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Alfalfa ( Medicago sativa L.) is often constrained by factors such as drought and salinity-alkalinity in cultivation. This study aims to investigate the differential response characteristics and physiological-ecological mechanisms of alfalfa during the germination stage under drought, bicarbonate, and drought-salt combined stress. Drought stress was simulated using PEG-6000 (0–20%), and salinity-alkalinity stress was simulated using NaHCO 3 (0–30 mM) to conduct germination tests on 12 alfalfa varieties. Based on the semi-inhibitory concentrations of germination and phenotypic indicators of the 12 alfalfa varieties under either drought or bicarbonate stress, drought-salt combined stress was applied. A membership function method was used to comprehensively evaluate the differential responses of the 12 alfalfa varieties to drought and salinity during germination, as well as the physiological-ecological response mechanisms of drought-tolerant, salt-tolerant, and drought-salt sensitive varieties. (1) There were significant differences among varieties in response to single drought or bicarbonate, identifying drought-tolerant varieties WL440HQ and WL363HQ, salt-tolerant variety WL525HQ, drought-sensitive 30°N, and salt-sensitive WL343HQ. Comprehensive evaluation indicated that the strongest and weakest drought and salt-resistant varieties were WL363HQ and WL319HQ, respectively; (2) Drought-salt combined stress exhibited antagonism at low concentrations and synergism at high concentrations; (3) The drought-salt tolerant variety WL363HQ adapted well to the combined stress through mechanisms involving SOD activity and soluble sugars, while the sensitive variety WL319HQ performed poorly. This study provides a scientific basis for elucidating the germination response mechanisms of plants under drought-salt combined stress and offers scientific support for planting in arid and saline-alkaline regions based on the selected drought- and salt-tolerant varieties.

In field conditions, especially in arid and semi-arid areas, tea plants are often simultaneously exposed to various abiotic stresses such as cold and drought, which have profound effects on leaf senescence process and tea quality. However, most studies of gene expression in stress responses focus on a single inciting agent, and the confounding effect of multiple stresses on crop quality and leaf senescence remain unearthed. Here, global transcriptome profiles of tea leaves under separately cold and drought stress were compared with their combination using RNA-Seq technology. This revealed that tea plants shared a large overlap in unigenes displayed \"similar\" (26%) expression pattern and avoid antagonistic responses (lowest level of \"prioritized\" mode: 0%) to exhibit very congruent responses to co-occurring cold and drought stress; 31.5% differential expressed genes and 38% of the transcriptome changes in response to combined stresses were unpredictable from cold or drought single-case studies. We also identified 319 candidate genes for enhancing plant resistance to combined stress. We then investigated the combined effect of cold and drought on tea quality and leaf senescence. Our results showed that drought-induced leaf senescence were severely delayed by (i) modulation of a number of senescence-associated genes and cold responsive genes, (ii) enhancement of antioxidant capacity, (iii) attenuation of lipid degradation, (iv) maintenance of cell wall and photosynthetic system, (v) alteration of senescence-induced sugar effect/sensitivity, as well as (vi) regulation of secondary metabolism pathways that significantly influence the quality of tea during combined stress. Therefore, care should be taken when utilizing a set of stresses to try and maximize leaf longevity and tea quality.

