Midday Leaf Research Articles

Mixing oak and pine trees in Mediterranean forests increases aboveground hydraulic dysfunctions.

Increasing tree species diversity in Mediterranean forests could reduce drought-induced hydraulic impairments through improved microclimate and reduced competition for water. However, it remains unclear if and how species diversity modulates tree hydraulic functions and how impacts may shift during the growing season. Using unmanaged Mediterranean forest stands composed of one (i.e., monospecific) or four (i.e., multispecific) tree species, we examined the seasonal dynamics of in-situ hydraulic traits (predawn and midday leaf water potential - Ψpd and Ψmd, xylem- and leaf-specific hydraulic conductivity - KS and KL, percentage loss of conductivity - PLC, specific leaf area - SLA, and Huber value - HV) in four co-existing Pinus and Quercus species over two years. We mainly observed adverse impacts of species diversity with lower Ψpd, Ψmd, KS, KL, and higher PLC in multispecific compared to monospecific stands, especially for the two pines. These impacts were observed all along the growing season but were stronger duringthe driest periods of the summer. Beneficial impacts of diversity were rare and only occured for oaks (Q. faginea) after prolonged and intense water stress. Our findings reveal that mixing oaks and pines could mainly enhance hydraulic impairments for all species during the dry season, suggesting a potential decline in mixed Mediterranean forests under future climate.

Assessing grapevine water status through fusion of hyperspectral imaging and 3D point clouds

Mild to moderate and timely water deficit is desirable in grape production to optimize fruit quality for winemaking. It is crucial to develop robust and rapid approaches to assess grapevine water stress for scheduling deficit irrigation. Hyperspectral imaging (HSI) has the potential to detect changes in leaf water status, but the robustness and accuracy are restricted in field applications. The varied leaf orientations can significantly affect how light interacts with the plant, ultimately influencing the reflectance properties. This study focused on developing an approach for detecting grapevine water status using HSI and 3D data. Leaf orientation parameters derived from 3D point clouds were integrated with spectral signatures to address the spectral variance caused by variations in leaf orientation. A water status assessment model was developed based on multiblock partial least squares (MBPLS) to estimate midday leaf water potential (ΨL) using spectral signatures and leaf orientation parameters. HSI and 3D point clouds of selected leaves were captured simultaneously in a vineyard during the 2021 growing season, and ΨL was measured as the ground truth to assess the model performance. Mean spectral reflectance was derived from the hyperspectral images, while leaf orientation parameters were extracted from 3D point cloud data. The dataset was split randomly into 70% training/calibration and 30% test datasets. The test result shows that the model estimated the ΨL with R2 = 0.89, RMSE=0.12 MPa and MAE=0.09 MPa. The leaf orientation parameters derived from 3D point clouds had a contribution of 6.25% in estimating ΨL. Moreover, it acted as an enhancing component that explained the spectral variance caused by variations in leaf orientation and improved the interpretation of the underlying relationship between spectral reflectance and vine water status.

Integrated Effects of Soil Moisture on Wheat Hydraulic Properties and Stomatal Regulation.

The development of water-saving management relies on understanding the physiological response of crops to soil drought. The coordinated regulation of hydraulics and stomatal conductance in plant water relations has steadily received attention. However, research focusing on grain crops, such as winter wheat, remains limited. In this study, three soil water supply treatments, including high (H), moderate (M), and low (L) soil water contents, were conducted with potted winter wheat. Leaf water potential (Ψleaf), leaf hydraulic conductance (Kleaf), and stomatal conductance (gs), as well as leaf biochemical parameters and stomatal traits were measured. Results showed that, compared to H, predawn leaf water potential (ΨPD) significantly reduced by 48.10% and 47.91%, midday leaf water potential (ΨMD) reduced by 40.71% and 43.20%, Kleaf reduced by 64.80% and 65.61%, and gs reduced by 21.20% and 43.41%, respectively, under M and L conditions. Although gs showed a significant difference between M and L, Ψleaf and Kleaf did not show significant differences between these treatments. The maximum carboxylation rate (Vcmax) and maximum electron transfer rate (Jmax) under L significantly decreased by 23.11% and 28.10%, stomatal density (SD) and stomatal pore area index (SPI) under L on the abaxial side increased by 59.80% and 52.30%, respectively, compared to H. The leaf water potential at 50% hydraulic conduction loss (P50) under L was not significantly reduced. The gs was positively correlated with ΨMD and Kleaf, but it was negatively correlated with abscisic acid (ABA) and SD. A threshold relationship between gs and Kleaf was observed, with rapid and linear reduction in gs occurring only when Kleaf fell below 8.70 mmol m-2 s-1 MPa-1. Our findings demonstrate that wheat leaves adapt stomatal regulation strategies from anisohydric to isohydric in response to reduced soil water content. These results enrich the theory of trade-offs between the carbon assimilation and hydraulic safety in crops and also provide a theoretical basis for water management practices based on stomatal regulation strategies under varying soil water conditions.

