Effect of seaweed (Ecklonia maxima) extract and legume-derived protein hydrolysate biostimulants on baby leaf lettuce grown on optimal doses of nitrogen under greenhouse conditions

In recent years, the demand for green leafy vegetables is increasing. In order to satisfy this trend, the leafy crops are cultivated under high energetic inputs, especially high doses of nitrogen (N) fertilization that leads to a nitrate accumulation in leaves, sometimes overcoming the legal threshold set by the European Community for their commercialization. The nitrate in leaves can be dangerous for human health because in the human body it can be converted into nitrite, which can cause methemoglobinemia or create cancer-causing compounds. In order to overcome this problem, a correct N management is needed, especially using technical means which can improve the nitrogen use efficiency. In this study, we evaluated the possible effect of two important plant biostimulants on yield and quality traits (nitrate, antioxidants activity, carotenoids) of baby leafy lettuce, grown in a greenhouse with three levels of nitrogen input. Nitrogen doses were 0, 10 (sub-optimal) and 20 (optimal) kg ha−1, N0, N10 and N20 respectively. The biostimulants were Ecklonia maxima seaweed extract (3 ml per liter) (named Bio 1), legume-derived protein hydrolysate (Bio 2) and non-treated control (Control). The treatments were distributed in a randomized complete-block design with three replications (3 N levels × 3 Biostimulant applications × 3 replications). Biostimulant applications of seaweed extracts and legume-derived protein hydrolysate improved yield and LAI: 13.4% and 12.0% increase over non treated plants, respectively. The highest yield was reached at 20 kg N ha−1. Application of foliar biostimulants stimulated the antioxidant systems of plants, improved leaves color and increased chlorophyll and carotenoids content. The nitrate concentration in leaves was increased under higher levels of N fertilization, meeting the EC legal limit at N20 treatment in plants sprayed by E. maxima seaweed extract. Therefore, in our growth conditions, it seems possible to reduce nitrogen input at 10 kg N ha−1, by applying additional applications of biostimulants to reduce the yield gap upon application of N20 treatment.