Autotoxicity and soil salinization are the main causes of continuous cropping obstacles of melon, which often occur at the same time and aggravate with the extension of cultivation years. Melatonin (MT) has a broad application prospect in improving plant resistance. The effects of exogenous MT on seed germination of melon ( Cucumis melo L.) under autotoxicity and saline-alkali combined stress were observed in this paper. Starch and reactive oxygen species metabolism were determined to analyze the physiological and biochemical basis of MT function. Cinnamic acid was used to simulate melon autotoxicity. The mixed solution of NaCl and NaHCO 3 was used to simulate saline-alkali stress. The effect of MT showed a concentration-dependent manner. 16 μM exogenous MT can effectively improve the germination rate, germination potential, and vigor index of melon seeds under combined stress, and increase radicle length and fresh weight. Seed vigor index was the most sensitive index, and MT treatment increased by 102.22% compared with combined stress. However, at high concentration, this stress relief effect can be offset or even reversed. MT can significantly shorten the time of seed breaking through the seed coat and improve the root growth inhibited by stress. MT enhanced α-amylase and β-amylase gene expression, antioxidant enzyme, and amylase activity. The contents of starch and soluble sugar changed accordingly. At the same time, the contents of malondialdehyde and reactive oxygen species were reduced. The expression analysis of 10 MT synthesis genes showed that MT treatment up-regulated eight genes. In general, by up-regulating the expression of amylase and some MT synthesis genes, enhancing the activities of antioxidant enzymes and amylase, reducing the content of malondialdehyde and reactive oxygen species, exogenous MT effectively increased the germination rate and germination potential of melon seeds under autotoxicity and saline-alkali combined stress, and significantly improved the quality of seed germination.

Plant responses to a combination of drought and bacterial pathogen infection, an agronomically important and altogether a new stress, are not well-studied. While occurring concurrently, these two stresses can lead to synergistic or antagonistic effects on plants due to stress-interaction. It is reported that plant responses to the stress combinations consist of both strategies, unique to combined stress and those shared between combined and individual stresses. However, the combined stress response mechanisms governing stress interaction and net impact are largely unknown. In order to study these adaptive strategies, an accurate and convenient methodology is lacking even in model plants like Arabidopsis thaliana. The gradual nature of drought stress imposition protocol poses a hindrance in simultaneously applying pathogen infection under laboratory conditions to achieve combined stress. In present study we aimed to establish systematic combined stress protocol and to study physiological responses of the plants to various degrees of combined stress. Here, we have comprehensively studied the impact of combined drought and Pseudomonas syringae pv. tomato DC3000 infection on A. thaliana. Further, by employing different permutations of drought and pathogen stress intensities, an attempt was made to dissect the contribution of each individual stress effects during their concurrence. We hereby present two main aspects of combined stress viz., stress interaction and net impact of the stress on plants. Mainly, this study established a systematic protocol to assess the impact of combined drought and bacterial pathogen stress. It was observed that as a result of net impact, some physiological responses under combined stress are tailored when compared to the plants exposed to individual stresses. We also infer that plant responses under combined stress in this study are predominantly influenced by the drought stress. Our results show that pathogen multiplication was reduced by drought stress in combined stressed plants. Combined stressed plants also displayed reduced ROS generation and declined cell death which could be attributed to activation of effective basal defense responses. We hypothesize a model on ABA mediated gene regulation to partly explain the possible mechanistic basis for reduced in planta bacterial numbers under combined stress over individual pathogen stress.

Plants are commonly exposed to abiotic and biotic stresses. We used 350 Arabidopsis thaliana accessions grown under controlled conditions. We employed genome-wide association analysis to investigate the genetic architecture and underlying loci involved in genetic variation in resistance to: two specialist insect herbivores, Pieris rapae and Plutella xylostella; and combinations of stresses, i.e. drought followed by P. rapae and infection by the fungal pathogen Botrytis cinerea followed by infestation by P. rapae. We found that genetic variation in resistance to combined stresses by drought plus P. rapae was limited compared with B. cinerea plus P. rapae or P. rapae alone. Resistance to the two caterpillars is controlled by different genetic components. There is limited overlap in the quantitative trait loci (QTLs) underlying resistance to combined stresses by drought plus P. rapae or B. cinerea plus P. rapae and P. rapae alone. Finally, several candidate genes involved in the biosynthesis of aliphatic glucosinolates and proteinase inhibitors were identified to be involved in resistance to P. rapae and P. xylostella, respectively. This study underlines the importance of investigating plant responses to combinations of stresses. The value of this approach for breeding plants for resistance to combinatorial stresses is discussed.