Seasonal Variations in Hydraulic Regulation of Whole-Tree Transpiration in Mongolian Pine Plantations: Insights from Semiarid Deserts in Northern China

Seasonal precipitation variance significantly alters soil water content, potentially inducing water stress and affecting plant transpiration in semiarid deserts. This study explored the effects of environmental variables and hydraulic conductance on whole-tree transpiration (ET) in Mongolian pines (Pinus sylvestris var. mongolica) across different forest stages in the semiarid deserts of Northern China. We measured ET using sap flow in mature (MMP), half-mature (HMP), and young (YMP) Mongolian pine plantations. Measurements included soil-leaf water potential difference (ΔΨ), atmospheric conditions, and soil moisture contents on sunny days, both in dry and wet periods. Seasonally variable rainfall distinctly affected soil moisture; during the dry periods, both stomatal and hydraulic conductance influenced ET, whereas stomatal conductance primarily regulated it during the wet periods. Discrepancies between predicted and measured ET were noticed: compared to the predicted ET, the measured ET was lower during dry periods while higher during wet periods. Hydraulic conductance (KT) increased with tree height (H) and ΔΨ. The KT values in the dry period were lower than those in the wet period, indicating that the hydraulic resistance in the dry period was higher. The hydraulic compensation occurred and was observed between 11:00 and 13:00, aligned with increased hydraulic resistance during dry periods. Decreasing hydraulic conductance intensified leaf water stress in dry periods, especially when photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) were heightened, potentially increasing stomatal sensitivity to drought, promoting water conservation and plant survival. A linear relationship between predawn and midday leaf water potentials was noticed, indicating extreme anisohydric behavior across forest stages during dry and wet periods. Although stomatal and hydraulic conductance influenced ET during the dry period, MMP and YMP were more susceptible to drought conditions. Understanding these dynamics could help evaluate semiarid desert ecological functions for water conservation amidst uneven seasonal precipitation in Northern China.

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.

Physiological responses of a black locust plantation to drought stress based on a throughfall exclusion experiment in semi-arid northwestern China

Abstract Drought poses a significant threat on the ecosystem stability of extensive areas of black locust (Robinia pseudoacacia L.) plantations in northwestern China. However, limited understanding of the physiological responses of black locust to drought has impeded the development of proactive measures to alleviate potential adverse effects of drought. This study investigated the physiological impacts of varying drought intensities, manipulated by a throughfall exclusion experiment, on a 20-year-old black locust plantation in northwestern China. The experiment involved 40% throughfall exclusion for moderate drought, 80% exclusion for extreme drought, and no exclusion for control. One year after the implementation of the experiment, both predawn (Ψpd) and midday (Ψmd) leaf water potentials were significantly lower under drought treatments compared to those in control (P < .01). Stomatal conductance (gs) exhibited a strong reduction, leading to decreased leaf transpiration and photosynthesis under drought. However, the reduction in gs did not effectively prevent the decrease in Ψmd. Instead, both Ψpd and Ψmd became more negative with increasing drought stress, but their difference remaining relatively constant (being ~1.1 MPa) across treatments. These results suggest that black locust adopts a balanced water regulatory strategy between isohydry and anisohydry to cope with drought stress. These results contribute to an enhanced understanding of the crucial physiological responses of black locust under drought stress, offering valuable insights for future management strategies aimed at sustaining the ecosystem stability of black locust plantations in an increasingly arid climate.

