Predawn Water Potential Research Articles

Water Stress Adaptations by Rhododendron arboreum Smith in The Himalayan Treeline Environment

Drought is amongst the most important climatic events which can severely impact natural ecosystems. The under-canopy Indian Himalayan tree species are less studied on water stress in harsh climatic conditions. Rhododendron arboreum is an ecologically and economically important tree species and distributed between 1500–3500 m asl in the Himalayan ecosystems. In this study, we have tried to assess the water stress adaptations by R. arboreum near the treeline ecotone located at 30°11´02˝N and 79°39´36˝E between 3100 and 3500 m asl elevation. Water potential (Ψ) was measured using a Pressure chamber and leaf conductance with the help of AP4 porometer. Soil moisture (Sm) across the depths was highly variable (p<0.05). The pre-dawn water potential (ΨPD) of R. arboreum tree varied between -0.12 and -0.91 MPa. ANOVA showed that the ΨPD of R. arboreum seedlings varied significantly across the years, seasons, and sites (p<0.05). The decline in osmotic potential at full turgor (Ψπ100) was -1.18 to -1.20 MPa in trees and -1.42 to -1.72 MPa in seedlings. The morning leaf conductance (gwAM) of R. arboreum trees ranged from 58.00 to 271.68 m mol m-2 sec-1. Correlation showed that the maximum leaf conductance in R. arboreum trees was observed when the ΨPD was -0.5 to -0.9 MPa. Water uptake function in plants is generally affected by winter frost which can change the Ψ, osmotic potential, and leaf conductance of plants. The ΨPD in the species was relatively high across all seasons in the treeline areas to disrupt physiological activities.

Water Affecting Photosynthesis of Seedlings Growing in the Oopen

An assessment of the decrease in the photosynthesis intensity due to the lack of moisture were carried out on 6-year-old oak, pine and spruce seedlings grown in containers in the open air within the Serebryanoborsky forestry district by the Institute of Forest Science of the RAS (Moscow region). We determined at which values of pre-dawn water potential (PWP) seedlings growing in the open are resistant to the lack of moisture. Almost all seedlings of these species with insufficient water supply have a depression of photosynthesis, which occurs more rapidly as the moisture deficit increases and at lower solar radiation. In case of an oak, when the PWP reaches –1.1 MPa, the intensity of photosynthesis decreases by half, and in case of pine and spruce — at a PVP equal to –0.8 MPa. For the oak, the intensity of photosynthesis drops to zero at –3.0 MPa, in pine — at –1.6 ÷ –1.8 MPa, in spruce — at –1.5 MPa. Thus, the most resistant to the lack of moisture is oak, then pine, and the most demanding in terms of water supply is spruce.

Dynamic soil hydraulic resistance regulates stomata.

The onset of stomatal closure reduces transpiration during drought. In seed plants, drought causes declines in plant water status which increases leaf endogenous abscisic acid (ABA) levels required for stomatal closure. There are multiple possible points of increased belowground resistance in the soil-plant atmospheric continuum that could decrease leaf water potential enough to trigger ABA production and the subsequent decreases in transpiration. We investigate the dynamic patterns of leaf ABA levels, plant hydraulic conductance and the point of failure in the soil-plant conductance in the highly embolism-resistant species Callitris tuberculata using continuous dendrometer measurements of leaf water potential during drought. We show that decreases in transpiration and ABA biosynthesis begin before any permanent decreases in predawn water potential, collapse in soil-plant hydraulic pathway and xylem embolism spread. We find that a dynamic but recoverable increases in hydraulic resistance in the soil in close proximity to the roots is the most likely driver of declines in midday leaf water potential needed for ABA biosynthesis and the onset of decreases in transpiration.

The rates of starch depletion and hydraulic failure both play a role in drought-induced seedling mortality

