Enhanced Levels of Combined Waterlogging and Heat Stress Exacerbate Anatomical Damage, Photosynthetic Impairment, and Oxidative Stress in Tomato

Global climate change increases concurrent waterlogging and heat waves, threatening crop production. Tomato, a widely cultivated vegetable crop, is susceptible to both stresses, but their combined effects on tomato responses remain poorly understood. This study analyzed the anatomical, physiological, and molecular responses of two tomato genotypes, ‘NT212’ and ‘NT606’ (sensitive and tolerant to combined waterlogging and heat stresses), under control (C), waterlogging (W), heat stress (H1, H2, and H3) (34/27 °C, 38/31 °C, 42/35 °C), and combined stress (WH1, WH2, and WH3) for 48 h. ‘NT212’ exhibited only abaxial epidermal peeling under H2, but severe anatomical degradation including abaxial epidermal peeling and sponge parenchyma cell lysis under WH2. The superoxide anion (O2•−) production rate and PsbS and HSP70 expression were significantly higher in ‘NT212’ under WH2 than W and H2. Moreover, as the stress intensity increased from WH1 to WH2 and WH3, the O2•− production rate and PsbS and HSP70 expression were significantly increased in ‘NT212’. In contrast, ‘NT606’ did not exhibit obvious cell lysis under any stress treatments. The O2•− production rate was highest in ‘NT606’ under WH3 among all the treatments. HSP70 expression was significantly upregulated in ‘NT606’ under H3 and WH3 compared with the other treatments. Consequently, ‘NT212’ exhibited severe wilting under the H2, H3, and all combined stresses, while ‘NT606’ showed severe wilting only under WH3. Thus, combined stress caused synergistic damage exceeding individual stress, with injury intensifying as temperature increased. This study provided vital knowledge for breeding climate-resilient tomato cultivars, especially under combined waterlogging and heat stress.