Monitoring cotton water status with microtensiometers

Stem water potential (Ψstem) is a key indicator for assessing plant water status, which is crucial in understanding plant health and productivity. However, existing measurement methods for Ψstem, characterized by destructiveness and intermittency, limit its applicability. Microtensiometers, an emerging plant-based sensor, offer continuous monitoring capabilities and have shown success in certain vine and tree species. In this study, we investigate the efficacy of microtensiometers ability to monitor the Ψstem of cotton (Gossypium hirsutum L.) under three distinct irrigation treatments in Maricopa, Arizona, an extremely hot, arid environment. We analyze the diurnal dynamics of Ψstem across the irrigation regimes and compare these measurements with midday leaf water potentials (Ψleaf) obtained using a dewpoint potentiometer. Our results demonstrate that the microtensiometer-derived Ψstem closely follows known diurnal patterns of Ψleaf, tracking with vapor pressure deficit (VPD) and responding to variations in irrigation levels and soil moisture content. Time cross-correlation analysis reveals an 80-minute lag in Ψstem response to changing VPD under non-water limiting conditions, which shortens under water-limiting conditions. Additionally, we establish a robust linear relationship (R2adj = 0.82) between Ψstem and Ψleaf, with this relationship strengthening as water availability decreases. Notably, we observe mean gradients of 1.2 and 0.06 MPa between soil vs. stem and stem vs. leaf water potentials, respectively. Moreover, Ψstem data proves to be more sensitive in distinguishing between irrigation treatments earlier in the growing season compared to Ψleaf, leaf temperature and leaf gas exchange parameters. These findings highlight the utility of microtensiometers as valuable tools for monitoring water status in smaller-stemmed row crops such as cotton.

Variation of leaf turgor and pressure parameters evaluation in drip-irrigated apple canopy

Leaf turgor pressure is a relevant metric for determining plant water content. To diagnose an apple tree water shortage, it is critical to precisely establish the features of leaf turgor pressure change in the canopy. In the current study, we continually monitored the leaf turgor pressure parameter (Pp) at different azimuthal, vertical, and radial locations of the apple canopy based on leaf patch clamp pressure (LPCP) probes. The results demonstrated that the Pp curves on sunny and rainy days were significantly different. A peak occurred on sunny days, during which the turgor pressure remained constant around midday. Under full irrigation, there was a greater change rate and daily maximum of Pp (maxPp) at the east (azimuthal), upper layer (vertical), and distal (radial) positions. While leaf turgor pressure dramatically decreased with incomplete watering, maxPp significantly increased at the central and lower layers of the east canopy, as well as on the proximal and middle of the radial parts. This spatial heterogeneity in canopy leaf turgor pressure was related to the soil water potential, midday leaf water potential, sponge tissue thickness, and canopy solar radiation. Then, the ideal drought stress indicator parameter (maxPp) and canopy turgor monitoring leaf location (the middle position of the eastern canopy central layer's radial branch) were determined by multiparameter assessment. The findings presented above serve as a theoretical foundation for the construction of an intelligent irrigation system based on apple tree water status monitoring.

Optimizing deficit drip irrigation to improve yield,quality, and water productivity of apple in Loess Plateau of China

