Light and nitrogen are the main factors commanding horticulture production. Therefore, this study aims at evaluating the effects of two different greenhouse cover films (clear-Film A and diffuse light-Film B) on yield and nutritive value of a ready-to-eat salad lamb’s lettuce (Valerianella locusta L.), grown under several nitrogen regimes (no nitrogen, sub-optimal and optimal N fertilization corresponding to 0, 25 and 50 kg ha-1, respectively). The combination N50 and Film B boosted SPAD index by 10.3% and yield by 91.9% compared to N0 × Film A. Concerning antioxidant activity and bioactive compounds, only total ascorbic acid was positively affected by Film B (9.4%), while there was no effect of this cover film on carotenoids and chlorophyllous pigments, which increased along the increment of nitrogen rates. Nitrate content in leaves was influenced by both factors and it showed increasing values, when nitrogen doses increased, reaching the highest value under N50-Film B conditions (3312.3 mg kg-1 fw). Our study showed that in the Mediterranean area, the use of greenhouse diffuse light film improved lamb’s lettuce yield, but on the other hand incurred significant increase in nitrate level, nonetheless remaining constantly under the legal threshold imposed by the commission regulation (EU). Secondary metabolites, such as total phenols and carotenoids, showed similar values under both films, instead the diffuse light film improved total ascorbic acid content.
Keywords: nitrate, antioxidant activity, color parameters, SPAD index, ascorbic acid, total phenols