A Graminaceae-derived protein hydrolysate and its fractions provide differential growth and modulate qualitative traits of lettuce grown under non-saline and mild salinity conditions

The modulation of plant secondary metabolism by the manipulation of nutrient solution (NS) electrical conductivity and biostimulant application has the potential to increase crop growth and the content of bioactive compounds, thus ameliorating the qualitative attributes of vegetables. A Graminaceae-derived protein hydrolysate (pH) and its molecular fractions PH1 (>10 kDa), PH2 (1 < x <10 kDa) and PH3 (<1 kDa) were applied on lettuce grown in a soilless system and irrigated with two levels of NaCl in the NS (0 and 30 mM). The different pH fractions provided distinct responses, with PH2 in saline conditions increasing lettuce fresh weight by 10.48%, and decreased Na, Cl, malate and citrate concentrations in the leaves by 37.24%, 15.45%,31.4% and 33.73% compared to control. Similar results were achieved by the fraction PH3. However, pH and its fractions PH2 and PH3 increased total flavonoids in salinity conditions, while pH and its fraction PH3 increased total phenolic acids in both saline and non-saline conditions. At the same time, all the 3 fractions of pH increased lutein in both conditions. Therefore, pH fractions should be casted, under tailored conditions, to achieve the desired growth and modulation of qualitative attributes. Nonetheless, omic approaches should be further recommended to support and clarify the mechanisms of these fractions.