ContextDeficit drip irrigation is an advanced agricultural technology widely promoted and applied around the world to maintain/increase yield and quality while simultaneously minimizing water consumption. However, there is still a lack of comprehensive research on the effects of deficit drip irrigation at different stages on apple yield, water productivity, and quality. ObjectiveThe aim was to investigate the effects of deficit drip irrigation treatments (DDITs) on apple yield, quality, and water productivity (WP), and optimize the deficit drip irrigation strategy for apples in the Loess Plateau of China. Methodsa two-year field experiment was conducted with 17 irrigation treatments, including a control treatment (CK, full irrigation (FI)) and four DDITs (D15%, D30%, D45%, D60%) during the bud burst to leafing stage (I), flowering to fruit set stage (II), fruit expansion stage (III), and fruit maturation stage (IV). The optimal irrigation management was determined using the fuzzy Borda combined model, which was based on four different single evaluation models of the TOPSIS method, principal component analysis (PCA), grey correlation analysis (GCA), and membership function analysis (MFA). ResultsThe DDITs at stage IV decreased midday leaf water potential (Ψs) by 2.57–20.00% compared with CK, and the difference of Ψs between IV-D30%, IV-D45%, IV-D60%, and CK were significant in two years (P<0.05). DDITs at stages I, II, and IV improved fruit weight (FW), fruit shape index (FSI), and fruit firmness (FN), but did not significantly affect fruit water content (FWC). The DDITs at stages III and IV significantly enhanced total soluble solids (TSS), soluble sugars (SS), and vitamin C (Vc) in 2021 and 2022, but a notable decrease in titratable acidity (TA) was observed. ConclusionsDDITs at stage IV exhibited the highest comprehensive benefit of apple, followed by DDITs at stages I and II, and DDITs at stage III had the lowest values. The DDITs of D15%, D30%, CK, and D15% at stages I, II, III, and IV were identified as the optimal irrigation strategy to improve apple yield, quality, and WP. ImplicationsOur findings can guide the development of appropriate irrigation strategies for apple cultivation in Loess Plateau of China, promoting efficient and sustainable apple production.

A trade-off between leaf hydraulic efficiency and safety across three xerophytic species in response to increased rock fragment content.

Limited information is available on the variation of plant leaf hydraulic traits in relation to soil rock fragment content (RFC), particularly for xerophytes native to rocky mountain areas. In this study, we conducted a field experiment with four gradients of RFC (0, 25, 50 and 75% ν ν-1) on three different xerophytic species (Sophora davidii, Cotinus szechuanensis and Bauhinia brachycarpa). We measured predawn and midday leaf water potential (Ψleaf), leaf hydraulic conductance (Kleaf), Ψleaf induced 50% loss of Kleaf (P50), pressure-volume curve traits and leaf structure. A consistent response of hydraulic traits to increased RFC was observed in three species. Kleaf showed a decrease, whereas P50 and turgor loss point (Ψtlp) became increasingly negative with increasing RFC. Thus, a clear trade-off between hydraulic efficiency and safety was observed in the xerophytic species. In all three species, the reduction in Kleaf was associated with an increase in leaf mass per area. In S. davidii, alterations in Kleaf and P50 were driven by leaf vein density (VLA) and Ψtlp. In C. szechuanensis, Ψtlp and VLA drove the changes in Kleaf and P50, respectively. In B. brachycarpa, changes in P50 were driven by VLA, whereas changes in both Kleaf and P50 were simultaneously influenced by Ψtlp. Our findings suggest that adaptation to increased rockiness necessarily implies a trade-off between leaf hydraulic efficiency and safety in xerophytic species. Additionally, the trade-off between leaf hydraulic efficiency and safety among xerophytic species is likely to result from processes occurring in the xylem and the outside-xylem hydraulic pathways. These findings contribute to a better understanding of the survival strategies and mechanisms of xerophytes in rocky soils, and provide a theoretical basis for the persistence of xerophytic species in areas with stony substrates.

Effects of foliar applications of a proline-rich specific yeast derivative on physiological and productive performances of field-grown grapevines (Vitis vinifera L.)