Key messageThe elapsed times to deplete starch concentrations and to reach a null hydraulic safety margin were related to tree seedling mortality under experimental drought. Starch concentration showed an accelerated decline across all species during the early stages of dehydration, while the concentrations of soluble sugars and total nonstructural carbohydrates remained stable. Concomitant carbohydrate depletion and hydraulic failure drive seedling mortality under drought.ContextCurrent upsurges of drought events are provoking impacts on tree physiology, resulting in forest mortality. Hydraulic dysfunction and nonstructural carbohydrate (NSC) depletion have been posited as the main mechanisms leading to plant mortality under drought.AimsThis study explores the dynamics of the two mortality-inducing processes during drought stress using an experimental approach with 12 evergreen tree species.MethodsSeedlings were subjected to drought until 100% mortality was observed. Midday (ΨMD) and predawn (ΨPD) water potentials, xylem pressure leading to a 50% loss of hydraulic conductivity (Ψ50), along with NSC concentrations in different organs (leaves, stems, and roots) were measured regularly during drought.ResultsTotal NSC concentrations and soluble sugar pools did not decline during drought. However, starch pools showed strong reductions early during drought stress as ΨPD decreased, and the time leading to starch depletion emerged as a strong mortality predictor. Ψ50 alone did not provide an accurate estimate of mortality, while the elapsed time to reach a null hydraulic safety margin (ΨMD—Ψ50 = 0) was related to seedling mortality.ConclusionAdopting a dynamic approach by estimating the times to consume both starch reserves and hydraulic safety margins is highly relevant to improve predictions of tree mortality under the current context of increasing global drought.

Drought Resilience in Oil Palm Cultivars: A Multidimensional Analysis of Diagnostic Variables.

Water scarcity is a significant constraint on agricultural practices, particularly in Colombia, where numerous palm cultivators rely on rainfed systems for their plantations. Identifying drought-tolerant cultivars becomes pivotal to mitigating the detrimental impacts of water stress on growth and productivity. This study scrutinizes the variability in drought responses of growth, physiological, and biochemical variables integral to selecting drought-tolerant oil palm cultivars in the nursery. A comprehensive dataset was compiled by subjecting seedlings of eleven cultivars to four soil water potentials (-0.05 MPa, -0.5 MPa, -1 MPa, and -2 MPa) over 60 days. This dataset encompasses growth attributes, photosynthetic parameters like maximum quantum yield and electron transfer rate, gas exchange (photosynthesis, transpiration, and water use efficiency), levels of osmolytes (proline and sugars), abscisic acid (ABA) content, as well as antioxidant-related enzymes, including peroxidase, catalase, ascorbate peroxidase, glutathione reductase, and superoxide dismutase. Principal Component Analysis (PCA) elucidated two principal components that account for approximately 65% of the cumulative variance. Noteworthy enzyme activity was detected for glutathione reductase and ascorbate peroxidase. When juxtaposed with the other evaluated cultivars, one of the cultivars (IRHO 7001) exhibited the most robust response to water deficit. The six characteristics evaluated (photosynthesis, predawn water potential, proline, transpiration, catalase activity, sugars) were determined to be the most discriminant when selecting palm oil cultivars with tolerance to water deficit.

Isohydricity and hydraulic isolation explain reduced hydraulic failure risk in an experimental tree species mixture.

Species mixture is promoted as a crucial management option to adapt forests to climate change. However, there is little consensus on how tree diversity affects tree water stress, and the underlying mechanisms remain elusive. By using a greenhouse experiment and a soil-plant-atmosphere hydraulic model, we explored whether and why mixing the isohydric Aleppo pine (Pinus halepensis, drought avoidant) and the anisohydric holm oak (Quercus ilex, drought tolerant) affects tree water stress during extreme drought. Our experiment showed that the intimate mixture strongly alleviated Q. ilex water stress while it marginally impacted P. halepensis water stress. Three mechanistic explanations for this pattern are supported by our modeling analysis. First, the difference in stomatal regulation between species allowed Q. ilex trees to benefit from additional soil water in mixture, thereby maintaining higher water potentials and sustaining gas exchange. By contrast, P. halepensis exhibited earlier water stress and stomatal regulation. Second, P. halepensis trees showed stable water potential during drought, although soil water potential strongly decreased, even when grown in a mixture. Model simulations suggested that hydraulic isolation of the root from the soil associated with decreased leaf cuticular conductance was a plausible explanation for this pattern. Third, the higher predawn water potentials for a given soil water potential observed for Q. ilex in mixture can-according to model simulations-be explained by increased soil-to-root conductance, resulting from higher fine root length. This study brings insights into the mechanisms involved in improved drought resistance of mixed species forests.

Dynamic regulation of water potential in Juniperus osteosperma mediates ecosystem carbon fluxes.

