Predawn Leaf Water Potential Research Articles

From the comfort zone to crown dieback: Sequence of physiological stress thresholds in mature European beech trees across progressive drought

Drought responses of mature trees are still poorly understood making it difficult to predict species distributions under a warmer climate. Using mature European beech (Fagus sylvatica L.), a widespread and economically important tree species in Europe, we aimed at developing an empirical stress-level scheme to describe its physiological response to drought. We analysed effects of decreasing soil and leaf water potential on soil water uptake, stem radius, native embolism, early defoliation and crown dieback with comprehensive measurements from overall nine hydrologically distinct beech stands across Switzerland, including records from the exceptional 2018 drought and the 2019/2020 post-drought period. Based on the observed responses to decreasing water potential we derived the following five stress levels: I (predawn leaf water potential >−0.4 MPa): no detectable hydraulic limitations; II (−0.4 to −1.3): persistent stem shrinkage begins and growth ceases; III (−1.3 to −2.1): onset of native embolism and defoliation; IV (−2.1 to −2.8): onset of crown dieback; V (<−2.8): transpiration ceases and crown dieback is >20%. Our scheme provides, for the first time, quantitative thresholds regarding the physiological downregulation of mature European beech trees under drought and therefore synthesises relevant and fundamental information for process-based species distribution models. Moreover, our study revealed that European beech is drought vulnerable, because it still transpires considerably at high levels of embolism and because defoliation occurs rather as a result of embolism than preventing embolism. During the 2018 drought, an exposure to the stress levels III-V of only one month was long enough to trigger substantial crown dieback in beech trees on shallow soils. On deep soils with a high water holding capacity, in contrast, water reserves in deep soil layers prevented drought stress in beech trees. This emphasises the importance to include local data on soil water availability when predicting the future distribution of European beech.

Effect of Propagation Method and Ploidy Level of Various Rootstocks on the Response of the Common Clementine (Citrus clementina Hort. ex Tan) to a Mild Water Deficit

Current climatic upheavals reduce water availability which impacts the growth and fruit quality of plants. In citrus crops, scion/rootstock combinations are used to ensure high fruit production and quality and a stress tolerance/resistance. Our objective was to assess the effect on the clementine scion (C) under natural mild water deficit of (i) polyploid rootstocks by comparing the allotetraploid FlhorAG1 (C/4xFLs; trifoliate orange + Willowleaf mandarin) with its diploid parents, trifoliate orange (C/2xTOs), and Willowleaf mandarin (C/2xWLs), and with a diploid genotype used as reference (Carrizo citrange, C/2xCCs), (ii) rootstock propagation methods by comparing trifoliate orange seedling (C/2xTOs) with cutting (C/2xTOc). A mild water deficit observed under orchard conditions during the summer period (July–August) induced a significant change in yield (except in C/2xTOs), fruit size, and quality. C/2xCCs, C/2xTOs, and C/2xWLs appeared less affected by water deficit as indicated by their lower reduction of predawn leaf water potential (Ψpd), relative water content (RWC), transpiration (E), and photosynthetic parameters (Pnet and gs). Their greater redox balance was probably due to their better antioxidant efficiency. Seedling rootstocks lead to a better adaptation of clementine scions to water deficit than cutting or allotetraploid rootstock. Improving the tolerance to water deficit requires taking into consideration the rootstock genotype, propagation method, and ploidy level.

Water relations, photosynthesis, xylem embolism and accumulation of carbohydrates and cyclitols in two Eucalyptus species (E. camaldulensis and E. torquata) subjected to dehydration–rehydration cycle

