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.