanus hybrida (London planetree) in the month of August. Other species with high daily transpiration rates included Ficus microcarpa (laurel fig), Gleditsia triacanthos (honeylocust), and Platanus racemosa (California sycamore). Despite irrigation and relatively large tree size, Brachychiton populneas (kurrajong), B. discolor (lacebark), Sequoia sempervirens (redwood), and Eucalyptus grandis (grand Eucalyptus) showed relatively low rates of transpiration, with values < 45 kg x tree(-1) x d(-1). When scaled to the plot level, transpiration rates were as high as 2 mm/d for sites that contained both species with high transpiration rates and high densities of planted trees. Because plot-level transpiration is highly dependent on tree density, we modeled transpiration as a function of both species and density to evaluate a likely range of values in irrigated urban forests. The results show that urban forests in irrigated, semi-arid regions can constitute a significant use of water, but water use can be mitigated by appropriate selection of site, management method, and species.

Abstract: Transgenic tomato (Solanum lycopersicum) plants expressing a fragment of the Sl SDH2-2 gene encoding the iron sulfur subunit of the succinate dehydrogenase protein complex in the antisense orientation under the control of the 35S promoter exhibit an enhanced rate of photosynthesis. The rate of the tricarboxylic acid (TCA) cycle was reduced in these transformants, and there were changes in the levels of metabolites associated with the TCA cycle. Furthermore, in comparison to wild-type plants, carbon dioxide assimilation was enhanced by up to 25% in the transgenic plants under ambient conditions, and mature plants were characterized by an increased biomass. Analysis of additional photosynthetic parameters revealed that the rate of transpiration and stomatal conductance were markedly elevated in the transgenic plants. The transformants displayed a strongly enhanced assimilation rate under both ambient and suboptimal environmental conditions, as well as an elevated maximal stomatal aperture. By contrast, when the Sl SDH2-2 gene was repressed by antisense RNA in a guard cell-specific manner, changes in neither stomatal aperture nor photosynthesis were observed. The data obtained are discussed in the context of the role of TCA cycle intermediates both generally with respect to photosynthetic metabolism and specifically with respect to their role in the regulation of stomatal aperture.

Abstract: Vegetation of dryland ecosystems is sensitive to precipitation pulses. Future climate scenarios suggest that the frequency and magnitude of precipitation events will change. How much and to what extent will these changes impact the hydrological cycle in creosotebush (Larrea tridentata) shrublands that dominate the three North American hot deserts? In this study, we examine the partitioning of precipitation inputs into bare soil evaporation (E) and transpiration (T) within creosotebush ecosystems at sites characterized by bimodal precipitation regimes: the Santa Rita Experimental Range (SRER) and the Walnut Gulch Experimental Watershed (WGEW). At both sites, during summer 2008, we measured evapotranspiration (ET) using eddy covariance, whole plant T using the heat-balance sap flow, and soil moisture at several depths. During the dry period preceding the summer monsoon, both ET and soil moisture were very low. With the onset of summer rains, E dominated ET; shrub transpiration did not respond to increases in soil moisture for approximately 3 more weeks. A series of large precipitation events increased moisture at deeper soil layers, and triggered T. Overall, ET was largely correlated to moisture levels in shallow soil layers typical of dryland ecosystems dominated by dry conditions, high evaporative demand, and poor soil infiltration. Under the current precipitation regime, characterized by many small storms and few large storms, soil moisture is low with most precipitation inputs lost as E. However, if climatic changes lead to less frequent but larger precipitation events, dryland communities could experience shifts in the partitioning of ET affecting the hydrologic budget of the ecosystem. Copyright © 2010 John Wiley & Sons, Ltd.

