Effects of Plant-Derived Protein Hydrolysates on Yield, Quality, and Nitrogen Use Efficiency of Greenhouse Grown Lettuce and Tomato

Plant-derived protein hydrolysates (PHs) are gaining global interest for their sustainability and positive effects on crops under abiotic stresses. However, the long-term effects of PHs on the yield and quality of greenhouse crops have not been described. Romaine lettuce (Lactuca sativa L.) and Micro-Tom tomato (Solanum lycopersicum L.) were grown with commercial growing media in 1 L pots and fertigated with four N levels (2, 5, 10, and 15 mM). PH (0 or 3 g/L) was applied as a foliar spray (PH-F) or root drench (PH-R) once every week. Compared to PH-F, PH-R effectively stimulated the yield, photosynthesis, water-use efficiency, chlorophyll contents, and antioxidant activities, and compounds regardless of N levels and species. Increasing the N level led to a total dry weight gain, and PH-R enhanced the lettuce shoot dry weight (+31%) and tomato fruit dry weight (+22%). PH-R also increased the fresh marketable yield of lettuce (+21%) and tomato (+32%). The increasing N level decreased antioxidant parameters in lettuce leaves, not in tomato fruits, whereas PH-R improved them in both species. PH-R significantly enhanced the N use and uptake efficiency. Taken together, our results suggested that the addition of PH-R effectively increased the N uptake and subsequently increased the lettuce and tomato yield and quality regardless of N levels