Resilience to xylem embolism and the accumulation of cyclitols are promising criteria for the selection of drought-resilient Eucalyptus species. Due to the aridity of the climate in the southern Mediterranean region, the choice of suitable species for afforestation projects is crucial. The present study aims to compare resilience mechanisms against drought in two frequently used Eucalyptus species (E. camaldulensis and E. torquata). Two-year-old self-rooted cuttings of the two species were grown in sand-filled pots and subjected to a dehydration period followed by rehydration. At regular intervals, water relations, gas exchange, chlorophyll fluorescence and xylem embolism were measured on these plants. In addition, carbohydrates and cyclitols were quantified in their leaves. The results revealed that E. camaldulensis was more resilient to drought than E. torquata. During the dehydration phase, plant water status, cell turgor, net photosynthetic rate (Pn) and photosynthetic machinery integrity were less affected in E. camaldulensis than E. torquata. After rehydration, these variables were at levels similar to those of control plants in the case of E. camaldulensis, but not in the case of E. torquata. The restoration rate was 96% and 88% for predawn leaf water potential (Ψpd), and it was 98% and 77% for Pn in E. camaldulensis and E. torquata, respectively. The resilience of E. camaldulensis against drought may be due to the efficiency of two determinant survival mechanisms: osmotic adjustment and resistance to xylem embolism. Indeed, E. camaldulensis was less vulnerable to drought-induced xylem embolism and more able to repair embolized xylem vessels than E. torquata. Xylem refilling reduced embolism rate (percent loss of conductivity) to 15% in E. camaldulensis, compared to 22% in E. torquata. Furthermore, E. camaldulensis was found capable of efficient osmotic adjustment than E. torquata by accumulating soluble carbohydrates, especially cyclitols. Indeed, under severe drought (Ψpd of − 7 MPa), E. camaldulensis accumulated 9.27 mg g− 1 DW of quercitol and 3.81 mg g− 1 DW of pinitol; it also had three times more myo-inositol in its leaves than E. torquata. We suggest the use of resilience to xylem embolism and the accumulation of cyclitols to screen Eucalyptus germplasm for drought resistance.

Tracking Seasonal and Interannual Variability in Photosynthetic Downregulation in Response to Water Stress at a Temperate Deciduous Forest

AbstractThe understanding and modeling of photosynthetic dynamics affected by climate variability can be highly uncertain. In this paper, we examined a well‐characterized eddy covariance site in a drought‐prone temperate deciduous broadleaf forest combining tower measurements and satellite observations. We find that an increase in spring temperature usually leads to enhanced spring gross primary production (GPP), but a GPP reduction in late growing season due to water limitation. We evaluated how well a coupled fluorescence‐photosynthesis model (SCOPE) and satellite data sets track the interannual and seasonal variations of tower GPP from 2007 to 2016. In SCOPE, a simple stress factor scaling of Vcmax as a linear function of observed predawn leaf water potential (ψpd) shows a good agreement between modeled and measured interannual variations in both GPP and solar‐induced chlorophyll fluorescence (SIF) from the Global Ozone Monitoring Experiment‐2 (GOME‐2). The modeled and satellite‐observed changes in SIFyield are ~30% smaller than corresponding changes in light use efficiency (LUE) under severe stress, for which a common linear SIF to GPP scaling would underestimate the stress reduction in GPP. Overall, GOME‐2 SIF tracks interannual tower GPP variations better than satellite vegetations indices (VIs) representing canopy “greenness.” However, it is still challenging to attribute observed SIF variations unequivocally to greenness or physiological changes due to large GOME‐2 footprint. Higher‐resolution SIF data sets (e.g., TROPOMI) already show the potential to well capture the downregulation of late‐season GPP and could pave the way to better disentangle canopy structural and physiological changes in the future.

Quantifying turgor loss point and leaf water potential across contrasting Eucalyptus clones and sites within the TECHS research platform

Understanding the mechanisms governing drought tolerance in highly productive clonal Eucalyptus plantations in Brazil will become increasingly important with climate change driven increases in temperature and drought events. We quantified how leaf water potential, hydraulic safety margin (mid-day – pre-dawn leaf water potential, Ψδ) and the physiological parameters obtained from standard pressure–volume curves differed between four contrasting Eucalyptus genotypes across a temperature and water availability gradient in Brazil. We hypothesized that genotypes developed in dry areas were more drought tolerant and would exhibit lower mid-day leaf water potentials and turgor loss points than clones developed in wetter regions. Recognizing that standard pressure volume curves are time consuming and may not be a suitable screening tool for forest managers, we also tested if a key physiological parameter from the pressure–volume curve (turgor loss point, πtlp) could be accurately estimated from an osmometer as has been found in other species. We found no support for our first hypothesis; physiological parameters determined (including turgor loss point) from the pressure volume curves were not associated with the supposed drought tolerance of any of the clones. Similarly, mid-day leaf water potentials were not directly correlated with drought tolerance. The lack of support for our hypothesis may be because our measurements were taken during periods of minimum water stress. However, we did find that, overall, turgor loss point tended to be lower at the dry compared to wetter sites we studied suggesting that it may be a useful tool for assessing drought tolerance of Eucalyptus plantations in the future. We also found that estimates of osmotic potential at full turgor were similar between the pressure volume curve and osmometer techniques and that turgor loss point can be accurately estimated with an osmometer in highly productive Eucalyptus genotypes (R2 = 0.79).

