Increasing drought stress due to climate warming has triggered various negative impacts on plantations in dryland areas, including growth reduction, crown dieback, and even tree mortality, with unavoidable consequences for forest ecosystems. However, how drought stress progressively led to the damage process from growth reduction to mortality for mature trees remains largely unclear, especially its varying soil moisture thresholds. Here we selected mature trees in larch (Larix principis-rupprechtii) plantations in the dryland areas of northwest China, and monitored the progressive tree responses in an extreme summer drought event in 2021, including transpiration, radial growth, leaf area index, discoloration, defoliation, crown dieback and tree mortality. The results showed strong responses of larch trees to summer drought, such as large stem shrinkage, dramatic decrease in transpiration and leaf area index, and obvious discoloration, defoliation, crown dieback and tree mortality at some sites. The intensity of tree responses mainly depended on soil moisture rather than meteorological factors and there were strong relationships between tree responses and relative soil water content (RSW) of 0-60 cm layers. Based on the trees responded to RSW, five soil drought stress levels or progressive mortality stages and their corresponding RSW thresholds were determined as following: no detectable hydraulic limitations (RSW>0.7, Level I), persistent stem shrinkage and onset of transpiration reduction (0.45<= 0.7, Level II), onset of slight discoloration and defoliation (0.35<= 0.45, Level III), onset of crown dieback and tree mortality (0.25<= 0.35, Level IV), and severe defoliation, crown dieback and tree mortality (RSW <= 0.25, Level V). This study showed that the trees responded to climatic drought were strongly regulated by soil moisture and thus were strongly site-specific. These findings will help to evaluate the degree and spatio-temporal distribution of tree damage and mortality in plantations under increasing climatic drought, particularly in dryland areas.

The pedunculate oak (Quercus robur L.) is a major tree species in Europe, but it has faced recent growth decline and dieback events in some areas resulting in economic and ecosystem losses. In the southeastern edge of its natural distribution in eastern Romania, rising temperatures since the 1980s, when a shift towards warmer and more arid conditions occurred, increased evaporative demand and triggered growth decline. We analyzed the adaptive potential of six oak stands (333 individual trees) with ages ranging between 97 and 233 years, located in three wet and three dry sites. Results showed unstable climate-growth correlations with a breakpoint after 1985 when climate warming intensified. Wet soil conditions from early spring to summer enhanced growth; on the contrary, a high evaporative demand linked to warmer conditions and greater potential evapotranspiration reduced growth, particularly in wet sites. After 1985, drought stress induced a reduction in latewood width in dry sites. The relationship between growth and summer-autumn drought intensified during the last decades in all sites. Warmer spring conditions negatively affected oak growth, particularly latewood production. Wet sites had lower resilience indices, and we also noted a post-1985 progressive reduction of growth resilience. Slow-growing trees from dry sites showed growth decline, which could be an early-warning signal of impending dieback and tree death. In contrast, fast-growing trees from wet sites showed sustained relative growth improvement, which was attributed to tree age and size effects. After 1985, the pedunculate oak is more vulnerable to drought damage in dry sites near the southeastern distribution limit in response to hotter winter-spring droughts.

In this study, the physiological response of potted apple trees to combined drought and heat stress was evaluated. After establishing different levels of soil water availability, the trees were exposed to a five-day simulated heatwave with daily maximum temperatures of 40 degrees C. Stem water potential, leaf gas exchange, chlorophyll fluorescence, and tree transpiration were monitored before, during and after the combined application of heat and water stress, therefore providing insights into the extent and rapidity of the recovery. Drought caused stomatal closure that limited net photosynthesis and transpiration both at leaf and at tree level, leading to structural damage through leaf loss. On drought-stressed plants, chlorophyll fluorescence was significantly reduced by heat stress, suggesting additional leaf damage although net photosynthesis was not lower than under drought stress alone. On the other hand, well-watered trees showed low midday stem water potentials and high transpiration rates during the heatwave, while net photosynthesis was not affected. Water use efficiency of well-watered trees at 33 degrees C was reduced to 60 % of that at 23 degrees C. After the heatwave, transpiration rate in well-watered trees immediately declined to pre-stress levels, underscoring the strong atmospheric control on transpiration in apple trees. In drought-stressed trees, predawn stem water potential reached pre-stress values already on the first day of recovery. Stomatal conductance, net photosynthesis, and chlorophyll fluorescence, however, required a longer period to recover, indicating that drought stress induced transient hydraulic limitations. Nevertheless, all parameters fully recovered within five days after the end of the heatwave, showing that apple trees can withstand periods of combined heat and drought stress. The key role of water in modulating the response to heat stress highlights the need for improved irrigation management in apple orchards under climate change.

