Shading and nitrogen management affect quality, safety and yield of greenhouse-grown leaf lettuce

• 0.9% of PAR reduction and 185 kg N ha−1 maximizes lettuce biomass yield.
• These N and PAR supply levels also correspond to the highest carotenoids accumulations.
• PAR reduction negatively influenced lettuce growth and polyphenols accumulation.
• PAR reduction stimulated minerals, NDF and ADF concentrations in lettuce leaves.
• Phytochemicals accumulation in leaves can be manipulated through N fertilization.

Two growing factors (light and nitrogen) supplied at optimal and sub-optimal levels were studied with the aim of evaluating their effects on greenhouse-grown lettuce’ biomass and quality performances. According to a split-plot design with three replications, greenhouse lettuce was subjected to sixteen experimental treatments consisting of four photosynthetically active radiation (PAR) availability levels (0, 50, 65 and 85% PAR reduction) and four nitrogen fertilization rates (0, 75, 150 and 300 kg N ha−1).Response surface methodology (RSM) allowed to predict the highest achievable dry biomass (10.54 g plant−1) at 0.9% PAR reduction and 185.4 kg N ha−1. This optimal light/N combination induced a nitrate concentration of 1176 and 1826 mg kg−1 fresh weight (FW) in the inner and outer leaves, respectively. Shading decreased both the total phenolic content (TPC) and antioxidant activity. High N rates lowered both TPC in fully light condition and antioxidant activity in shading environment. The highest chlorophyll (Chl) concentrations were obtained with the combination shading/high N availability (at 85% PAR reduction and 300 kg N ha−1) with values of 1.938 and 1.716 mg g−1 FW for Chla and Chlb, respectively.In general, the results highlighted the potential for sustainable lettuce production, considering both economic and nutritional yields, i.e. providing high nutritionally dense products, slightly affecting the harvested biomass.