Some plants exhibit dynamic hydraulic regulation, in which the strictness of hydraulic regulation (i.e. iso/anisohydry) changes in response to environmental conditions. However, the environmental controls over iso/anisohydry and the implications of flexible hydraulic regulation for plant productivity remain unknown. In Juniperus osteosperma, a drought-resistant dryland conifer, we collected a 5-month growing season time series of in situ, high temporal-resolution plant water potential ( ) and stand gross primary productivity (GPP). We quantified the stringency of hydraulic regulation associated with environmental covariates and evaluated how predawn water potential contributes to empirically predicting carbon uptake. Juniperus osteosperma showed less stringent hydraulic regulation (more anisohydric) after monsoon precipitation pulses, when soil moisture and atmospheric demand were high, and corresponded with GPP pulses. Predawn water potential matched the timing of GPP fluxes and improved estimates of GPP more strongly than soil and/or atmospheric moisture, notably resolving GPP underestimation before vegetation green-up. Flexible hydraulic regulation appears to allow J. osteosperma to prolong soil water extraction and, therefore, the period of high carbon uptake following monsoon precipitation pulses. Water potential and its dynamic regulation may account for why process-based and empirical models commonly underestimate the magnitude and temporal variability of dryland GPP.

Lianas in tropical dry seasonal forests have a high hydraulic efficiency but not always a higher embolism resistance than lianas in rainforests.

Lianas have higher relative abundance and biomass in drier seasonal forests than in rainforests, but whether this difference is associated with their hydraulic strategies is unclear. Here, we investigate whether lianas of seasonally dry forests are safer and more efficient in water transport than rainforest lianas, explaining patterns of liana abundance. We measured hydraulic traits on five pairs of congeneric lianas of the tribe Bignonieae in two contrasting forest sites: the wet 'Dense Ombrophilous Forest' in Central Amazonia (~2 dry months) and the drier 'Semideciduous Seasonal Forest' in the inland Atlantic Forest (~6 dry months). We also gathered a broader database, including 197 trees and 58 liana species from different tropical forests, to compare hydraulic safety between habits and forest types. Bignonieae lianas from both forests had high and similar hydraulic efficiency but exhibited variability in resistance to embolism across forest types when phylogenetic relationships were taken into account. Three genera had higher hydraulic safety in the seasonal forest than in the rainforest, but species across both forests had similar positive hydraulic safety margins despite lower predawn water potential values of seasonal forest lianas. We did not find the safety-efficiency trade-off. Merging our results with previously published data revealed a high variability of resistance to embolism in both trees and lianas, independent of forest types. The high hydraulic efficiency of lianas detected here probably favours their rapid growth across tropical forests, but differences in hydraulic safety highlight that some species are highly vulnerable and may rely on other mechanisms to cope with drought. Future research on the lethal dehydration threshold and the connection between hydraulic resistance strategies and liana abundance could offer further insights into tropical forest dynamics under climatic threats.

Shrub interaction with oak seedlings under high herbivory pressure: The role of ontogeny, drought, and plant competition

Land use changes occurring during the last decades have caused new challenges in forest management of Mediterranean ecosystems, such as overpopulation of wild ungulates. This is causing serious impacts on forest regeneration, particularly increased seedling mortality of dominant tree species. This study aims to unravel whether oak seedlings could be facilitated by non-palatable shrubs under herbivory and drought stress to improve assisted oak regeneration. For this purpose, we planted one- and two-year-old seedlings of an evergreen oak (Quercus ilex L.) in a Mediterranean open shrubland, considering three factors: herbivore exposure (with or without herbivores), seedling age (one- vs. two-year-old seedlings), and shrub cover (seedlings planted under non-palatable shrubs Cistus ladanifer or in open microsites). We evaluated seedling survival and performance monthly along the growing season (February-September), and measured physiological traits in May, just prior to the drought season. Our results revealed that herbivores, mostly wild boar (Sus scrofa), were the main cause of mortality from February to late May, while drought was responsible of mortality from early June to September. Almost all seedlings died within the first year of plantation after a severe long summer. Leaf predawn water potential and leaf stomatal conductance of one-year-old seedlings were lower under shrubs than in open microsites, a negative effect of shrubs that was not evident in the two-year-old seedlings. Plant photosynthetic performance of two-year old seedlings was higher under shrubs than in open microsites. Interestingly, the net effect of shrubs as facilitators (measured monthly through chlorophyll content) was greater in areas with no herbivores, probably due to the strong difference in herbaceous plant cover between open areas and under shrubs. Our results suggest that facilitation of Q. ilex seedlings by C. ladanifer shrub is weak under herbivory pressure and strongly depends on seedling age as shrubs seem to mostly compete for water with one-year-old seedlings and alleviate photoinhibition in two-year-old seedlings. This study reveals the possible use of two-year-old seedlings under very specific conditions, but some important inconveniences may arise as they are more stressed in the open microsites (ungulate induced mortality was similar for one and two-year old oak seedlings) and require more resources (e.g., time, energy and water) before plantation than one-year-old seedlings. We conclude that the age of plant material must be taken into account when planning oak restoration practices that can potentially benefit from the facilitative effects of shrubs under moderate biotic and abiotic stress.

