Continuous and pulsed LED applications on red and green lettuce (Lactuca sativa L. var. capitata) for pre- and post-harvest quality and energy cost assessments

Abstract

• 1 kHz pulsed frequency has been tested as an alternative to continuous LED for lettuce quality retention. • Effect of pulsed LED is cultivar dependent. • Remarkable reduction in energy consumption can be achieved through pulsed LED lighting. • Continuous LED has produced more secondary metabolites than pulsed LED in red lettuce. Light manipulation for plant cultivation is a critical area of study in controlled environment agriculture (CEA), where a variety of artificial lighting conditions are often employed in plant factories, growth chambers and greenhouses. With this aim of manipulation, two lettuce cultivars, green and red, were treated with two different LED treatments: a continuous LED in which an average photosynthetic photon flux density (PPFD) at the seedling level was maintained at 228 µmole s¯¹ m¯², a photoperiod of 16 h was set for a growing cycle of 30 days, and a dynamic mode pulsed LED treatment with a pulsed frequency set at 1 kHz with a duty cycle of 50 % along with the PPFD at 228 µmole s¯¹ m¯², a 16-hour photoperiod, and a growing cycle of 30 days. Sampling for quality assessment was done at harvesting (T0) and the effect of pre-harvest LED application was analyzed after 7 days of cold storage (T7). A significant reduction in the average fresh weight of both cultivars was seen under pulsed LED while a significant increase in leaf length was noticed among the treatments in red lettuce. Both treatments resulted in non-significant variations for photosynthetic pigments: total chlorophyll and carotenoids, while no significant differences were seen in terms of phenolic index and anthocyanin production in green lettuce. Red lettuce, however, yielded a significantly higher phenolic index for continuous LED at T0, which significantly declined at T7. In green lettuce, nitrate production underwent no significant differences under both treatments and time points; however, pulsed LED in red lettuce yielded significantly higher nitrate than continuous LED at T0. At both timepoints, no marked changes were seen in terms of total sugars in green lettuce, while a significant reduction in sugar was recorded under pulsed LED treatment. On the other hand, a significant decline in total sugars was noticed between the timepoints for red lettuce under continuous LED treatment, while no such variations were seen in red lettuce. Similarly, at T0 both green and red lettuce showed no remarkable increment or decline for sucrose, while it significantly declined between timepoints for red lettuce. Non-destructive analysis was carried out to investigate the health status of lettuce plants where the green lettuce under pulsed LED accumulated higher anthocyanins at both T0 and T7 and higher chlorophyll at T7 than red lettuce under the same LED application. Significantly higher anthocyanins were also seen at T7 between the two treatments in green lettuce. Non-significant differences, however, were found between the treatments and timepoints for both the photochemical maximum quantum efficiency of photosystem II (Fv/Fm ratio) and the overall performance index (PI) of leaves. This research demonstrated the significance of artificial light modification from continuous to pulsed LED to save energy costs and a step forward towards retaining the quality of the produce in this dynamic mode.

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