Foliar applications of a vegetal-derived protein hydrolysate alone or in combination with calcium alleviate heat stress in lettuce by boosting leaf antioxidant activity.

Introduction: The increase in temperatures caused by climate change represents a major abiotic stress factor negatively affecting yield and quality traits of leafy vegetable crops. The use of protein hydrolysate-based products has been recently proposed as a tool to mitigate the negative effects of heat stress on several crops. Methods: Multiple foliar applications of a vegetal derived protein hydrolysate alone (PH) or enriched with calcium (Ca-PH) were tested, together with an untreated control, on lettuce plants subjected to a heat stress event (43/30 °C day/night for 3 consecutive days). Morpho-physiological traits were monitored over the trial through a phenotyping platform and a Multi Pigment Meter along with destructive measurements of plant biomass and biochemical and mineral analysis of leaf tissues. Results: Both protein hydrolysate-based products mitigated heat stress by enhancing fresh weight and dry weight of lettuce shoots in comparison with control treatment. Moreover, both protein-hydrolysate-based products promoted plant uptake of key nutrients like Ca, P, and Mn and the antioxidant activity (FRAP and DPPH) in lettuce leaves. Ca-PH was more effective than PH in promoting a fast recovery of lettuce plants (Digital Biomass) after heat stress as a result of a pre-stress accumulation of proline. Conclusion: In conclusion, foliar applications of a vegetal-derived protein hydrolysate alone or in combination with Ca can be considered a sustainable approach to counteract the negative effects of heat stress induced by climate change in lettuce crops.