The Ecophysiological Response of Olive Trees under Different Fruit Loads.

Olive trees have a unique reproductive pattern marked by biennial fruiting. This study examined the repercussions of alternate fruit bearing on the water relations of olive trees and the associated ecophysiological mechanisms. The experiment spanned two consecutive years: the "ON" year, characterized by a high crop load, and the "OFF" year, marked by minimal fruit production. Key ecophysiological parameters, including sap flow, stomatal conductance, and photosynthetic rate, were monitored in both years. Pre-dawn water potential was measured using continuous stem psychrometers and the pressure chamber technique. Biochemical analyses focused on non-structural carbohydrate concentrations (starch, sucrose, and mannitol) and olive leaves' carbon-stable isotope ratio (δ13C). Results revealed a higher leaf gas exchange rate during the "ON" year, leading to an average 29.3% increase in water consumption and a 40.78% rise in the photosynthetic rate. Higher water usage during the "ON" year resulted in significantly lower (43.22% on average) leaf water potential. Sucrose and starch concentrations were also increased in the "ON" year, while there were no significant differences in mannitol concentration. Regarding the carbon-stable isotope ratio, leaves from the "OFF" year exhibited significantly higher δ13C values, suggesting a higher resistance to the CO2 pathway from the atmosphere to carboxylation sites compared to the "ON" year plants.

Differentiated responses of plant water use regulation to drought in Robinia pseudoacacia plantations on the Chinese Loess Plateau

Robinia pseudoacacia plantations play an important role in improving the ecological environment of the Chinese Loess Plateau (CLP). However, drought stress is emerging as the major threat of sustainable growth in R. pseudoacacia plantations under the background of global warming and increasing water scarcity. Investigating the responses of plant water use to drought and the associated regulating mechanism in R. pseudoacacia is helpful for improving understanding of plant survival strategies and developing sustainable forest management practices under climate change. In this study, we monitored the canopy transpiration (Tr) dynamics with synchronous observations of soil water content and leaf water potentials in R. pseudoacacia plantations during two different hydrological years (2021 and 2022) at two sites featuring semihumid (Changwu) and semiarid (Mizhi) climate conditions. Results showed that normalized Tr exhibited stronger relationships with meteorological variables at the Changwu site than Mizhi site, as well as under non-drought condition compared to drought condition. The canopy stomatal conductance (Gc) decreased significantly with increasing vapor pressure deficit (VPD) and soil drought at both sites. The sensitivity of Gc to VPD revealed more strict stomatal regulation of transpiration in response to drought at the Changwu site, and less strict stomatal regulation at the Mizhi site. Relationship between midday and predawn water potentials indicated a partial isohydric strategy in response to drought, and reflected stomatal closure tends to occur more rapidly than hydraulic conductivity loss in R. pseudoacacia. These results suggest that the Tr and Gc values of R. pseudoacacia and their sensitivity to climate weakened as soil drought progresses and varied with different climatic conditions, and R. pseudoacacia exhibited flexible stomatal regulation of transpiration and water use strategies in response to drought.

Can Grapevine Leaf Water Potential Be Modelled from Physiological and Meteorological Variables? A Machine Learning Approach.