Abstract: Chickpea (Cicer arietinum L.) is often exposed to terminal drought, and deep, profuse rooting has been proposed as the main breeding target to improve terminal drought tolerance. This work tested whether plant water use at the vegetative stage and under non-limited water conditions could relate to the degree of sensitivity of chickpea to terminal drought. Transpiration response to a range of vapour pressure deficits under controlled and outdoor conditions was measured with canopy conductance using gravimetric measurements and thermal imagery in eight chickpea genotypes with comparable phenology and contrasting seed yield under terminal drought in the field. Additionally, the response of plant growth and transpiration to progressive soil moisture depletion was assayed in the same genotypes. Drought-tolerant genotypes had a lower canopy conductance under fully irrigated conditions at the vegetative stage; this trend was reversed at the early pod filling stage. While two sensitive genotypes had high early growth vigour and leaf development, there was a trend of lower growth in tolerant genotypes under progressive soil drying than in sensitive ones. Tolerant genotypes also exhibited a decline of transpiration in wetter soil compared to sensitive genotypes. Canopy conductance could be proxied by measuring leaf temperature with an infrared camera, although the relationship lost sensitivity at the pod filling stage. This work suggests that some traits contribute to water saving when water does not limit plant growth and development in drought-tolerant chickpea. It is hypothesised that this water would be available for the reproduction and grain filling stages.

Abstract: The impact of water deficit on stomatal conductance (g(s)), petiole hydraulic conductance (K(petiole)), and vulnerability to cavitation (PLC, percentage loss of hydraulic conductivity) in leaf petioles has been observed on field-grown vines (Vitis vinifera L. cv. Chasselas). Petioles were highly vulnerable to cavitation, with a 50% loss of hydraulic conductivity at a stem xylem water potential (Ψ(x)) of -0.95 MPa, and up to 90% loss of conductivity at a Ψ(x) of -1.5 MPa. K(petiole) described a daily cycle, decreasing during the day as water stress and evapotranspiration increased, then rising again in the early evening up to the previous morning's K(petiole) levels. In water-stressed vines, PLC increased sharply during the daytime and reached maximum values (70-90%) in the middle of the afternoon. Embolism repair occurred in petioles from the end of the day through the night. Indeed, PLC decreased in darkness in water-stressed vines. PLC variation in irrigated plants showed the same tendency, but with a smaller amplitude. The Chasselas cultivar appears to develop hydraulic segmentation, in which petiole cavitation plays an important role as a 'hydraulic fuse', thereby limiting leaf transpiration and the propagation of embolism and preserving the integrity of other organs (shoots and roots) during water stress. In the present study, progressive stomatal closure responded to a decrease in K(petiole) and an increase in cavitation events. Almost total closure of stomata (90%) was measured when PLC in petioles reached >90%.

Abstract: In 2007, an extreme drought and acute heat wave damaged ecosystems across the southeastern US, including a 19-year-old Liquidambar styraciflua L. (sweetgum) tree plantation exposed to long-term elevated CO2 treatments. Stem sap velocities in trees exposed to ambient (A) or elevated (E) CO2 were analyzed to assess potential interactions between CO2 and these weather extremes. Leaf temperature (Tleaf) and net carbon uptake (GPP) were modeled based on patterns of sap velocity to estimate indirect impacts of CO2-reduced transpiration on premature leaf senescence. Elevated CO2 reduced sap flow by 28% during early summer, and by up to 45% late in the drought during record-setting high air temperatures. Canopy transpiration and conductance declined more rapidly in ECO2 plots, resulting in ECO2 Tleaf up to 45 C, which was 1-2 C greater than ACO2 Tleaf. Pre-drought GPP was ~7% greater in ECO2 plots, then declined to 30% less than ACO2 GPP as the drought progressed. Leaf abscission peaked during this period, and was 30% greater for ECO2 trees. While ECO2 can reduce leaf-level water use under droughty conditions, acute drought or heat conditions may induce excessive stomatal closure that could offset benefits of ECO2 to temperate forest species during extreme weather events.