Honey Mesquite Water Relations and Gas Exchange Following Herbicide-Induced Morphological Change

Honey mesquite (Prosopis glandulosa Torr.) may maintain apical dominance after a treatment that causes partial top-kill (PTK) and leaves canopies with “stem flagging.” In contrast, top-killing treatments stimulate multistemmed regrowth (i.e., basal sprouting; BSP). Because this difference may impact competition with grasses, a better understanding of physiology associated with PTK and BSP canopies compared with untreated canopies is needed. We quantified predawn leaf water potential (ΨPD), leaf-level gas exchange rates (photosynthesis [A] and stomatal conductance [gs]), and whole-tree stomatal conductance (Gs) of untreated, PTK, and BSP mesquites 2−11 yr after aerial herbicide application in a north Texas savanna. Total leaf area was nearly 4 × greater in untreated and BSP compared with PTK trees. In a few situations where soil moisture stress was greatest, untreated mesquites exhibited more negative ΨPD and lower leaf-level gas exchange than did PTK mesquites. BSP mesquites occasionally had greater ΨPD and leaf-level gas exchange than untreated mesquites. Since imbalances in root-to-shoot ratios caused by PTK were largely not manifested at the leaf-physiology level, PTK mesquites likely adjusted rapidly to herbicide disturbance. When gs was scaled to the whole canopy, Gs estimates were 3–4 × greater in untreated and BSP than PTK trees. Thus, canopy leaf area was the primary driver of differences in whole-canopy Gs. It is LIKELY that transpiration would be lower in a stand of untreated mesquites compared with PTK mesquites of similar stand density and basal areas before treatment, as results from comparing untreated and PTK mesquites indicate. Partially top-killed trees that maintain apical dominance and do not resprout should compete less with grasses for water and light and facilitate species diversity and heterogeneity.

Molecular basis of rootstock-related tolerance to water deficit in Vitis vinifera L. cv. Sangiovese: A physiological and metabolomic combined approach

The rootstock M4 (V. vinifera × V. berlandieri) × V. berlandieri cv. Resseguier n.1) is a recent selection reported to confer improved drought tolerance to grafted V. vinifera scions, a very desired feature in the era of global warming. Therefore, a short-term study was performed on a batch of 12 potted cv. Sangiovese vines grafted either on M4 or on the drought susceptible SO4 rootstock. Ecophysiological assessments as whole canopy net CO2 exchange rate (NCER), transpiration (Tc), and pre-dawn leaf water potential (Ψpd) and UHPLC-ESI/QTOF-MS metabolomics were then used to investigate the different vine responses during water limiting conditions. Water stress was induced by applying 50 % of estimated daily water use from days of year 184–208.M4 was able to deliver similar CO2, at a significantly reduced water use, compared to SO4 grafting. In turn, this resulted in enhanced canopy water use efficiency (NCER/Tc ratio) quantified as +15.1 % during water stress and +21.7 % at re-watering. Untargeted metabolomics showed a similar modulation of brassinosteroids and ABA between the two rootstocks, whereas the up accumulation of cytokinins and gibberellins under drought was peculiar of M4 grafted vines. The increase in gibberellins, together with a concurrent down accumulation of chlorophyll precursors and catabolites and an up accumulation of folates in M4 rootstock suggests that the capacity of limiting reactive-oxygen-species and redox imbalance under drought stress was improved. Finally, distinctive osmolyte accumulation patterns could be observed, with SO4 investing more on proline and glycine-betaine content and M4 primarily showing polyols accumulation.