Climate change is affecting global viticulture, increasing heatwaves and drought. Precision irrigation, supported by robust water status indicators (WSIs), is inevitable in most of the Mediterranean basin. One of the most reliable WSIs is the leaf water potential (Ψleaf), which is determined via an intrusive and time-consuming method. The aim of this work is to discern the most effective variables that are correlated with plants' water status and identify the variables that better predict Ψleaf. Five grapevine varieties grown in the Alentejo region (Portugal) were selected and subjected to three irrigation treatments, starting in 2018: full irrigation (FI), deficit irrigation (DI), and no irrigation (NI). Plant monitoring was performed in 2023. Measurements included stomatal conductance (gs), predawn water potential Ψpd, stem water potential (Ψstem), thermal imaging, and meteorological data. The WSIs, namely Ψpd and gs, responded differently according to the irrigation treatment. Ψstem measured at mid-morning (MM) and mid-day (MD) proved unable to discern between treatments. MM measurements presented the best correlations between WSIs. gs showed the best correlations between the other WSIs, and consequently the best predictive capability to estimate Ψpd. Machine learning regression models were trained on meteorological, thermal, and gs data to predict Ψpd, with ensemble models showing a great performance (ExtraTrees: R2=0.833, MAE=0.072; Gradient Boosting: R2=0.830; MAE=0.073).

How are physiological responses to drought modulated by water relations and leaf economics’ traits in woody plants?

Species’ drought resistance is determined by a combination of multiple traits and their plastic response. However, a clear understanding of how these traits are coordinated and modulate plant responses to drought is still lacking.Here we used a water exclusion experiment on 20 Mediterranean woody species to evaluate a new framework to study plant drought responses, in which relatively constant functional traits modulate the physiological responses to a given drought exposure. In particular, we assessed how the response of stomatal conductance (Gs), leaf relative water content (RWC) and leaf water potential difference (ΔΨ) to leaf predawn water potential (Ψpd) were modulated by commonly used functional traits. The latter included hydraulic and water relations attributes such as turgor loss point in leaves (Ψtlp), vulnerability to xylem embolism in stems (Ψ50), hydraulic conductivity per unit sapwood (Ks), hydraulic conductivity per unit leaf area (Kl, hydraulic sufficiency) and Huber value (Hv), as well as specific leaf area (SLA) as a proxy for the leaf economics spectrum.Our results show that functional traits are highly coordinated, defining two main axes: the first related to drought tolerance and resource-use strategies and the second characterising the hydraulic sufficiency of leaves. These two axes, as well as many of the underlying functional traits, showed a significant interaction with Ψpd in explaining physiological responses to drought, thus modulating Gs, RWC, and ΔΨ responses. Drought-tolerant species showed a less stringent regulation of water use and leaf water potential but were more effective at regulating leaf RWC. Also, the leaf water potential at turgor loss (Ψtlp) emerged as an important trait orchestrating plant responses to drought.This study highlights that a clear separation between slow-varying traits (Ψtlp, Ψ50, Ks, Kl, Hv, SLA), physiological responses (Gs, RWC, ΔΨ) and tissue-level exposure to drought (Ψpd) improves our understanding of plant drought resistance strategies.

An analysis of the effects of water regime on grapevine canopy status using a UAV and a mobile robot

In this paper, we propose a novel approach for analyzing the effects of water regime on grapevine canopy status using robotics as an aid for monitoring and mapping. Data from an unmanned aerial vehicle (UAV) and a ground mobile robot are used to obtain multispectral images and multiple vegetation indexes, and the 3D reconstruction of the canopy, respectively. Unlike previous works, sixty vegetation indexes are computed precisely by using the projected area of the vineyard point cloud as a mask. Extensive experimental tests on repeated plots of Pinot gris vines show that the GDVI, PVI, and TGI vegetation indexes are positively correlated with the water potential: GDVI (R2=0.90 and 0.57 for the stem and pre-dawn water potential, respectively), PVI (R2=0.90 and 0.57), TGI (R2=0.87 and 0.77). Furthermore, the canopy volume and the canopy area projected on the ground are impacted by the water status, as well as stem and pre-dawn water potential measurements. The results obtained in this work demonstrate the feasibility of the proposed approach and the potential of robotic technologies, supporting precision viticulture.

Carbon starvation following a decade of experimental drought consumes old reserves in Pinus edulis.