Drought stress significantly inhibits the growth of Astragalus mongholicus, leading to reduced biomass, decreased photosynthetic efficiency, and exacerbated oxidative damage. In our study, the accumulation of saponins and flavonoids in Astragalus roots markedly increased under moderate drought stress. These secondary metabolites further reshaped the rhizosphere microbial community structure, significantly increasing its diversity and interaction network complexity. Notably, drought stress enriched beneficial bacterial genera such as Rhizobium and Pseudomonas in the rhizosphere soil. Combined with the isolation of culturable microorganisms and the cooccurrence network of the rhizosphere bacterial community, we constructed a 13-strain synthetic community (SynCom) and simplified it to 7 strains. Compared with the noninoculated control, under moderate drought stress, inoculation with the simplified SynCom significantly increased plant growth, increasing the aboveground fresh weight by 50.10 %, dry weight by 55.29 %, and underground fresh weight by 76.40 %. Similarly, plants treated with the synthetic community presented significant increases in aboveground fresh weight and dry weight compared with those of the noninoculated control, with increases of 46.98 % and 61.54 %, respectively. Moreover, inoculation with the simplified community significantly reduced the content of malondialdehyde (MDA) and improved the catalase (CAT) and peroxidase (POD) activities and leaf photosynthetic parameters (Fv/ Fm and Y(II)) of Astragalus. Our findings provide new insight into improving the yield and quality of Astragalus and highlight the potential of synthetic rhizosphere microbial communities for assisting plants in coping with abiotic stress.

Drought significantly reduces cotton boll yields across various fruiting branches (FBs). Potassium (K) application can partially mitigate the drought-induced damage by modifying the biosynthesis of photoassimilates in the leaf subtending to cotton boll (LSCB) and facilitating their transport to the subtending bolls, although its effects vary among FBs. The underlying mechanisms remain unclear. To investigate this, potting experiments were conducted at three soil relative water content (SRWC): 75 +/- 5 % (W75), 60 +/- 5 % (W60), and 45 +/- 5 % (W45), along with K rates of 0 (K0), 150 (K150) and 300 (K300) kg K2O ha-1. Compared to W75, the W60 and W45 treatments reduced the photosynthesis of LSCBs in different FBs, adversely affecting carbohydrate accumulation in the subtending cotton bolls. K application can mitigate this negative impact, with the most pronounced effects observed in the middle and upper FBs. K application (K150 and K300) enhanced the net photosynthetic rate, stomatal conductance, maximum mass yield of PSII and chlorophyll content of LSCB in the middle and upper FBs compared to K0 under drought conditions. Additionally, K application significantly increased K content in LSCBs within the middle and upper FBs, which in turn elevated sucrose phosphate synthase (SPS), and sucrose synthase (SuSy) activities, reducing the conversion of sucrose into starch, ultimately facilitating carbohydrate exports to the subtending bolls. In summary, we propose a model that elucidates how K application mitigates drought damage by enhancing the exports of photoassimilates from the middle and upper FBs to their respective subtending cotton bolls.

Drought stress is becoming a structural phenomenon in cropping systems challenged by climate change and soil fertility degradation. A balanced fertilization strategy based on nitrogen, phosphorus, and potassium as well as on silicon supplementation was tested as an efficient practice to improve maize tolerance to short-term drought stress. Three fertilization strategies (control: treatment with zero NPK fertilizer application; NPK: granular NPK fertilizer, and NPK + Si: granular NPK fertilizer enriched with 5% silicon) were evaluated under three irrigation regimes simulating three probable water deficit levels in the Mediterranean climate (I1, well-watered conditions: 80% of soil field capacity; I2, medium drought stress: 60% of soil field capacity; and I3, severe drought stress: 30% of soil field capacity). Drought stress was applied at V10 growth stage of maize and maintained for 15 days, then plants were rewatered according to the optimal irrigation regime. Results showed that medium and severe drought stress down-regulated maize plant growth and yield, especially under nutrient deficient conditions (control). Plants amended with NPK and NPK + Si recorded higher chlorophyll a pigment content (+ 22 to + 64%), stomatal conductance (+ 6 to 24%), and leaf relative water content (+ 7 to 23%) than those of the control, depending on the drought stress level. Silicon supplementation attenuated the down-regulation effects of drought stress on maize photosynthesis and biomass accumulation by improving stomatal conductance and electron transfer efficiency between PSII and PSI. Silicon supply improved the performance index for energy conservation from photons absorbed by PSII to the reduction of intersystem electron acceptors (PIabs) and reduced the dissipation energy flux (DIo/RC), responsible for the protection of PSII from photo-damage under drought stress, which resulted in significant enhancement of maize photosynthesis recovery and grain yield (+ 59 to 69%). Findings from the present study demonstrate that granular NPK-fertilizer fortified with silicon could be an efficient strategy to increase maize photosynthesis performance, plant growth, and productivity under short-term drought stress conditions.