Relationships between chlorophyll content of vine leaves, predawn leaf water potential at veraison, and chemical and sensory attributes of wine.

Water deficit and iron deficiency (iron chlorosis) are common environmental stresses that affect grapevine production in the Mediterranean area. Studies on the impact of both stresses, when they act simultaneously, are rare. The main objective of this investigation was to evaluate the combined effects of the incidence of iron chlorosis and the vine water status on quality of Tempranillo wine. For this, 20 non-irrigated vineyard subzones (10 m × 10 m each), from non-affected to moderately affected by iron chlorosis, were monitored in the Ribera del Duero area (north-central Spain) during two consecutive seasons. Factorial analyses of variance were performed to study the effects of predawn leaf water potential and foliar chlorophyll content, both measured at veraison, on chemical and sensory characteristics of wine. With an impact much greater than water status, the incidence of iron stress decreased pH of the wine and enhanced sensory attributes as tonality, layer intensity, flavour intensity, and persistence in the mouth. There were increases in red colour, astringency, and persistence of the wine associated with chlorosis, although they might be restricted in water-deficit conditions. The results have demonstrated that mild to moderate iron stress can have positive effects on chemical and sensory attributes of Tempranillo wine. Measurements of foliar chlorophyll content at veraison could be very useful to map quality potential in rainfed vineyards affected by iron deficiency. © 2020 Society of Chemical Industry.

Effect of Soil Moisture and Determination of Critical Soil Moisture Contents of Cassava

The aims of this research were to investigate the effects of soil moisture on cassava growth and physiological processes and to determine the critical soil moisture contents. To fulifll the research objectives, cassava was grown under five levels of soil moisture, including 50, 40, 30 and 20% of soil available water holding capacity (AWHC) along with control (no irrigation). Physiological traits and plant growth parameters were measured. The results showed that cassava grown under 50% of AWHC exhibited the highest photosynthesis rate, stomatal conductance, predawn leaf water potential and chlorophyll activity. All physiological traits decreased significantly when the moisture content was less than 40 and 20% of AWHC in sandy clay loam and loamy sand soil, respectively. Predawn leaf water potential was used to determine the critical point of soil moisture. It was found that the critical soil moisture contents were 39.0 and 15.7% of AWHC in sandy clay loam and loamy sand soil, respectively.

Waterlogging tolerance in apple trees grafted on rootstocks from G, CG, and M series

This study was performed to evaluate the waterlogging tolerance of Cornell-Geneva (G11, G202, G214, G935, CG4814, and CG5087), M26, and M9 apple rootstocks. After grafting ‘Fuji’ scions on each type of rootstock, grafted trees were planted in 17-L pots and grown in a greenhouse under well-irrigated conditions. Sixteen weeks after planting, grown trees were divided into two groups: one group was drip-irrigated daily with 2 L of water (control treatment, CT), and the other group was waterlogged by repeating flooding and drainage at 1- to 3-day intervals for 4 weeks (waterlogging treatment, WT). After the cessation of flooding, trees were irrigated as in CT for 18 days. Trees grafted on G202, G214, and M9 had markedly lower leaf water potential than CT trees on the 27th day of flooding; predawn leaf water potential was − 1.26 to − 1.45 MPa in WT trees and − 0.30 to − 0.32 MPa in CT trees, and midday leaf water potential was − 2.85 to − 3.03 MPa in WT trees and − 1.83 to − 1.87 MPa in CT trees. This difference persisted until the 18th day after the cessation of flooding. The net photosynthetic rate and stomatal conductance of trees grafted on these rootstocks were also extremely low in WT trees, and they did not recover to the corresponding levels in CT trees until the 18th day after the cessation of flooding. Among WT trees, the height, trunk cross-sectional area, and dry weight of G202, G214, and M9 trees were markedly lower than those of trees grafted on other rootstocks, whereas CG4814 trees showed the least reduction in these parameters. The defoliation percentages of G202, G214, and M9 trees were 22%, 23%, and 35%, respectively, in WT trees, whereas trees grafted on other rootstocks had 4 to 9% defoliation. Thus, G202 and G214 trees showed similar sensitivity levels as M9 trees, whereas CG4814 trees were more resistant to flooding than M26 trees, and G11, G935, and CG5087 trees showed a waterlogging tolerance comparable to M26 trees.

