Plant biostimulants from seaweeds or vegetal proteins enhance the salinity tolerance in greenhouse lettuce by modulating plant metabolism in a distinctive manner

The use of plant biostimulants such as seaweed extracts (SWE) and protein hydrolysates (PH) has grown in the recent years due to their beneficial effects on yield under both optimal and sub-optimal conditions such as salt stress. The comprehension of the mode of action of these two important categories of biostimulants on plant performance will allow to use them more efficiently under different growth conditions. This study aimed to examine the efficacy of a seaweed and plant-based biostimulants on greenhouse lettuce (Lactuca sativa L.) grown under non-saline (0 mM NaCl) and saline conditions (40 mM NaCl) in terms of growth, yield, SPAD index, leaf mineral composition and metabolomic profiling. Shoot fresh weight of lettuce was reduced by 15.3% under 40 mM of NaCl. Nonetheless, it was boosted by both used biostimulants by 9 and 18%, on average, under 0 and 40 mM NaCl salinity, respectively. Na content under saline conditions was reduced in the presence of the biostimulants treatment, where PH reduced it significantly by 15.6% and SWE by 9.4%. On the other hand, Cl content was significantly reduced only under PH treatment. Both biostimulants elicited a broad metabolic reprogramming, involving the accumulation of stress-related compounds such as glucosinolates, terpenoid phytoalexins, and jasmonates. Interestingly, distinctive metabolomic signatures could be observed following the application of the different biostimulants under salinity conditions. In more detail, PH promoted the accumulation of glucosinolates and phytoalexins precursors, while SWE induced a down accumulation of secondary metabolites. Our findings indicate different processes being modulated by PH and SWE, with possible synergistic effects, thus paving the way towards integrated strategies to alleviate the detrimental effects of salinity in lettuce.