The occurrence of heat waves in vineyards can damage the integrity of photosynthetic systems, impair yield and affect fruit quality. The frequency of these events is increasing due to global warming, and this is the subject of growing concern among the players in the wine sector. Canopy applications of specific yeast derivatives (SYD) are proposed to enhance plant physiology under abiotic stresses and preserve yield and fruit composition. This study aimed at assessing the effects of foliar application of a proline-rich SYD formulate, LalVigne ProHydro™ (LPH), on field-grown vines cv. Barbera. In a two-year experiment, vines subjected to multiple LPH foliar applications were compared to untreated vines (C). Gas exchange rates and vegetative and productive performances were monitored together with leaf proline, carotenoids and H2O2 concentrations. The data demonstrate that the formulate improved midday leaf water potential (+0.14 MPa), stomatal conductance (+0.15 mol m−2 s−1) and assimilation rates (+2.8 μmol m−2 s−1) under summer stress conditions. Interestingly, LPH leaves showed a dramatically higher concentration of proline (+600 %) and a significantly lower H2O2 content (-26 %) as compared to C leaves. The data suggest that this could be related to a priming effect of LPH application. Moreover, in SYD vines, cluster sunburn occurrence was reduced by 69 %, compared to C vines. At harvest, LPH improved vine yield (+1.27 kg), and LPH grapes had a significantly lower sugar concentration (-2.09 °Brix). The study demonstrates for the first time the potential benefits of proline-rich SYD application in vineyards subjected to multiple summer stress. Further studies should focus on the treatment effects on proline biosynthesis and gene expression.

Drought effects on water status, plant hydraulics and growth in Eucalyptus grandis and clonal hybrids with red gums and E. urophylla

Afforestation with Eucalyptus species occupies over a million hectares in Uruguay, and E. grandis is one of the most cultivated species, mainly for the timber industry. However, the increasing frequency of drought and heat waves in the region raises the need to evaluate hybrids with red gums (E. camaldulensis and E. tereticornis), since they are known to sustain gas exchange and growth even under severe drought conditions. Hybrids with E. urophylla are also evaluated for plantations in subtropical regions. This study aimed to compare the effects of drought on plant water status, hydraulics, and growth of E. grandis (one clone; G), E. grandis × camaldulensis (one clone; GC), E. grandis × tereticornis (one clone; GT), and E. grandis × urophylla (two clones; GU1 and GU2). Two drought-stress cycles were applied to six-month rooted cuttings from mid-spring to early fall under greenhouse environmental conditions. Predawn leaf water potential (?pd), midday leaf water potential (?md), relative leaf water content (RWC), stomatal conductance (gs), and plant growth were measured across each cycle. Daily fluctuation of Ψ was calculated as ΔΨ = Ψpd - Ψmd. Specific (kS) and leaf-specific (kL) hydraulic conductivities and specific leaf area (SLA) were measured at the end of the experiment. Percentage loss of hydraulic conductivity (PLC) was calculated after removing the xylem embolism. Preliminary results suggest that both red-gum hybrids had higher PLC under moderate-to-severe drought, as a consequence of maintaining high water potential gradients to sustain transpiration and carbon fixation throughout the experiment. Under the well-watered condition, no clonal difference was observed. Also, GT showed a three-fold increase in cavitation and had 1.5 times stomatal opening (gs) than the most drought-sensitive clone (E. grandis, G), and exhibited the lowest reduction (17%) in diameter growth under drought. Also, GC attained the highest height at the end of the study, 40% higher than E. grandis, and increased by 25% the SLA under drought. GU clones reached the highest diameter at the end of the study. Correlations among functional variables are explored, and the water supply capacity to the foliage among clones is discussed for a better understanding of drought resistance strategies and growth performance of the different taxa.

Comparative effects of drought stress on leaf gas exchange, foliar ABA and leaf orientation in four grapevine cultivars grown in Northern Italy.

Drought tolerance varies greatly across Vitis vinifera cultivars, depending on physiological responses and structural and morphological adaptations. In this study, responses to water stress were examined in three extensively cultivated varieties from Northern Italy. Over the course of two seasons, mature potted vines were subjected to a 12 or 13-day period of water restriction. Vine water relations were investigated using measures of water potential, gas exchanges, and leaf ABA content. Leaf angle response to increasing water stress was analysed in the four cultivars as a mechanism that improves stress tolerance. Different physiological responses were observed among cultivars, suggesting a near-isohydric water-use strategy for Moscato and a near-anisohydric one for Garganega, Glera and Merlot. Results of leaf ABA analysis highlighted a variability among the studied varieties, indicating higher contents and lower sensitivity to ABA for the anisohydric ones. In all varieties, a similar increase in midday leaf inclination was observed in response to decreasing stem water potentials, indicating that leaf angle adjustments may represent a common adaptive response to drought. These findings increase the understanding of the leaf physiological and structural mechanisms that contribute to water stress tolerance in grapevine, supporting a more efficient cultivar selection to cope with the expected changes in Mediterranean climate.