Applying plant-based irrigation scheduling to assess water use efficiency of cotton following a high-biomass rye cover crop

BackgroundThis study addressed the potential of combining a high biomass rye winter cover crop with predawn leaf water potential (ΨPD) irrigation thresholds to increase agricultural water use efficiency (WUE) in cotton. To this end, a study was conducted near Tifton, Georgia under a manually-controlled, variable-rate lateral irrigation system using a Scholander pressure chamber approach to measure leaf water potential and impose varying irrigation scheduling treatments during the growing season. ΨPD thresholds were − 0.4 MPa (T1), − 0.5 MPa (T2), and − 0.7 MPa (T3). A winter rye cover crop or conventional tillage were utilized for T1-T3 as well.ResultsReductions in irrigation of up to 10% were noted in this study for the driest threshold (− 0.7 MPa) with no reduction in lint yield relative to the − 0.4 MPa and − 0.5 MPa thresholds. Drier conditions during flowering (2014) limited plant growth and node production, hastened cutout, and decreased yield and WUE relative to 2015.ConclusionsWe conclude that ΨPD irrigation thresholds between − 0.5 MPa and − 0.7 MPa appear to be viable for use in a ΨPD scheduling system with adequate yield and WUE for cotton production in the southeastern U.S. Rye cover positively impacted water potential at certain points throughout the growing season but not yield or WUE indicating the potential for rye cover crops to improve water use efficiency should be tested under longer-term production scenarios.

Consequences of pine colonization in dry oak woodlands: effects on water stress

The potential impacts of species colonization on the structure and functioning of ecosystems are poorly understood. We propose a novel approach for understanding the consequences of habitat colonization, highlighting the influence of colonists on the availability of limiting resources to resident species. We studied how colonization of dry oak woodlands by pines (Pinus halepensis) is affecting water stress of resident oaks (Quercus calliprinos). We monitored predawn leaf water potential (PLWP) of oaks monthly for 2 years. Using maximum likelihood and multi-model inference, we evaluated how the PLWP of oaks was affected by pine colonists. The influence of colonizing pines on PLWP of resident oaks varied in time and space from negative to positive depending on season, oak size, pine size, and proximity to pines presence. The water stress of oaks increased along the dry season (− 1.5 to − 4.5 MPa), with small oaks becoming more severely stressed than large ones (up to 60% difference). During the dry season, neighboring pine colonists increased the water stress of oaks (up to − 0.4 MPa difference), but during the wet season, they reduced the water stress mainly for large oaks. Our findings indicate that pine colonization differentially affects water limitation for resident oaks with implications for future development and regeneration. The influence of pine colonists shifted from positive to negative along an increasing water stress gradient, contrary to predictions by the stress gradient hypothesis. Our work demonstrates how colonization by non-resident species can influence key ecosystem processes through the redistribution of limiting resources. Identifying these processes is fundamental for understanding the consequences of colonization, mitigating these influences, and predicting future change in the structure and function of ecosystems.

Photosynthetic traits and plant–water relations of two apple cultivars grown as bi-leader trees under long-term waterlogging conditions