Shifts in the age or turnover time of non-structural carbohydrates (NSC) may underlie changes in tree growth under long-term increases in drought stress associated with climate change. But NSC responses to drought are challenging to quantify, due in part to large NSC stores in trees and subsequently long response times of NSC to climate variation. We measured NSC age (Δ14 C) along with a suite of ecophysiological metrics in Pinus edulis trees experiencing either extreme short-term drought (-90% ambient precipitation plot, 2020-2021) or a decade of severe drought (-45% plot, 2010-2021). We tested the hypothesis that carbon starvation - consumption exceeding synthesis and storage - increases the age of sapwood NSC. One year of extreme drought had no impact on NSC pool size or age, despite significant reductions in predawn water potential, photosynthetic rates/capacity, and twig and needle growth. By contrast, long-term drought halved the age of the sapwood NSC pool, coupled with reductions in sapwood starch concentrations (-75%), basal area increment (-39%), and bole respiration rates (-28%). Our results suggest carbon starvation takes time, as tree carbon reserves appear resilient to extreme disturbance in the short term. However, after a decade of drought, trees apparently consumed old stored NSC to support metabolism.

Short and long-term acclimation to water status at leaf and plant level of fungus-tolerant genotypes

Today’s viticulture faces the double challenge of reducing chemical input while adapting plant material to future climate conditions. Adopting fungus disease-tolerant varieties appears to be a long-term solution as long as they are performant under abiotic constraints, such as drought. This study aimed to study the effects of water deficit (WD) and characterise the different strategies adopted to cope with drought in six new fungus disease-tolerant varieties selected by INRAE compared to Syrah. During five consecutive seasons (2018 to 2022), a gradient of WD was applied in field conditions and monitored through vine predawn water potential measurements. Grape development was non-destructively tracked by imaging to determine the arrest of berry phloem unloading. All variables were collected at the single plant level. The impacts of WD on leaf gas exchange and intrinsic water use efficiency (WUEi), vegetative development and yield components were assessed, as well as the genotypic sensitivity to WD at the leaf, vegetative and reproductive levels. All variables were negatively impacted by WD, except for intrinsic water use efficiency and total non-structural carbohydrate concentrations. Genotypes were differently impacted by WD at the physiological and vegetative levels, while no genotype-dependent response was observed for most reproductive variables. Generally, genotypes with the highest potential (highest intercept) in a certain variable were also the most sensitive to water deficit (higher slope). The most sensitive genotype regarding physiological and reproductive variables was G5, which showed higher reductions in berry weight, photosynthesis and WUEi, contrasting with Syrah, 3159B and Floreal. In this study, we observed a diversity of strategies to cope with WD in the long term, where some genotypes opted to reduce most variables (G5 and Artaban) or to maintain functioning at higher levels (Syrah and 3176N). Others displayed mixed responses: either reducing vegetative growth more than yield (3159B) or vice-versa (G14).

An experimental assessment of carbon and nitrogen allocation in Pinus pinea populations under drought stress and rewatering treatment

Global drought effects will increase with climate change and, accordingly, plant species will be affected at different scales. Carbon (C) and nitrogen (N) are critical for plant growth and development. Drought stress disturbs the process of adjusting C and N acquisition and allocation in plants. The present study aimed to determine the N (%), C (%), C/N ratio, carbon mass (CM, g), and nitrogen mass (NM, g) in different plant organs (needle, shoot, stem, and root) in 2-years-old stone pine (Pinus pinea L.) seedlings from seven different populations of Türkiye under drought stress and rewatering treatment. The seedlings were subjected to drought until a predawn water potential between − 4.5 MPa to − 4.0 MPa. The return of the seedlings to their pre-drought level was determined by using resistance (RS), recovery (RC), and resilience (RSL) indices. The N concentration in each organ showed significant differences between the populations in response to drought while the C concentration differed between the populations only in roots. In drought, the N (%) and NM (g) values in needles increased in the populations and the increase was 50% higher in northern populations. The N (%) and NM (g) values in shoots increased almost twice in response to drought. Although C (%) and CM (g) decreased in needles and shoots in drought, they increased in stems and roots. The stem C/N ratio increased by more than 100% in some populations in drought. Root C/N ratios increased by more than 50% in drought, while they generally decreased below control values with rewatering. The C/N ratio in the stem showed great resistance and recovery, while resilience stayed negative. In conclusion, stone pine populations generally demonstrated high recovery in terms of N concentration in roots after drought. Compared to northern populations, southern populations showed more resistance to drought in terms of needle and shoot N concentration. The needle and shoot N (%) of northern populations demonstrated high resilience capacity.

Response of Maize (Zea mays L.) to Drought under Salinity and Boron Stress in the Atacama Desert.

