Altering light–dark cycle at pre-harvest stage regulated growth, nutritional quality, and photosynthetic pigment content of hydroponic lettuce

Lettuce exposed to red (R) and blue (B) LEDs (4R:1B, 200 μmol·m −2 ·s −1 ) were cultivated hydroponically in an environmentally controlled plant factory to examine the effects of light–dark cycle (LDC) conversion at pre-harvest stage on the growth, phytochemical, and photosynthetic pigment accumulation of lettuce. Lettuce plants of four groups were firstly cultivated with the same LDC of 16/8 h (light/dark) for fourteen days. Six days before harvest, the LDC of three groups were altered to 8/4 h (T12), 24/12 h (T36), and 32/16 h (T48), respectively. The last group had invariable photoperiod and was concerned as control (CK, 16/8 h). The results showed that T12 and T36 significantly improved while T48 reduced leaf area and shoot fresh and dry weight of lettuce compared with the CK. The plants treated with altered LDC had higher phytochemicals (soluble sugar, soluble protein, and ascorbate) and photosynthetic pigments (chlorophyll and carotenoids) contents than the CK, but the starch and nitrate contents of the CK were higher. These results suggested that the yield and nutritional quality of lettuce can be increased by changing the LDC properly in the later growth stage. But if the light–dark period was too long, it would have negative effects on the growth of lettuce. In addition, shorten the LDC at pre-harvest stage promoted better production yield and nutritional quality based on the same electricity consumption.