With global climate change, the frequency and intensity of waterlogging events are increasing due to frequent and heavy precipitation. Little is known however about the response of plants to repeated waterlogging stress events. The aim is to clarify physiological regulation mechanisms of tomato plants under repeated waterlogging stress, and whether Trichoderma harzianum can alleviate waterlogging injury. We identified two genotypes of tomato, ‘MIX-002’ and ‘LA4440’, as waterlogging tolerant and sensitive genotypes, respectively, based on plant biomass accumulation. The two tomato genotypes were subjected to a waterlogging priming treatment for 2 days (excess water for 1 cm above substrate surface) followed by a recovery stage for 2 days, and then a second waterlogging stress for 5 days (excess water for 1 cm above substrate surface) followed by a second recovery stage for 3 days. Leaf physiological, plant growth parameters, and the expression of five key genes were investigated. We found that the two genotypes responded differently to waterlogging priming and stress in terms of photosynthesis, reactive oxygen species (ROS), and osmotic regulatory mechanisms. Waterlogging stress significantly increased H₂O₂ content of ‘MIX-002’, while that of ‘LA4440’ had no significant change. Under waterlogging stress, photosynthesis of the two genotypes treated with waterlogging priming returned to the control level. However, Trichoderma harzianum treatment during the second recovery stage did not show positive mitigative effects. The plants of ‘LA4440’ with priming showed lower peroxidase (POD) activity and proline content but higher H₂O₂ content than that without priming under waterlogging stress. Under waterlogging stress with priming as compared to without priming, SODCC2 was downregulated in two tomatoes, and AGR2 and X92888 were upregulated in ‘MIX-002’ but downregulated in ‘LA4440’. Overall, the two tomato genotypes exhibited distinct photosynthetic, ROS and osmotic regulatory mechanisms responding to the waterlogging stress. Waterlogging priming can induce stress memory by adjusting stomatal conductance, sustaining ROS homeostasis, regulating osmotic regulatory substances and key gene expressions mediated by H₂O₂, and thus alleviate the damage on tomato photosynthesis when waterlogging reoccurred.

As a beneficial fungus, Trichoderma harzianum (T. harzianum) has been widely applied for growth promotion and biocontrol. Recently, it has attracted much attention with regard to improving stress tolerance in plants under abiotic stress. In this paper, the multiple mechanisms of T. harzianum for alleviating abiotic stress damage in plants are reviewed. T. harzianum can regulate the synthesis of key phytohormones, such as abscisic acid (ABA), indole-3-acetic acid (IAA), etc., thereby enhancing the physiological response ability of plants under stress conditions such as drought, salt stress, and high temperature. These are associated with antioxidant system regulation in plants, which reduces levels of reactive oxygen species (ROS) and oxidative damage and maintains intracellular redox balance. T. harzianum can also improve plant nutrient uptake and root development, secondary metabolism, soil environment and structure, and expression of related genes. In addition, in this paper, the characteristics of T. harzianum application in field and horticultural crops are summarized and compared, revealing differences in the methods, concentrations, time, and effects of applying T. harzianum to various crops. We further explore the synergistic regulation effect of T. harzianum and plant–microbiome interaction on the stress microenvironment. Future perspectives on the molecular mechanism of T. harzianum and its field application potential are discussed. This review provides a theoretic and practical reference for the application of T. harzianum in agricultural production.

Under natural conditions, abiotic stresses that limit plant growth and development tend to occur simultaneously, rather than individually. Due to global warming and climate change, the frequency and intensity of heat and salt stresses are becoming more frequent. Our aim is to determine the response mechanisms of tomato to different intensities of combined heat and salt stresses. The physiological and morphological responses and photosynthesis/reactive oxygen species (ROS)-related genes of tomato plants were compared under a control, heat stress, salt stress (50/100/200/400 mM NaCl), and a combination of salt and heat stresses. The stomatal conductance (g_s) of tomato leaves significantly increased at a heat + 50 mM NaCl treatment on day 4, but significantly decreased at heat + 100/200/400 mM NaCl treatments, compared with the control on days 4 and 8. The O₂^·− production rate of tomato plants was significantly higher at heat + 100/200/400 mM NaCl than the control, which showed no significant difference between heat + 50 mM NaCl treatment and the control on days 4 and 8. Ascorbate peroxidase 2 was significantly upregulated by heat + 100/200/400 mM NaCl treatment as compared with heat + 50 mM NaCl treatment on days 4 and 8. This study demonstrated that the dominant effect ratio of combined heat and salt stress on tomato plants can shift from heat to salt, when the intensity of salt stress increased from 50 mM to 100 mM or above. This study provides important information for tomato tolerance improvement at combined heat and salt stresses.

Waterlogging stress caused by concentrated and heavy rainfall has become an increasingly popular abiotic stress, especially for tomato production. The aim is to clarify the leaf physiological and biochemical responses as well as flower and fruit set of tomato plants at reproductive stage under waterlogging stress. Six tomato genotypes including 'MIX-002', 'LA4440', 'Fenbeibei', 'Hezuo 908', 'Jinzhu' and 'Ruifen 882' at anthesis stage were treated under control and waterlogging stress. We found that plant height, stem diameter, inflorescences number, single fruit weight and fruit weight per plant of all the six tomato genotypes significantly decreased under waterlogging stress as compared with the control. Chlorophyll a content of 'MIX-002', 'LA4440' and 'Jinzhu' and total chlorophyll content of 'MIX-002', 'LA4440' and 'Ruifen 882' were significantly lower under waterlogging conditions than the control. Waterlogging stress induced low net photosynthetic rate of 'MIX-002' and decreased transpiration rate of six genotypes except 'Fenbeibei'. Antioxidant enzymes activity and proline content responded to waterlogging stress by up/down regulation, resulting in high malondialdehyde and H₂O₂ content in leaves of six genotypes. Overall, waterlogging stress caused low gas diffusion, negatively affected pigment content, decreased transpiration rate, and induced antioxidant system disorder in tomato plants at anthesis stage. These adverse effects contributed to few buds and flowers and finally resulted in yield loss of tomato plants under waterlogging conditions. The significant correlation between malondialdehyde and H₂O₂ content and tomato yield indicated that the malondialdehyde and H₂O₂ content in leaves could be applied to predict the tomato yield under waterlogging condition.

Frequent extreme weather events exacerbate agricultural abiotic stress, with drought causing widespread yield loss. Tomato, a globally important vegetable sensitive to water deficit, has been predominantly studied under single-drought scenarios that poorly reflect recurrent field conditions. This study investigated physiological and molecular responses of two tomato genotypes to repeated drought stress. Results showed that the drought-sensitive genotype ‘TGTB’ exhibited faster ABA accumulation and more pronounced ABA-mediated stomatal closure. During the second drought cycle, stomatal pore length and width were significantly smaller than during the first drought, indicating a strong stress memory effect. In contrast, the drought-tolerant ‘LA1598’ showed minimal memory responses. Under extreme drought stress, primed and non-primed ‘TGTB’ plants showed significantly lower H₂O₂ content than controls, whereas primed ‘LA1598’ plants maintained a significantly lower O₂^·− production rate than non-primed plants during both extreme drought cycles. Antioxidant enzyme systems contributed to ROS homeostasis, supported by the regulation of key drought-responsive genes. This study demonstrates genotype-dependent memory capacity and reveals that drought priming enhances repeated drought tolerance through ABA-regulated stomatal adjustment. These findings provide a theoretical basis for improving tomato resilience to recurrent drought and supporting breeding of drought-tolerant varieties.