Waterlogging has been one of the major problems in plant cultivation worldwide. As its frequency and severity are increasing in the era of climate change, waterlogging tolerance of fruit cultivars is an important factor that should be considered before plantation. In this study, we investigated leaf morphological traits, photosynthetic responses, and plant–water relations of the two most popular apple cultivars in Korea, Hongro (medium maturing) and Fuji (late maturing), under long-term waterlogging conditions. Experimental bi-leader trees grafted on a single M.9 rootstock were subjected to two irrigation regimes (well-irrigated and waterlogged) for 75 days. Waterlogging stress adversely affected most of the morphological, physiological, and anatomical characteristics as well as the plant–water relations in both cultivars; the negative effects of waterlogging were more pronounced in Hongro than in Fuji. Both cultivars experienced a significant decrease in leaf size, whereas leaf length was reduced significantly in Hongro and leaf width in Fuji. Predawn leaf water potential (ΨPD) decreased with waterlogging, but midday leaf water potential (ΨMD) was more significantly decreased in Hongro than in Fuji under waterlogging. Net photosynthetic rate (Pn) and stomatal conductance (gs) were significantly reduced in both cultivars; the reduction was greater in Hongro than in Fuji. A more pronounced reduction in vessel density, vessel diameter, and stomatal density (Sd) was observed in Hongro than in Fuji, resulting in greater reduction in daily sap flow and hydraulic conductivity in Hongro than in Fuji. These results indicate that Fuji was better adapted to waterlogging, as indicated by fewer changes in morphological, physiological, and anatomical responses when compared with those of Hongro. Our data suggest that the lower reduction in vessel density, vessel area, and vessel diameter in the Fuji cultivar under waterlogging are major factors that improve plant–water relations and photosynthetic performance, which in turn determine the leaf morphological characteristics and vegetative growth under prolonged waterlogging.

Differences in Near Isohydric and Anisohydric Behavior of Contrasting Poplar Hybrids (I-101 (Populus alba L.) × 84K (Populus alba L. × Populus glandulosa Uyeki)) under Drought-Rehydration Treatments

Carbon starvation and hydraulic failure are considered important factors in determining the mechanisms associated with tree mortality. In this study, iso/anisohydric classification was used to assess drought resistance and mortality mechanisms in two contrasting poplar species, as it is generally believed that isohydric species are more susceptible to carbon starvation, while anisohydric species are more susceptible to hydraulic failure. However, these assumptions are rarely tested in poplar genotypes with contrasting growth strategies. Thus, we subjected potted poplar genotypes (I-101 (Populus alba L.) × 84K (Populus alba L. × Populus glandulosa Uyeki)) with fast and slow growth rates to drought–rehydration treatments. The slow-growing genotype maintained higher stomatal conductance and lower predawn leaf water potential than the fast-growing genotype, thus exhibiting a near-anisohydric stomatal behavior throughout the treatment period. The nonstructural carbohydrate (NSC) content indicated that the two genotypes had the same trend of carbon change (e.g., the NSC content in the leaves increased with drought and then decreased). However, when NSC content data were combined with the growth and photosynthetic data, it was observed that the slow-growing genotype mobilized carbon to maintain hydraulic safety, while the NSC content of the fast-growing genotype among tissues was static. The percent loss of hydraulic conductivity in the branches during treatments indicated that the fast-growing genotype could recover more quickly from xylem embolism than the slow-growing genotype. The slow-growing genotype with a slow growth recovery after rehydration showed a significant increase in carbon consumption, combined with a significant increase in the hydraulic safety threshold value, indicating that there may be drought tolerance. In comparison, the fast-growing genotype showed a faster hydraulic recovery ability that had no effect on the NSC content in the leaves and roots. Our findings demonstrate intraspecific isohydric behavior in poplar; however, the trade-off between carbon distribution and stomatal regulation should be considered separately within genotypes of the same species. In addition, NSC plays an important role in water–carbon balance in the drought–rehydration cycle.

Physiological approach to decipher the drought tolerance of a soybean genotype from Brazilian savana

Drought is one of the major constraints for soybean production in Brazil. In this study we investigated the physiological traits of two soybean parental genotypes under progressive soil drying and rewetting. The plants were evaluated under full irrigation (control) conditions and under water deficit imposed by suspending irrigation until the plants reached predawn leaf water potentials (Ψam) of −1.0 MPa (moderate) and −1.5 MPa (severe). Physiological analyses showed that these genotypes exhibit different responses to water deficit. The Embrapa 48 genotype reached moderate and severe water potential two days after the BR16 genotype and was able to maintain higher levels of A, ETR and ΦPSII even under deficit conditions. This result was not related to changes in gs, 13C isotopic composition and presence of a more efficient antioxidant system. In addition, Fv/Fm values did not decrease in Embrapa 48 genotype in relation to irrigated condition showing that stress was not causing photochemical inhibition of photosynthesis. The greater reduction in the relative growth of the shoots, with concomitant greater growth of the root system under drought, indicates that the tolerant genotype is able to preferentially allocated carbon to the roots, presenting less damage to photosynthesis. Therefore, the physiological responses revealed that the tolerant genotype postponed leaf dehydration by a mechanism involving a more efficient use and translocation of water from root to shoot to maintain cell homeostasis and photosynthetic metabolism under stress.

