Evaluation of different molecular weight fractions of soybean protein hydrolysate on lettuce

Protein hydrolysates effectively promote growth and nutrient uptake, enhance tolerance to abiotic stresses and positively influence yield and quality parameters. It is also widely reported that some biological properties of protein hydrolysates are strongly related to molecular weights (MW) of their constituted peptides. In this work, a commercial soybean hydrolysate (SH) obtained through enzymatic hydrolysis was firstly fractionated by ultrafiltration with molecular weight cut-off membranes of 1, 5, and 10 kDa. The biostimulant properties of the obtained fractions, SH_F1 (< 1 kDa), SH_F2 (1–5 kDa), SH_F3 (5–10 kDa) and SH_F4 (> 10 kDa), and the entire hydrolysate (SH) were investigated on lettuce (2 kg ha − 1 ), through destructive and non-destructive measurements. Non-destructive measurements (chlorophyll, flavonol and anthocyanin content), performed twice (14 days apart), shown no significative differences among treatments. Destructive measurements were performed at harvest; SH_F2 (the fraction which contains peptides with molecular weights between 1 and 5 kDa) induced an increase of both chlorophylls a + b (+ 22.5% and + 25.2%) and carotenoids (+ 27% and + 33%) with respect to the control (deionised water) and the entire hydrolysate (SH), respectively. The same trend was observed for total sugars: the entire hydrolysate (SH) and the fraction SH_F2 led to a significant increase in total sugars content, 3.29 ± 0.1 mg g − 1 and 2.92 ± 0.2 mg g − 1 , respectively, with respect to the control (2.31 ± 0.3 mg g − 1 ). This study successfully confirms the biological significance of compounds with low molecular weights, including peptides, sugars and other metabolites, in mediating plant responses to protein hydrolysates.