Abstract: The results of research into the water relations of cocoa are reviewed in the context of drought mitigation and irrigation need. Background information on the centres of production of the cocoa tree, and the role of water in crop development and growth processes, is followed by reviews of the effects of water stress on stomatal conductance, leaf water status and gas exchange, together with drought tolerance, crop water use and water productivity. Leaf and shoot growth occur in a series of flushes, which are synchronized by the start of the rains following a dry season (or an increase in temperature), alternating with periods of ‘dormancy’. Flowering is inhibited by water stress but synchronous flowering occurs soon after the dry season ends. Roots too grow in a rhythmic pattern similar to that of leaf flushes. Roots can reach depths of 1.5–2.0 m, but with a mass of roots in the top 0.2–0.4 m, and spread laterally >5 m from the stem. Stomata open in low light intensities and remain fully open in full sunlight in well-watered plants. Partial stomatal closure begins at a leaf water potential of about −1.5 MPa. Stomatal conductance is sensitive to dry air, declining as the saturation deficit increases from about 1.0 up to 3.5 kPa. Net photosynthesis and transpiration both consequently decline over a similar range of values. Little has been published on the actual water use of cocoa in the field. Measured ETc values equate to <2 mm d−1 only, whereas computed ETc rates of 3–6 mm d−1 in the rains and <2 mm d−1 in the dry season have also been reported. Despite its sensitivity to water stress, there is too a paucity of reliable, field-based published data of practical value on the yield responses of cocoa to drought or to irrigation. With the threat of climate change leading to less, or more erratic, rainfall in the tropics, uncertainty in yield forecasting as a result of water stress will increase. Social, technical and economic issues influencing the research agenda are discussed.

Abstract: The role of root systems in drought tolerance is a subject of very limited information compared with above-ground responses. Adjustments to the ability of roots to supply water relative to shoot transpiration demand is proposed as a major means for woody perennial plants to tolerate drought, and is often expressed as changes in the ratios of leaf to root area (A(L):A(R)). Seasonal root proliferation in a directed manner could increase the water supply function of roots independent of total root area (A(R)) and represents a mechanism whereby water supply to demand could be increased. To address this issue, seasonal root proliferation, stomatal conductance (g(s)) and whole root system hydraulic conductance (k(r)) were investigated for a drought-tolerant grape root system (Vitis berlandieri×V. rupestris cv. 1103P) and a non-drought-tolerant root system (Vitis riparia×V. rupestris cv. 101-14Mgt), upon which had been grafted the same drought-sensitive clone of Vitis vinifera cv. Merlot. Leaf water potentials (ψ(L)) for Merlot grafted onto the 1103P root system (-0.91±0.02 MPa) were +0.15 MPa higher than Merlot on 101-14Mgt (-1.06±0.03 MPa) during spring, but dropped by approximately -0.4 MPa from spring to autumn, and were significantly lower by -0.15 MPa (-1.43±0.02 MPa) than for Merlot on 101-14Mgt (at -1.28±0.02 MPa). Surprisingly, g(s) of Merlot on the drought-tolerant root system (1103P) was less down-regulated and canopies maintained evaporative fluxes ranging from 35-20 mmol vine(-1) s(-1) during the diurnal peak from spring to autumn, respectively, three times greater than those measured for Merlot on the drought-sensitive rootstock 101-14Mgt. The drought-tolerant root system grew more roots at depth during the warm summer dry period, and the whole root system conductance (k(r)) increased from 0.004 to 0.009 kg MPa(-1) s(-1) during that same time period. The changes in k(r) could not be explained by xylem anatomy or conductivity changes of individual root segments. Thus, the manner in which drought tolerance was conveyed to the drought-sensitive clone appeared to arise from deep root proliferation during the hottest and driest part of the season, rather than through changes in xylem structure, xylem density or stomatal regulation. This information can be useful to growers on a site-specific basis in selecting rootstocks for grape clonal material (scions) grafted to them.

Abstract: Plant physiological adaptation to the global rise in atmospheric CO(2) concentration (CO(2)) is identified as a crucial climatic forcing. To optimize functioning under rising CO(2), plants reduce the diffusive stomatal conductance of their leaves (g(s)) dynamically by closing stomata and structurally by growing leaves with altered stomatal densities and pore sizes. The structural adaptations reduce maximal stomatal conductance (g(smax)) and constrain the dynamic responses of g(s). Here, we develop and validate models that simulate structural stomatal adaptations based on diffusion of CO(2) and water vapor through stomata, photosynthesis, and optimization of carbon gain under the constraint of a plant physiological cost of water loss. We propose that the ongoing optimization of g(smax) is eventually limited by species-specific limits to phenotypic plasticity. Our model reproduces observed structural stomatal adaptations and predicts that adaptation will continue beyond double CO(2). Owing to their distinct stomatal dimensions, angiosperms reach their phenotypic response limits on average at 740 ppm and conifers on average at 1,250 ppm CO(2). Further, our simulations predict that doubling today's CO(2) will decrease the annual transpiration flux of subtropical vegetation in Florida by ≈60 W·m(-2). We conclude that plant adaptation to rising CO(2) is altering the freshwater cycle and climate and will continue to do so throughout this century.

