High daily light integral at end of production improves lettuce nutritional quality

• End-of-Production lighting can be used to improve lettuce nutritional quality. • High daily light integral (DLI) increases carbohydrates and vitamin C content. • With same DLI, longer photoperiod and lower light intensity improves quality. End-of-Production (EoP) lighting, which involves adjusting light intensity or spectrum a few days before harvest, is particularly effective in vertical farms and greenhouses using artificial lighting. Increased EoP light intensity has been found to improve lettuce nutritional quality and to extend the shelf life, but the simultaneous changes in daily light integral (DLI) and cumulative light sum (CLS, cumulative light sum received during the EoP phase) complicate the understanding of these effects. This study aims to investigate the effects of different EoP light factors, including light intensity, photoperiod, DLI and CLS on the nutritional quality of lettuce ( Lactuca sativa L.), focusing on carbohydrates and total ascorbic acid (Vitamin C, TAsA) levels. We applied six EoP light treatments with varying light intensities (200, 240, 300, 360 and 450 μmol m -2 s -1 ) and photoperiods (16, 20 and 24 h d -1 ) over six days. DLI was either 17 or 26 mol m -2 d -1 . High DLI in EoP lighting increased carbohydrates and TAsA levels in lettuce, regardless of whether the high DLI was achieved through longer photoperiod or higher light intensity. At a given DLI, longer photoperiod with lower light intensity resulted in higher carbohydrates and TAsA levels compared to shorter photoperiod with higher light intensity. Prolonging EoP lighting period (e.g. from 2 to 6 days) with low DLI did not affect nutritional quality despite higher CLS. Our findings show that the most effective way to enhance lettuce nutritional quality is through EoP lighting with higher DLI created by a longer photoperiod.