Bark wounding triggers gradual embolism spreading in two diffuse-porous tree species.

Xylem transport is essential for the growth, development and survival of vascular plants. Bark wounding may increase the risk of xylem transport failure by tension-driven embolism. However, the consequences of bark wounding for xylem transport are poorly understood. Here, we examined the impacts of the bark wounding on embolism formation, leaf water potential and gas exchange in the terminal branches of two diffuse-porous tree species (Acer platanoides L. and Prunus avium L.). The effects of bark removal were examined on field-grown mature trees exposed to increased evaporative demands on a short-term and longer-term basis (6h vs 6 days after bark wounding). Bark removal of 30% of branch circumference had a limited effect on the xylem hydraulic conductivity when embolized vessels were typically restricted to the last annual ring near the bark wound. Over the 6-day exposure, the non-conductive xylem area had significantly increased in the xylem tissue underneath the bark wound (from 22-29% to 51-52% of the last annual ring area in the bark wound zone), pointing to gradual yet relatively limited embolism spreading to deeper xylem layers over time. In both species, the bark removal tended to result in a small but non-significant increase in the percent loss of hydraulic conductivity compared with control intact branches 6 days after bark wounding (from 6 to 8-10% in both species). The bark wounding had no significant effects on midday leaf water potential, CO2 assimilation rates, stomatal conductance and water-use efficiency of the leaves of the current-year shoot, possibly due to limited impacts on xylem transport. The results of this study demonstrate that bark wounding induces limited but gradual embolism spreading. However, the impacts of bark wounding may not significantly limit water delivery to distal organs and leaf gas exchange at the scale of several days.

Реакция на черешови сортове и хибриди към ниски зимни температури и засушаване

The response of some introduced sweet cherry cultivars and hybrids to abiotic stressors was studied. Resistance to low winter temperatures was monitored in laboratory conditions (2020-2021), and their sensitivity to drought in natural conditions (2018-2019). From the three levels of controlled freezing (-15ºС; -20ºС; -25ºС) it was found that the cultivars Bigalise pozna, Bigalise, and the hybrids 8-102 and № 5752 had increased cold hardiness of the regenerative organs. Van compact and Techlovan had proved to be the most sensitive at these levels of freezing. The drought resistance was determined by visual symptoms of moisture deficiency and midday leaf water potential (Ψ midday) of the trees. Under the conditions of the experiment, all the studied cultivars experience water stress, which was from “moderate” to “very high”. The Bigalise and Techlovan cultivars stand out with increased drought resistance in both years of study. The visual symptoms of moisture deficiency did not fully correspond to the established levels of water stress. The twisting of the leaves is a feature aimed at shading part of the leaves and reducing overheating.

Assessing the Water Status and Leaf Pigment Content of Olive Trees: Evaluating the Potential and Feasibility of Unmanned Aerial Vehicle Multispectral and Thermal Data for Estimation Purposes

Global warming presents a significant threat to the sustainability of agricultural systems, demanding increased irrigation to mitigate the impacts of prolonged dry seasons. Efficient water management strategies, including deficit irrigation, have thus become essential, requiring continuous crop monitoring. However, conventional monitoring methods are laborious and time-consuming. This study investigates the potential of aerial imagery captured by unmanned aerial vehicles (UAVs) to predict critical water stress indicators—relative water content (RWC), midday leaf water potential (ΨMD), stomatal conductance (gs)—as well as the pigment content (chlorophyll ab, chlorophyll a, chlorophyll b and carotenoids) of trees in an olive orchard. Both thermal and spectral vegetation indices are calculated and correlated using linear and exponential regression models. The results reveal that the thermal vegetation indices contrast in estimating the water stress indicators, with the Crop Water Stress Index (CWSI) demonstrating higher precision in predicting the RWC (R2 = 0.80), ΨMD (R2 = 0.61) and gs (R2 = 0.72). Additionally, the Triangular Vegetation Index (TVI) shows superior accuracy in predicting the chlorophyll ab (R2 = 0.64) and chlorophyll a (R2 = 0.61), while the Modified Chlorophyll Absorption in Reflectance Index (MCARI) proves most effective for estimating the chlorophyll b (R2 = 0.52). This study emphasizes the potential of UAV-based multispectral and thermal infrared imagery in precision agriculture, enabling assessments of the water status and pigment content. Moreover, these results highlight the vital importance of this technology in optimising resource allocation and enhancing olive production, critical steps towards sustainable agriculture in the face of global warming.