FLOWERING PERCENTAGE IN ARABICA COFFEE CROPS DEPENDS ON THE WATER DEFICIT LEVEL APPLIED DURING THE PRE-FLOWERING STAGE1

ABSTRACT Nonuniform flowering leads to uneven ripening of fruits, which impairs harvesting efficiency and the quality of the coffee. The aim of this study was to determine the water deficit level required to break flower bud dormancy of Coffea arabica and to evaluate its effects on gas exchange, photosynthetic pigment levels, coffee yield, and fruit maturation. After a growth period of 18 months in 200 L pots maintained under greenhouse conditions, water deficit treatments were imposed by withholding watering from plants exhibiting at least a 60% rate of "E4 stage" flower buds. When five groups of six coffee plants reached the pre-dawn leaf water potential (Ψwpd) of -0.04, -0.65, -1.43, -1.96, and -2.82 MPa, the leaf gas exchange was measured and leaf disks were collected to quantify the photosynthetic pigment levels, after which, watering was resumed. The rate of opened flowers increased with the reduction of Ypd based on the mathematical model, Y = 67.064 + 20.660 x ln(-Ψwpd). The leaf gas exchange was strongly affected by water deficit levels, without any alterations in the photosynthetic pigment levels. Coffee yield was not affected by the treatments but the ripe stages of cherries increased slightly with the reduction in Ψwpd. The water deficit level applied at the pre-flowering stage determined the percentage of flowering in C. arabica.

The importance of cuticular permeance in assessing plant water–use strategies

Accurate understanding of plant responses to water stress is increasingly important for quantification of ecosystem carbon and water cycling under future climates. Plant water use strategies can be characterized across a spectrum of water stress responses, from tight stomatal control (isohydric) to distinctly less stomatal control (anisohydric). A recent and popular classification method of plant water use strategies utilizes the regression slope of predawn and midday leaf water potentials, σ, to reflect the coupling of soil water availability (predawn leaf water potential) and stomatal dynamics (daily decline in leaf water potential). This type of classification is important in predicting ecosystem drought response and resiliency. However, it fails to explain the relative stomatal responses to drought of Acer sacharrum and Quercus alba, improperly ranking them on the spectrum of isohydricity. We argue this inconsistency may be in part due to the cuticular conductance of different species. We used empirical and modeling evidence to show that plants with more permeable cuticles are more often classified as anisohydric; the σ values of those species were very well correlated with measured cuticular permeance. Furthermore, we found that midday leaf water potential in species with more permeable cuticles would continue to decrease as soils become drier, but not in those with less permeable cuticles. We devised a diagnostic parameter, Γ, to identify circumstances where the impact of cuticular conductance could cause species misclassification. The results suggest that cuticular conductance needs to be considered to better understand plant water use strategies and to accurately predict forest responses to water stress under future climate scenarios.

A New Low-Cost Device Based on Thermal Infrared Sensors for Olive Tree Canopy Temperature Measurement and Water Status Monitoring