Abstract The devastating effects and extent of abiotic stress on cereal production continue to increase globally, affecting food security in several countries, including Tunisia. Heat waves and the scarcity of rainfall strongly affect durum wheat yields. The present study aims to screen for tolerance to combined water and heat stresses in durum wheat at the juvenile stage. Three combined treatments were tested, namely: T0 (100% field capacity (FC) at 24 °C), T1 (50% FC at 30 °C), and T2 (25% FC at 35 °C). The screening was carried out based on morphological, physiological, and biochemical criteria. The results showed that the combined stress significantly affected all the measured parameters. The relative water content (RWC) decreased by 37.6% under T1 compared to T0. Quantum yield (Fv/m) and photosynthetic efficiency (Fv/0) decreased under severe combined stress (T2) by 37.15% and 37.22%, respectively. Under T2 stress, LT increased by 63.7%. A significant increase in osmoprotective solutes was also observed, including proline, which increased by 154.6% under T2. Correlation analyses of the combination of water and heat stress confirm that the traits RWC, chlorophyll b content, Fv/m, proline content, Fv/0 and leaf temperature can be used as reliable screening criteria for the two stresses combined. The principal component analysis highlighted that Aouija tolerates the two levels of stresses studied, while the genotypes Karim and Hmira are the most sensitive. The results show that the tolerance of durum wheat to combined water and heat stress involves several adaptation mechanisms proportional to the stress intensity. This study aims to select contrasting drought- and heat-stress-tolerant durum wheat genotypes from a Landraces gene pool using physiological and biochemical traits. The results showed that landraces have a much greater genetic tolerance than the improved variety to mitigate combined stress constraints. Indeed, the landraces Aouija and Hedhba were identified as tolerant to drought and heat stress. Besides, the improved variety Karim and the landrace Hmira are the most sensitive to stress conditions. Our data showed that chlorophyll fluorescence parameters, relative water content, chlorophyll b content, proline content, and leaf temperature can be used as reliable screening criteria for the two stresses combined.

In nature, plants are exposed to multiple and simultaneous abiotic stresses that influence their growth, development, and reproduction. In the last years, the study of combined stresses has aroused the interest to know the physiological and molecular responses, because these new stress conditions are probed to be different from the sum of the individual stress. We are interested in the study of the acclimation of plants growing under the combination of cold and water deficit stresses prevalent in cold-arid or semi-arid climates worldwide. We hypothesized that the reproduction of the acclimated plants will be compromised and affected. A rabidopsis plants were submitted to long-term combined stress from the beginning to the reproductive stage, when floral bud was visible, until the silique development. Our results demonstrate severe morphoanatomical changes after acclimation to combined stress. Inflorescence stem morphology was altered having a delayed bolting and a limited growth. Flowering and silique formation were delayed, and a higher size in the corolla and the petals was observed. Flower and silique number were severely diminished as a result of combined stress, unlike acclimated plants to individual cold stress. These traits were recovered after deacclimation to optimal conditions and plants achieved similar silique production as control plants. The long-term stress results suggest that there is not a single dominant stress, but there is an alternating dominance depending on the structure or the plant stage development evaluated.

In nature, plants are constantly exposed to a varied abiotic and biotic stresses or their combinations, limiting the productivity of major crops, including wheat. Combinations of drought and soil-borne Fusarium-instigated diseases are the most common combinations of stresses, significantly reducing wheat yield around the world. Here, were analyzed the potential of application of endophytic bacteria Bacillus subtilis (strain 10–4) together with the natural signal molecule salicylic acid (SA) to improve growth and tolerance of Triticum aestivum L. (wheat) plants under combined drought and Fusarium culmorum-instigated root rot (FRR) stresses. It was revealed that pre-sowing treatment with B. subtilis 10–4, SA, and B. subtilis 10–4 + SA, both under normal and combined drought conditions, notably reduced (by 50–80% or more) the incidence of FRR development in wheat plants, with the most notable effect for B. subtilis 10–4 + SA (wherein disease symptoms were almost absent). Moreover, B. subtilis 10–4, SA, and especially B. subtilis 10–4 + SA increased plant growth (root and shoot length, fresh and dry biomass) under normal (up to 20–50%), drought (up to 15–40%), FRR (up to 15–30%), and combined drought + FRR stresses (up to 20%), with the maximum effect for B. subtilis 10–4 + SA. Additionally, B. subtilis 10–4, SA, and B. subtilis 10–4 + SA decreased stress (drought, FRR, and combined drought + FRR)-instigated lipid peroxidation and osmotic damages of plant cells. The findings indicate that endophytic bacteria B. subtilis 10–4 alone and in a mixture with SA may be used as an effective eco-friendly agent to improve wheat growth and tolerance under the influence of drought, FRR, and combinations of these stresses.