Leaf morpho-physiological traits of Populus sibirica and Ulmus pumila in different irrigation regimes and fertilizer types.

The impacts of climate change, such as increased soil dryness and nutrient deficiency, highlight the need for environmentally sustainable restoration of forests and groundwater resources. However, it is important to consider that extensive afforestation efforts may lead to a depletion of groundwater supply due to higher evapotranspiration rates, exacerbating water scarcity issues. Consequently, we conducted a study to examine how the fast-growing tree species Populus sibirica (Horth ex Tausch) and Ulmus pumila (L.) respond morpho-physiologically to varying watering regimes and types of fertilizers, aiming to better understand their specific water and nutrient requirements. We used two-year-old nursery-growth seedlings (N=512) of P.sibirica and U. pumila with initial root collar diameter (RCD) and the height of 0.51±0.02mm and 68±2.94 cm and 0.33±0.01mm and 51±1.14 cm, respectively. The leaf area (LA), specific leaf area (SLA), chlorophyll concentration, stomatal conductance (gs), chlorophyll fluorescence, and predawn and midday leaf water potential were measured across treatments. Four different irrigation regimes and two different fertilizer types were applied: no irrigation (control, 0 L h-1), 2 L h-1 = 0.25 mm m-2, 4L h-1 = 0.5mm m-2, 8 L h-1 = 1.0 mm m-2 and 120 g and 500 g tree-1 of NPK and compost (COMP). Twelve plots (600 m2) were established in the study site for each species and treatments. During the first growing season (2021), the LA of P. sibirica was larger in the 4-8 L h-1 without fertilizer, but it was smaller in the 4 L h-1+ COMP during the second growing season (2022). The 2 L h-1 without fertilizer and 2 L h-1 + NPK had larger LA compared with the control (CONT) for the first and second growing seasons, respectively, for U. pumila. P. sibirica seedlings at 4 L h-1 without fertilizer had the highest SLA for 2021 and at 2 L h-1 + NPK for 2022, whereas CONT and 4 L h-1 had the highest SLA than the other treatments for 2021 and 2022 growing seasons, respectively, for U. pumila. The chlorophyll concentration of P. sibirica seedlings in the first year was generally higher in CONT, while the 2 L h-1 without any fertilizer yielded a significantly higher chlorophyll concentration of U. pumila. Chlorophyll fluorescence parameters (PIABS and Fm) were generally lower in CONT with/without NPK or COMP for both species. The CONT with NPK/COMP generally had a higher gs compared with the other treatments in both experimental periods for U. pumila, whereas CONT and 2 L h-1+ NPK-treated P. sibirica seedlings had a significantly greater gs during the first year and second year, respectively. The predawn and midday leaf water potentials of both species were generally the lowest in CONT, followed by 2 L h-1+ NPK/COMP during the first growing season, but a different pattern was observed during the second growing season. Overall, the morpho-physiological traits of the two species were affected by watering and fertilizer treatments, and the magnitude of the effects varied depending on growing season, amount of irrigation, and fertilizer type, and their interactions.

Quantifying the effects of precipitation exclusion and groundwater drawdown on functional traits of Haloxylon ammodendron — How does this xeric shrub survive the drought?

