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.

中文翻译：

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红色和绿色生菜 （Lactuca sativa L. var. capitata） 的连续和脉冲 LED 应用，用于收获前和收获后的质量和能源成本评估


植物栽培的光操纵是受控环境农业 （CEA） 的一个关键研究领域，植物工厂、生长室和温室经常采用各种人工照明条件。为了这个操作的目的，用两种不同的 LED 处理了绿色和红色两个生菜品种：连续 LED，其中幼苗水平的平均光合光子通量密度 （PPFD） 保持在 228 μmole s ̄¹ m ̄²，光周期为 16 h，生长周期为 30 天， 以及动态模式脉冲 LED 处理，脉冲频率设置为 1 kHz，占空比为 50%，PPFD 为 228 μmole s ̄¹ m ̄²，光周期为 16 小时，生长周期为 30 天。在收获时 （T0） 进行质量评估采样，并在冷藏 7 天 （T7） 后分析收获前 LED 应用的效果。在脉冲 LED 下，两个品种的平均鲜重均显著降低，而红莴苣的处理中叶片长度显著增加。两种处理均导致光合色素：总叶绿素和类胡萝卜素无显著变化，而绿生菜的酚类指数和花青素产生方面没有显著差异。然而，红生菜在 T0 时连续 LED 的酚类指数显著升高，在 T7 时显著下降。在绿生菜中，硝酸盐的产生在处理和时间点下没有显著差异;然而，在 T0 时，红生菜中的脉冲 LED 产生的硝酸盐明显高于连续 LED。 在这两个时间点，青生菜中的总糖含量均未发生显著变化，而在脉冲 LED 处理下，糖分显著降低。另一方面，在连续 LED 处理下，红生菜的时间点之间观察到总糖分显着下降，而红生菜中没有观察到这种变化。同样，在 T0 时，绿色和红色生菜的蔗糖没有明显的增加或减少，而红色生菜的时间点之间显着下降。进行无损分析以调查生菜植株的健康状况，在脉冲 LED 下，绿色生菜在脉冲 LED 下在 T0 和 T7 处积累的花色苷和在 T7 处的叶绿素都高于红生菜。在两种处理之间，绿莴苣在 T7 处也观察到花青素显着升高。然而，在处理和时间点之间，光系统 II 的光化学最大量子效率 （Fv/Fm 比率） 和叶片的整体性能指数 （PI） 均存在不显著差异。这项研究证明了从连续 LED 到脉冲 LED 的人造光改性对于节省能源成本的重要性，以及在这种动态模式下保持产品质量的一步。