Mycorrhizal associations play a crucial role in afforestation efforts, as they enhance the acquisition of nutrients and water, thereby supporting seedling establishment. However, the influence of nitrogen (N) forms in the soil, particularly the organic N, on the formation of mycorrhizal associations and their subsequent effects on seedling morpho-physiology remains poorly understood. In this study, we examine the mycorrhizal colonization, along with morpho-physiological and functional traits, in Pinus cooperi seedlings following fertilization with organic N in controlled nursery conditions. A factorial experiment was performed with Pinus cooperi C. E. Blanco seedlings using two N sources: organic N (amino acids) and inorganic N (NH4NO3) and two N doses: low and high (60 vs 200 mg N seedling-1). Seedlings were inoculated with a mixture of native fungi, but the phylogenetic analysis showed that Suillus placidus (Bonord.) Singer was the only species colonizing roots. Organic N promoted similar morphology and nutritional status as inorganic N, though at a low N rate, it improved root growth and mycorrhizal colonization. High N fertilization improved seedling growth and nutritional status but reduced mycorrhizal colonization. Mycorrhizal colonization improved needle P concentration, delayed plant desiccation, and reduced root cellular damage when seedlings were subjected to desiccation, though it decreased plant growth and needle N concentration. We conclude that organic N fertilization improves mycorrhization of P. cooperi with S. placidus, but the fertilization dose should be adjusted to meet species-specific requirements in order to optimize plant quality and promote afforestation success.

Carbon monoxide (CO) is known primarily as a globally emitted by-product of incomplete combustion from the industry and biomass burning. However, CO is also produced in living plants and acts as a stress-signalling molecule in animals and plants. While CO emissions from soil and litter decomposition have been studied, research on the CO flux from living vegetation is scarce, particularly under field conditions. Here, we present a year-long field study on the effects of light, heat, and seasonal drought on leaf CO production and flux using automated twig chambers on mature Pinus halepensis trees grown under summer-droughted and nondroughted (irrigated) conditions. We found CO buildup in drought-stressed tree leaves, with emissions linked to the heat-controlled biogenic production of CO rather than to photodegradation. In irrigated trees, CO fluxes occurred through open stomata, whereas in droughted trees, CO buildup overcame stomatal closure to result in a flux. The results support the role of CO in heat stress response and the likely mitigation of damage induced by reactive oxygen species. We highlight the need for further research into the mechanistic basis for CO flux from living plants.

BackgroundDue to global warming and gradual climate change, plants are subjected to a wide range of environmental stresses, adversely affecting plant growth and production worldwide. Plants have developed various mechanisms to overpower these abiotic stresses, including salt stress, drought, and high light intensity. Apart from their own defense strategies, plants can get help from the beneficial endophytic bacteria inside host plants and assist them in enduring severe growth conditions. A previously isolated plant endophytic bacteria, Burkholderia seminalis 869T2, from vetiver grass can produce auxin, synthesize siderophore, and solubilize phosphate. The B. seminalis 869T2 can colonize inside host plants and increase the growth of bananas, Arabidopsis, and several leafy vegetables.ResultsWe further demonstrated that different growth parameters of Arabidopsis and pak choi plants were significantly increased after inoculating the B. seminalis 869T2 under normal, salt, and drought stress conditions compared to the mock-inoculated plants. Both transcriptome analysis and quantitative real-time PCR results showed that expression levels of genes related to phytohormone signal transduction pathways, including auxin, gibberellin, cytokinin, and abscisic acid were altered in Arabidopsis plants after inoculated with the strain 869T2 under salt stress, in comparison to the mock-inoculated control with salt treatments. Furthermore, the accumulation levels of hydrogen peroxide (H2O2), electrolyte leakage (EL), and malondialdehyde (MDA) were lower in the 869T2-inoculated Arabidopsis and pak choi plants than in control plants under salt and drought stresses.ConclusionsThe plant endophytic bacterium strain B. seminalis 869T2 may affect various phytohormone responses and reduce oxidative stress damage to increase salt and drought stress tolerances of host plants.

Fodder soybean (Glycine max L.) with high protein and yield is a popular forage grass in northeast China. Seasonal drought inhibits its growth and development during seedling stage. The objective of this study was to observe morpho-physiological changes in fodder soybean seedlings under melatonin (MT) treatments and identify appropriate concentration to alleviate the drought damage. Two varieties commonly used in northeast China were treated with 0, 50, 100, and 150 mu M melatonin at soil water content of 30%. The results indicated that applying melatonin enhanced height, biomass and altered root morphology of fodder soybean seedlings under water-deficient conditions. The treatments with melatonin at different concentrations significantly reduced the contents of H2O2, O2- and MDA, while boosting the capacity of the antioxidant defense system and the content of osmotic adjustment substances. Meanwhile, increases in light energy capture and transmission efficiency were observed. Furthermore, treatment with melatonin regulated the expression levels of genes associated with photosynthesis and the antioxidant defense system. Notably, 100 mu M melatonin treatment produced the most favorable effect in all treatments under drought conditions. These research results provide new information for enhancing the drought tolerance of fodder soybean using chemical measures.