Abstract: Summary • Understory plants are subjected to highly intermittent light availability and their leaf gas exchanges are mediated by delayed responses of stomata and leaf biochemistry to light fluctuations. In this article, the patterns in stomatal delays across biomes and plant functional types were studied and their effects on leaf carbon gains and water losses were quantified. • A database of more than 60 published datasets on stomatal responses to light fluctuations was assembled. To interpret these experimental observations, a leaf gas exchange model was developed and coupled to a novel formulation of stomatal movement energetics. The model was used to test whether stomatal delays optimize light capture for photosynthesis, whilst limiting transpiration and carbon costs for stomatal movement. • The data analysis showed that stomatal opening and closing delays occurred over a limited range of values and were strongly correlated. Plant functional type and climate were the most important drivers of stomatal delays, with faster responses in graminoids and species from dry climates. • Although perfectly tracking stomata would maximize photosynthesis and minimize transpiration at the expense of large opening costs, the observed combinations of opening and closure times appeared to be consistent with a nearoptimal balance of carbon gain, water loss and movement costs.

Abstract: Drought stress is known to limit plant performance in Mediterranean-type ecosystems. We have investigated the dynamics of the hydraulics, gas exchange and morphology of six co-existing Mediterranean woody species growing under natural field conditions during a drought that continued during the entire summer. Based on the observed minimum leaf water potentials, our results suggest that the six co-existing species cover a range of plant hydraulic strategies, from isohydric to anisohydric. These differences are remarkable since the selected individuals grow within several meters of each other, sharing the same environment. Surprisingly, whatever the leaf water potentials were at the end of the dry period, stomatal conductance, photosynthesis and transpiration rates were relatively similar and low across species. This result contradicts the classic view that anisohydric species are able to maintain gas exchange for longer periods of time during drought stress. None of the plants showed the expected structural acclimation response to the increasing drought (reduction of leaf-to-sapwood area ratio), thereby rejecting the functional equilibrium hypothesis for our study system. Instead, three of the six species increased photosynthetic area at the branch level. The observed dissimilar patterns of gas exchange, hydraulics and morphology across species seem to be equally successful given that photosynthesis at the leaf level was maintained at similar rates over the whole dry period.

Abstract: With global climate change, drought may become more com-mon in the future (IPCC 2007). Several factors will promote more frequent droughts: earlier snowmelt, higher temperatures and higher variability in precipitation. For ecosystems where the water cycle is dominated by snowmelt, warmer tempera-tures bring earlier melt (Stewart et al. 2005) and longer, drier snow-free periods (Westerling et al. 2006). Higher tempera-tures will also increase vapor pressure deficit if evapotranspira-tion does not also increase. Greater variability in precipitation has two implications for plant water balance: longer periods without water, and less captured in the soil in the more intense storms.Many recent tree mortality events have been linked to drought (Allen et al. 2010), but how much do we really know about the drought responses of trees? Physiological responses of trees to low soil water potential vary, but have been fairly well characterized (McDowell et al. 2008)—they vary from concomitantly lower leaf water potential (anisohydric response) to maintenance of leaf water potential above a threshold (isohydric response) with various strategies in between. Many tree species shed leaves during a strong drought, reducing transpiration and photosynthesis, and adjust partitioning to roots and storage (McDowell et al. 2008). Drought also ‘weakens’ trees and makes them more susceptible to insect attacks and pathogens (McDowell et al. 2008). Growth can be reduced through impairment of cell division and cell expansion (Hsiao 197 3), which occurs at a lower water stress threshold than does photosynthetic inhibi-tion (Hsiao et al. 1976). In fact, a variety of physiological processes respond at different plant water potentials (Hsiao 1973, Ditmarova et al. 2010), so that the severity of the drought will influence the physiological response. Drought is also relative—species adapted to low precipitation climates can survive low soil water potentials that would kill or seriously wound trees in more mesic climates and trees in mesic cli-mates might suffer from ‘drought’ that would be normal to trees in xeric climates. Finally, we know that there are many mechanisms of drought tolerance, and that many of these involve coordination within the whole tree.Two papers in this issue try to untangle some of the complex and variable responses of trees to drought. Larcheveque et al. (2011 ) droughted two different hybrid poplar saplings (each with