In recent years, many olive orchards, which are a major crop in the Mediterranean basin, have been converted into intensive or super high-density hedgerow systems. This configuration is more efficient in terms of yield per hectare, but at the same time the water requirements are higher than in traditional grove arrangements. Moreover, irrigation regulations have a high environmental (through water use optimization) impact and influence on crop quality and yield. The mapping of (spatio-temporal) variability with conventional water stress assessment methods is impractical due to time and labor constraints, which often involve staff training. To address this problem, this work presents the development of a new low-cost device based on a thermal infrared (IR) sensor for the measurement of olive tree canopy temperature and monitoring of water status. The performance of the developed device was compared to a commercial thermal camera. Furthermore, the proposed device was evaluated in a commercially managed olive orchard, where two different irrigation treatments were established: a full irrigation treatment (FI) and a regulated deficit irrigation (RDC), aimed at covering 100% and 50% of crop evapotranspiration (ETc), respectively. Predawn leaf water potential (ΨPD) and stomatal conductance (gs), two widely accepted indicators for crop water status, were regressed to the measured canopy temperature. The results were promising, reaching a coefficient of determination R2 ≥ 0.80. On the other hand, the crop water stress index (CWSI) was also calculated, resulting in a coefficient of determination R2 ≥ 0.79. The outcomes provided by the developed device support its suitability for fast, low-cost, and reliable estimation of an olive orchard’s water status, even suppressing the need for supervised acquisition of reference temperatures. The newly developed device can be used for water management, reducing water usage, and for overall improvements to olive orchard management.

Fragmentation reduces severe drought impacts on tree functioning in holm oak forests

Fragmentation and increased summer drought are two main threats to Mediterranean forests. Forest fragmentation has many negative impacts on forests but could attenuate water stress on Mediterranean species by reducing intraspecific competition or improving soil properties at forest edges. However, little is known about the combined effects of drought and fragmentation on tree functioning. We evaluated the effect of forest fragmentation on tree functioning under severe drought periods in continental holm oak forests (Quercus ilex). We monitored the functional response of focal trees before and during summer drought in two regions of the Iberian Peninsula with contrasting climates. Forest interiors, edges and small forest fragments were compared. Predawn leaf water potential (Ψpd), leaf stomatal conductance to water vapor (gs), maximum leaf photochemical efficiency (Fv/Fm), ground (Fo) and maximum (Fm) fluorescence in dark adapted, fully-developed leaves were assessed in the field. Trees located at forest interiors and edges showed strong stomatal closure and clear symptoms of photochemical inhibition, revealing that the trees were operating at predawn water potential beyond tolerance thresholds. In contrast, trees located in small forest fragments were able to maintain high levels of functionality for all ecophysiological parameters during the drought. These differences in tree functioning among fragments were more noticiable in the driest region. Our results prove that forest fragmentation attenuates drought impacts on tree functioning, and that these positive effects become more important during extremely dry periods. These novel findings are essential for realistic predictions of the functionality of fragmented forests under increasing and more severe droughts.

Distinct xylem responses to acute vs prolonged drought in pine trees.

Increasing dryness challenges trees' ability to maintain water transport to the leaves. Most plant hydraulics models use a static xylem response to water stress. Yet, in reality, lower soil moisture and warmer temperatures during growing seasons feed back onto xylem development. In turn, adjustments to water stress in the newly built xylem influence future physiological responses to droughts. In this study, we investigate the annual variation of anatomical traits in branch xylem in response to different soil and atmospheric moisture conditions and tree stress levels, as indicated by seasonal predawn leaf water potential (ΨL,pd). We used a 6-year field experiment in southwestern USA with three soil water treatments applied to Pinus edulis Engelm trees-ambient, drought (45% rain reduction) and irrigation (15-35% annual water addition). All trees were also subject to a natural 1-year acute drought (soil and atmospheric) that occurred during the experiment. The irrigated trees showed only moderate changes in anatomy-derived hydraulic traits compared with the ambient trees, suggesting a generally stable, well-balanced xylem structure under unstressed conditions. The artificial prolonged soil drought increased hydraulic efficiency but lowered xylem construction costs and decreased tracheid implosion safety ((t/b)2), suggesting that annual adjustments of xylem structure follow a safety-efficiency trade-off. The acute drought plunged hydraulic efficiency across all treatments. The combination of acute and prolonged drought resulted in vulnerable and inefficient new xylem, disrupting the stability of the anatomical trade-off observed in the rest of the years. The xylem hydraulic traits showed no consistent direct link to ΨL,pd. In the future, changes in seasonality of soil and atmospheric moisture are likely to have a critical impact on the ability of P. edulis to acclimate its xylem to warmer climate. Furthermore, the increasing frequency of acute droughts might reduce hydraulic resilience of P. edulis by repeatedly creating vulnerable and less efficient anatomical structure.