Biostimulatory Action of Vegetal Protein Hydrolysate and the Configuration of Fruit Physicochemical Characteristics in Grafted Watermelon

Abstract

Foliar application of a vegetal-derived protein hydrolysate as a biostimulant was assessed for possible interaction with the ripening of diploid watermelon grafted onto interspecific hybrid rootstock. Assessment encompassed crop performance; fruit morphometric and sensory quality traits; soluble carbohydrates; macrominerals; and bioactive composition at 10, 20, 30, 40, and 50 days post anthesis (dpa). The biostimulant effect on yield components was confounded by the vigorous rootstock effect. Pulp firmness declined precipitously with cell enlargement from 10 to 30 dpa, and the biostimulant phyto-hormonal potential on firmness and rind thickness was masked by grafting. Pulp colorimetry was determined solely by ripening and peaked at 40 dpa. The biostimulant effect reduced lycopene content by 8% compared to the control. Total sugars coevolved with soluble solids content, peaked at 30 dpa, and then stabilized. Fructose and glucose prevailed during rapid fruit growth from 10 to 30 dpa and sucrose prevailed at advanced ripeness between 40–50 dpa, whereas acidity peaked at 20 dpa and then decreased. Potassium, which was the most abundant micromineral, peaked before full ripeness at 30 dpa. The biostimulant effect on the watermelon fruit ripening process is not granted, at least regarding the conditions this study was carried out under. The absence of biostimulant effect might relate to rootstock vigorousness, the grafted watermelon physiology, or the type of biostimulant used.

Combined Effect of Biostimulants and Mineral Fertilizers on Crop Performance and Fruit Quality of Watermelon Plants

“…Interestingly, the highest total sugar content was detected in the W6 treatment, due to a balanced content of individual free sugars. In contrast to our study, Soteriou et al [42] did not record a significant effect of protein hydrolysates application on the free sugar content of watermelon fruit, whereas a biostimulant that included polysaccharides, vitamins, amino acids, potassium oxide and polypeptides increased glucose and fructose content in pepper fruit (Capsicum annuum L. cv Palermo) [76]. A similar effect was recorded for the application of free amino acids, which increased monosaccharides content in tomato fruit [77], whereas Peris-Felipo et al [58] suggested that only fructose content was affected by silicon application on strawberry plants.…”

“…Interestingly, no significant effect on fruit firmness was recorded for the tested biostimulants in our study, since, according to the literature, Si application is expected to increase fruit firmness due to its involvement in cell wall structure and the induced biosynthesis of lignin and hemicellulose [69,70]. However, our results are in agreement with the study of Soteriou et al [42], who also did not record any significant effect of biostimulant application on the firmness of watermelon fruit flesh, which highlights the impacts the application dose and biostimulant composition may have on fruit quality parameters. The effect of biostimulant formulation application on the proximal composition of watermelon fruit is presented in Table 6.…”

“…In other studies where the effect of biostimulants on fruit quality was evaluated, a varied impact was recorded. For example, Soteriou et al [42] did not record a significant effect of protein hydrolysates on either soluble solids content or titratable acidity of watermelon fruit, while similar results have been suggested for the titratable acidity of other fruit, such as tomato (Solanum lycopersicum L.) and melon (Cucumis melo L.) [64,65]. On the other hand, Salim et al [59] reported that Si application may improve total soluble sugar content in squash leaves under either normal or deficit irrigation, since Si is associated with tolerance to abiotic stressors and its application induces osmoprotective mechanisms.…”

“…The same type of biostimulants also had a positive effect on alleviating the negative impact of drought stress on watermelon transplants [41]. On the other hand, Soteriou et al [42] suggested that the application of protein hydrolysates did not affect yield parameters, and most of the fruit quality features assessed in grafted watermelon plants were due to the high vigor of the rootstock, which concealed any impacts of the biostimulant formulation.…”

Influence of the application of humic substances on the growth of watermelon and melon seedlings

“…To overcome such limitations when seeking the production of high-quality seedlings, the use of substances with biostimulant properties is an interesting technology in sustainable agriculture to face problems related to fertilizer use in agricultural crops (Nardi et al, 2021). Humic substances (HS) have been used to supply nutrients and/ or stimulate the production of phytohormones that lead to root growth that the plant would otherwise not achieve (El-Hai et al, 2019;Qin & Leskovar, 2020a;Soteriou et al, 2021). Previous studies have demonstrated the effect of humic substances as seedling growth modulators, among other beneficial effects (Gomes Júnior et al, 2019;Qin & Leskovar, 2020a;Silva et al, 2022;Targino et al, 2023).…”