The Lluta valley in northern Chile is a hyper-arid region with annual precipitation lower than 1.1 mm, and high levels of boron (B) from alluvial deposits are present together with other salts that originated from the Cretaceous. Under these abiotic conditions, the 'lluteño' maize (Zea mays L.) is of interest because it has adapted to the Lluta valley with high salinity levels and B excess in the soil and irrigation water. Water and salt stress coincide in heavily irrigated hyper-arid agricultural areas, yet they are usually studied in isolation. We investigated in field conditions the combined effects of drought (22 days with no irrigation) under salinity (ECe: 5.5 mS cm-1; Na+: 17.8 meq L-1) and B (21.1 meq L-1) stress on physiology, growth, yield, and hourly water relations. The results allow to hypothesize that the measurement of the pre-dawn water potential represents the balance between the water potential of the soil and the root. Besides, under drought a significant effect of irrigation and time interaction was observed presenting a high differential between the leaf and stem water potential in both phenological stages. Furthermore, a decrease in net assimilation was observed, and it could be explained in part by non-stomatal factors such as the high radiation and temperature observed at the end of the season. Despite the drought, the cobs did not present a significantly lower quality compared to the cobs of plants without stress.

Physiological responses of three field-grown species (Ceratonia siliqua, Eucalyptus camaldulensis, and Moringa oleifera) to water deficits in a Mediterranean semi-arid climate

Reforestation of degraded drylands calls for the selection of species with the capacity to withstand water scarcity. In this current study we have assessed, the physiological responses of three field-grown species (Ceratonia siliqua, Eucalyptus camaldulensis and Moringa oleifera) to water deficits in semi-arid regions in order to suggest a potential species for rehabilitation programs. The physiological behavior of the given species was studied in three irrigation schemes: subsurface drip irrigation (applied weekly), tank irrigation (applied monthly), and unirrigated plants. In a stressed state, an assessment of relative water content (RWC), water potential (pre-dawn water potential PWP and midday water potential MWP) and stomatal conductance revealed three contrasting physiological responses. First, C. siliqua stomata remained open with a high RWC at low water potentials. Consequently, this species tolerated water deficits by decreasing its leaf water potential, primarily associated with osmotic adjustment. On the other hand, E. camaldulensis was found to be a drought-avoider species, mutated to a water-saving strategy by complete stomatal closure. Finally, for the extreme case, M. oleifera showed leaf shedding under water deficit conditions. These different physiological responses allowed these species to survive water deficits, and consequently, could be considered suitable candidates for rehabilitating degraded semi-arid areas.

Drought impairs herbivore-induced volatile terpene emissions by ponderosa pine but not through constraints on newly assimilated carbon.

Volatile terpenes serve multiple biological roles including tree resistance against herbivores. The increased frequency and severity of drought stress observed in forests across the globe may hinder trees from producing defense-related volatiles in response to biotic stress. To assess how drought-induced physiological stress alters volatile emissions alone and in combination with a biotic challenge, we monitored pre-dawn water potential, gas-exchange, needle terpene concentrations and terpene volatile emissions of ponderosa pine (Pinus ponderosa) saplings during three periods of drought and in response to simulated herbivory via methyl jasmonate application. Although 3-, 6- and 7-week drought treatments reduced net photosynthetic rates by 20, 89 and 105%, respectively, the magnitude of volatile fluxes remained generally resistant to drought. Herbivore-induced emissions, however, exhibited threshold-like behavior; saplings were unable to induce emissions above constitutive levels when pre-dawn water potentials were below the approximate zero-assimilation point. By comparing compositional shifts in emissions to needle terpene concentrations, we found evidence that drought effects on constitutive and herbivore-induced volatile flux and composition are primarily via constraints on the de novo fraction, suggesting that reduced photosynthesis during drought limits the carbon substrate available for de novo volatile synthesis. However, results from a subsequent 13CO2 pulse-chase labeling experiment then confirmed that both constitutive (<3% labeled) and herbivore-induced (<8% labeled) de novo emissions from ponderosa pine are synthesized predominantly from older carbon sources with little contribution from new photosynthates. Taken together, we provide evidence that in ponderosa pine, drought does not constrain herbivore-induced de novo emissions through substrate limitation via reduced photosynthesis, but rather through more sophisticated molecular and/or biophysical mechanisms that manifest as saplings reach the zero-assimilation point. These results highlight the importance of considering drought severity when assessing impacts on the herbivore-induced response and suggest that drought-altered volatile metabolism constrains induced emissions once a physiological threshold is surpassed.