Background Affected by global warming, freeze-thaw occurs more frequently in Northeast China. Meanwhile, as a major grain-producing area, this region is influenced by the invasive plant Solanum rostratum Dunal. Moreover, due to the long-term application of chemical fertilizers, soil cadmium pollution has been aggravated. Therefore, crops in Northeast China may suffer from compound stress simultaneously. However, the impact of combined stress on plants has not been given enough attention, and the interrelationships between different stresses have not been thoroughly studied. This experiment adopted the indoor simulation method. By determining the changing trends and amplitudes of relative conductivity (RC), soluble protein (SP), chlorophyll (Chl), malondialdehyde (MDA) content, and the activities of superoxide dismutase (SOD) and peroxidase (POD), the influence effects of the combined stress of freeze-thaw, S. rostratum extract, and cadmium chloride on the growth and metabolism of seedlings could be judged. Results Under the combined stress of freeze-thaw, cadmium chloride, and S. rostratum extract, the growth of rye seedlings was inhibited; The relative conductivity (RC) increased by 1.92–71.07%, and the content of malondialdehyde (MDA) increased by 17.34–28.11%; The soluble protein (SP) content decreased by 17.82–22.14%; The activities of superoxide dismutase (SOD) and peroxidase (POD) both increased, but POD activity was inhibited at the lowest point of freeze-thaw (-5℃); The chlorophyll (Chl) content decreased by 9.68–19.67%. Conclusion Stress affects osmotic pressure, and seedlings need to accumulate osmoregulatory substances to maintain cell osmotic balance. Compared to a single stress factor, the combined stress of freeze-thaw, cadmium chloride, and S. rostratum extract further enhanced the physiological damage to plants. This compound stress leads to electrolyte leakage, intensified membrane lipid peroxidation, inhibition of protein synthesis, increased osmotic pressure, and disruption of cell osmotic balance. Combined stress further promotes the accumulation of reactive oxygen species (ROS) in the seedlings, leading to oxidative damage and inhibiting photosynthesis.

Rhodotorula glutinis is an important oleaginous yeast that can synthesize various valuable compounds, including carotenoids, lipids, and exopolysaccharides. The effect of combined heat stress and glucose starvation on carotenoid biosynthesis in R. glutinis was investigated in this study. Carotenoid production in R. glutinis was promoted by heat stress, and this effect was further enhanced when glucose starvation was applied to the strain. The results of multiomics analysis revealed that the effects of heat stress and glucose starvation on promoting carotenoid biosynthesis appeared to be additive, with the combined stress leading to a further increase in reactive oxygen species (ROS) levels and a reduction in enzymatic antioxidant capacity, while carotenoid biosynthesis was prioritized simultaneously. The key responses of R. glutinis to combined stress include the regulation of the cell cycle and energy metabolism, maintenance of membrane integrity, an increase in ROS scavenging capacity, and non-enzymatic antioxidant activity. Additionally, several candidate genes and metabolites associated with the combined stress response were identified. To summarize, we provided new insights into optimizing fermentation processes for increased carotenoid production in Rhodotorula glutinis and established a molecular basis for further genetic engineering to increase carotenoid yield.