Effects of constant versus fluctuating red–blue LED radiation on yield and quality of hydroponic purple-leaf lettuce

This study investigated the effects of constant light and alternating relatively high-intensity (500 µmol m−2 s−1) and low-intensity (150 µmol m−2 s−1) red–blue LEDs (4R:1B) on the biomass production and quality of hydroponic purple-leaf lettuce (Lactuca sativa L. cv. ‘Zishan’) in an environmentally controlled plant factory. Four treatments were set up to separate 1 h of high light into four different alternating frequencies in a 24-h light−dark cycle (16/8 h): one time (A1), three times (A3), six times (A6), and twelve times (A12). In addition, one constant light treatment with the same daily light integral (DLI, 9.8 mol m−2 per day) as other treatments was set as the control (CK, 170 µmol m−2 s−1). The results indicated that A6 significantly reduced shoot fresh weight and increased the root–shoot ratio of lettuce compared with CK, but there was no significant difference among other treatments. Alternating light treatments did not promote the accumulation of soluble sugar, soluble protein, and phenolic substances compared with CK. Meanwhile, A12 significantly promoted the accumulation of total ascorbate (TA) in lettuce leaves compared with other treatments but decreased ascorbate/TA ratio. Above all, under the same DLI condition, alternating high and low light did not have obvious positive effects on biomass production and the accumulation of nutrient substance in lettuce under constant light was better than that under alternating light. Therefore, compared with the fluctuating radiation with the same DLI, constant radiation is a better choice for lettuce production.