Abstract: We studied xylem anatomy and hydraulic architecture in 14 transgenic insertion events and a control line of hybrid poplar (Populus spp.) that varied in lignin content. Transgenic events had different levels of down-regulation of two genes encoding 4-coumarate:coenzyme A ligase (4CL). Two-year-old trees were characterized after growing either as free-standing trees in the field or as supported by stakes in a greenhouse. In free-standing trees, a 20 to 40% reduction in lignin content was associated with increased xylem vulnerability to embolism, shoot dieback and mortality. In staked trees, the decreased biomechanical demands on the xylem was associated with increases in the leaf area to sapwood area ratio and wood specific conductivity (k(s)), and with decreased leaf-specific conductivity (k(l)). These shifts in hydraulic architecture suggest that the bending stresses perceived during growth can affect traits important for xylem water transport. Severe 4CL-downregulation resulted in the patchy formation of discoloured, brown wood with irregular vessels in which water transport was strongly impeded. These severely 4CL-downregulated trees had significantly lower growth efficiency (biomass/leaf area). These results underscore the necessity of adequate lignification for mechanical support of the stem, water transport, tree growth and survival.

Abstract: Recent studies have shown that planted pine stands exhibit higher evapotranspiration (ET) and are more sensitive to climatic conditions compared with hardwood stands. Whether this is due to management and stand effects, biological effects or their interaction is poorly understood. We estimated growing season canopy- and sap flux-scaled leaf-level transpiration (Ec and EL) in five major overstory species over 3 years. Four hardwood species, Liriodendron tulipifera, Carya spp., Quercus rubra and Quercus prinus, were measured in an unmanaged watershed. Pinus strobus was measured in an adjacent planted pine watershed. We hypothesized that (1) species would differ in EL and stomatal conductance (GS), and their relationship with vapour pressure deficit (D); and (2) differences in growing season ET between the stands would result primarily from the differences in interception (Ei). Growing season ET in the planted pine stand exceeded hardwood ET by twofold during all 3 years. Transpiration and Ei contributed similarly to the ET difference, suggesting that physiological differences were equally as important as structural factors to the overall difference in ET. Among species, mean EL and GS differed significantly, as did their relationship with D. EL and GS of oaks and hickories were least responsive to changing D, while L. tulipifera and P. strobus were most responsive. This species-level understanding of variation in EL and GS and their interactions with climatic driving variables has important implications for predicting watershed-level responses to stand management, species invasion and loss, and climate variability. Published in 2010. This article is a US Government work and is in the public domain in the USA.

Abstract: Global climate change projections suggest an increasing frequency of droughts and extreme rain events in the steppes of the Eurasian region. Using the eddy covariance method, we measured carbon and water balances of a meadow steppe ecosystem in Northeast China during 2 years which had contrasting precipitation patterns in spring seasons in 2007 and 2008. The meadow steppe sequestrated only 64Ð2 gC m 2 year 1 in 2007 compared to 160Ð5 gC m 2 year 1 in 2008, due to a severe spring drought in 2007. The 2007 spring drought resulted in a dramatic reduction of leaf area index (LAI) and aboveground net primary productivity (ANPP). However, the meadow steppe still acted as a carbon sink in 2007. The strength of the sink was much greater than that in the typical steppes in Central Mongolia and Inner Mongolia. Spring drought also caused a reduction of plant transpiration (Tr) and total ecosystem evapotranspiration (ET). However, the suppression of ET in 2007 was relatively small in comparison to gross ecosystem productivity (GEP) reduction. Thus, ecosystem water use efficiency (WEU) (GEP/ET) in 2007 was reduced to 5Ð0 gCO2 kg 1 H2O or 75% of that of 2008. We concluded that spring drought detrimentally impacted meadow steppe ecosystem by reducing leaf areas, biomass, GEP, WUE and associated increases in soil evaporation (Es) that might aggravate soil salinization of the Songnen Plain. Copyright  2011 John Wiley & Sons, Ltd.