Effect of Lignosulphonate-Based Biostimulants and Grafting on Watermelon Plant Performance Under Open-Field Conditions for Two Consecutive Years

Lignosulfonate-based biostimulants (LS) are recognized to play a role as chelating and careering agents for micronutrients. Concomitantly, vegetable grafting is a recognized technique with which to improve yield and fruit quality and increase plant tolerance to abiotic and biotic distresses. The objective of this study was to evaluate how the integration of these complementary approaches could affect agronomic attributes, colorimetric parameters, qualitative and nutritional characteristics, and the physiological traits of watermelon plants. The combined effects of LS biostimulant and grafting (non-grafting, self-grafting, or grafting onto ‘RS 841’ rootstock) were evaluated in a two-year (2024 and 2025) open-field study. Plots treated with LS biostimulant yielded 5% more than control plots, whereas grafted plots yielded between 12 and 13% more than self-grafted and non-grafted plots, respectively. In addition, biostimulant treatment reduced physiological traits such as stomatal conductance and transpiration, whereas grafting only enhanced stomatal conductance. Furthermore, LS biostimulant increased β-carotene (+25%), lycopene (+21%) and carotenoid (+21%) contents, while grafting had the same effect and increased β-carotene (+55%), lycopene (+50%) and carotenoids (+48%) when compared with the control treatment. Results indicated that the effects of the biostimulant and grafting technique were dependent on the year. Accordingly, this study suggested that for yield, physiological traits, quality, nutritional characteristics and colorimetric parameters, the year of experimentation should be no taken into consideration, whereas for percentage of fruit dry matter, solid soluble content, pulp firmness, and epicarp thickness, the year of experimentation should be taken into consideration in order to improve crop quality.