Contrasting effects of serotonin and melatonin on rocket yield and quality under different water availability...

Biostimulants have emerged as an effective tool to reduce the negative impact of water scarcity on plants, improving or maintaining crop yield and vegetable quality. Here, a native Mediterranean species, rocket (Eruca vesicaria subsp. sativa), was treated with 100 μM of the indoleamines serotonin and melatonin as potential biostimulant ingredients, with the aim of improving production and quality under two different water availability conditions. However, melatonin did not have any positive impact on rocket yield or quality, and serotonin applications unexpectedly reduced rocket yield and quality, particularly under water deficit. Yield was reduced by 28% and visual quality was also negatively affected by serotonin treatments in plants subjected to low water availability, as shown by a darker green color at harvest. Among the mineral elements and low-molecular-weight antioxidants analyzed, serotonin application improved only the concentrations of S, Mn, Cu and Zn under water deficit conditions, an effect that did not compensate for losses in yield and visual quality. Our study revealed unexpected effects of indoleamines on rocket yield and quality, showing overall negligible effects of melatonin and even negative effects of serotonin. The physiological basis underlying these results and their implications for the use of indoleamines as biostimulants are discussed.