Young Tomato Plants Respond Differently under Single or Combined Mild Nitrogen and Water Deficit: An Insight into Morphophysiological Responses and Primary Metabolism

Tomato responses to nitrogen, drought and combined stresses: Shared and specific effects on vascular plant anatomy, nutrient partitioning and amino acids profile

Crops are often subjected to various abiotic stresses and interactions between them may occur, but how plants cope with them remains poorly understood. This study explored how combined nitrogen and drought stress impact tomato vascular stem anatomy, nutrient partitioning and amino acids profile. Tomato seedlings were exposed to control (CTR; 100N + 100%W), N stress (N; 50%N), drought stress (W; 50%W), or combined stress (N + W; 50%N+50%W) for 27 days. All treatments similarly reduced the phloem and xylem areas. Plants under N + W stress exhibited increased root synthesis of asparagine and arginine (up to 230% compared to W stress and 66% compared to N stress) and showed a higher reallocation and synthesis of osmolytes such as K+ and proline, respectively. This, along with the specific increase in other amino acids related to osmoregulation (alanine, tyrosine and phenylalanine), contributed to an enhanced stomatal closure and lower transpiration rate compared to W stressed plants. Conversely, N stressed plants responded mainly through N remobilization from the photosynthetic machinery, leading to decreased chlorophyll content (up to 32%) and photosynthetic rate (up to 57%). Under single W stress, plants invested more in the root system as a strategy to increase W and nutrients' uptake, compared to those grown under N + W stress, and maintained the photosynthetic rate at the level of CTR plants. It is concluded that tomato plants employed distinct mechanisms for reallocating nitrogen and regulating osmosis to withstand either single or combined stresses and that amino acids and nutrients' homeostasis have an important role in these processes.

Effects of Foliar Selenium Application on Oxidative Damage and Photosynthetic Properties of Greenhouse Tomato under Drought Stress

Both drought stress and exogenous selenium (Se) cause changes in plant physiological characteristics, which are key factors affecting crop yield. Although Se is known to be drought-resistant for crops, its internal physiological regulatory mechanisms are not clear. This study analyzed the effects of selenium application (SeA) on antioxidant enzyme activities, osmoregulatory substance contents, and photosynthetic characteristics of greenhouse tomatoes under drought stress and related physiological mechanisms. The results showed that drought stress induced oxidative damage in cells and significantly increased the content of the membrane lipidation product malondialdehyde (MDA) and the osmoregulatory substance proline (p < 0.001) compared with the adequate water supply. The proline content of severe drought stress (W1) was 9.7 times higher than that of the adequate water supply (W3), and foliar SeA increased glutathione peroxidase (GSH-PX) activity, and SeA induced different enzymatic reactions in cells under different drought stresses; catalase (CAT) was induced under severe drought stress (p < 0.01) and was significantly increased by 32.1% compared with the clear water control, CAT. Peroxidase (POD) was induced under adequate water supply conditions (p < 0.01), which was significantly increased by 15.2%, and SeA attenuated cell membrane lipidation, which reduced MDA content by an average of 21.5% compared with the clear water control, and also promoted photosynthesis in the crop. Meanwhile, through the entropy weighting method analysis (TOPSIS) of the indexes, the highest comprehensive evaluation score was obtained for the S5W3, followed by the S2.5W3 treatment. Therefore, this study emphasized the importance of SeA to reduce oxidative damage and enhance photosynthesis under drought stress.