The increasing frequency of drought and decline in groundwater levels are causing ecophysiological changes in woody plants, particularly in desert ecosystems in arid regions. However, the combined effects of meteorological and hydrological droughts on perennial desert plants, especially phreatophytes, remain poorly understood. To address this knowledge gap, we conducted a 5-year precipitation exclusion experiment at two sites with contrasting groundwater depths in the Gurbantunggut Desert located in northwest China. Our study aimed to investigate the impacts of precipitation exclusion and groundwater depth decline on multiple traits of H. ammodendron. We found that long-term precipitation exclusion enhanced midday leaf water potential, stomatal conductance, chlorophyll content, root nonstructural carbohydrates concentration, leaf starch concentration, but decreased water use efficiency. Groundwater drawdown decreased predawn and midday leaf water potentials, maximum net photosynthetic rate, stomatal conductance, Huber value, stem water δ18O, but enhanced water use efficiency and branch nonstructural carbohydrates concentration. A combination of precipitation exclusion and groundwater depth decline reduced Huber value, but did not show exacerbated effects. The findings demonstrate that hydrological drought induced by groundwater depth decline poses a greater threat to the survival of H. ammodendron than future changes in precipitation.

Limits of thermal and hydrological tolerance in a foundation tree species (Populus fremontii) in the desert southwestern United States.

Populus fremontii is among the most dominant, and ecologically important riparian tree species in the western United States and can thrive in hyper-arid riparian corridors. Yet, P. fremontii forests have rapidly declined over the last decade, particularly in places where temperatures sometimes exceed 50°C. We evaluated high temperature tolerance of leaf metabolism, leaf thermoregulation, and leaf hydraulic function in eight P. fremontii populations spanning a 5.3°C mean annual temperature gradient in a well-watered common garden, and at source locations throughout the lower Colorado River Basin. Two major results emerged. First, despite having an exceptionally high Tcrit (the temperature at which Photosystem II is disrupted) relative to other tree taxa, recent heat waves exceeded Tcrit , requiring evaporative leaf cooling to maintain leaf-to-air thermal safety margins. Second, in midsummer, genotypes from the warmest locations maintained lower midday leaf temperatures, a higher midday stomatal conductance, and maintained turgor pressure at lower water potentials than genotypes from more temperate locations. Taken together, results suggest that under well-watered conditions, P. fremontii can regulate leaf temperature below Tcrit along the warm edge of its distribution. Nevertheless, reduced Colorado River flows threaten to lower water tables below levels needed for evaporative cooling during episodic heat waves.

A shift from isohydric to anisohydric water-use strategy as a result of increasing drought stress for young apple trees in a semiarid agroforestry system

Prolonged drought due to global warming can have significant effects on tree growth and sustainability by changing physiological traits. Agroforestry is considered climate-smart and can help buffer the effects of extreme climates. However, it remains unclear as to how trees in agroforestry physiologically adapt to prolonged drought in semiarid regions. Here, we report results from a three-year rainfall exclusion experiment (an extreme natural drought occurred during the experiment) designed to understand the physiological mechanism of young apple trees responding to different degrees of prolonged drought (moderate drought by reducing rainfall by 15% and severe drought by reducing rainfall by 25%) in an alley agroforestry system constituting of apple trees and oil crops. We found that the measured physiological traits for young apple tree under agroforestry were not clearly different from those under monoculture, although soil water content in the top 80 cm was reduced by 9.3%. Under moderate drought conditions, the apple trees in agroforestry implemented a conservative water-use strategy. Stomatal conductance, photosynthesis, and leaf transpiration were reduced by 15.5%, 3.8%, and 12.6%, respectively, whereas pre-dawn (Ψpd) and midday (Ψmd) leaf water potential stabilized, indicating a clear isohydric behavior. Under severe drought conditions, however, the apple trees still maintained normal stomatal opening to significantly (p<0.01) increase photosynthesis (9.8%) and leaf transpiration (12%) at the expense of reducing Ψpd (58.1%) and Ψmd (25.4%), showing anisohydric behavior which can place apple trees at risk. Reduction of aboveground biomass and greater numbers of fine roots in deeper soils to explore deep-layer soil water could explain such risky behavior by apple trees suffering severe drought. The findings here provide new insights into the mechanism through which the coordination between canopy water consumption and aboveground/belowground biomass redistribution of trees underpin the physiological adaptation of trees to different degrees of prolonged drought in semiarid regions.