Ascophyllum nodosum and Silicon-Based Biostimulants Differentially Affect the Physiology and Growth of Watermelon Transplants under Abiotic Stress Factors: The Case of Drought

Climate change is an inevitable process characterized by an abrupt increase in global temperature and a decrease in precipitations leading to drought incidents. Biostimulants could be a valuable tool for mitigating these harsh conditions. The objective of our study was to test the efficiency of two biostimulants, a silicon-based seaweed and the seaweed Ascophyllum nodosum, to mitigate the drought stress endured by watermelon transplants during the first few weeks after transplanting. In order to achieve this, three water treatments (100%, 75%, and 50% of field capacity) were applied in pots. Important growth parameters (leaf number, fresh weight, and plant area) deteriorated depending on water availability. This was also the case for the root system development displayed by root dry weight, total length, and surface area. It is the first time the OJIP transient has been evaluated after the application of A. nodosum for drought-stressed plants. Chlorophyll fluorescence parameters showed that the photosynthetic apparatus was more stressed when A. nodosum was applied, especially in the harshest conditions (i.e., 50% field capacity). Overall, the silicon-based biostimulant failed to demonstrate drought-mitigating potential compared to the non-treated counterparts. On the other hand, A. nodosum alleviated the negative effects of water deficit, especially in the harshest conditions.