Responses of butter leaf lettuce to mixed red and blue light with extended light/dark cycle period

To investigate the effects of extended light/dark (L/D) cycle periods (relative to the diurnal L/D cycle) on lettuce and explore the potential advantages of abnormal L/D cycles, butter leaf lettuce was grown in a plant factory with artificial light (PFAL) and exposed to mixed red (R) and blue (B) LED light with different L/D cycles that were respectively 16 h light/8 h dark (L16/D8, as control), L24/D12, L48/D24, L96/D48, and L120/D60. The results showed that all the abnormal L/D cycles increased shoot dry weight (DW) of lettuce (by 34–83%) compared with the control, and lettuce DW increased with the L/D cycle period prolonged.

The contents of soluble sugar and crude fiber in lettuce showed an overall upward trend with the length of the L/D cycle extended, and the highest vitamin C content, as well as low nitrate content, were both detected in lettuce treated with L120/D60. The light use efficiency (LUE) and electric use efficiency (EUE) of lettuce reached the maximum (respectively 5.37% and 1.76%) under L120/D60 treatment, and so were DW, Assimilation rate (A), RC/CS, ABS/CS, TRo/CS, and DIo/CS, indicating that a longer L/D cycle period was beneficial for the assimilation efficiency and dry matter accumulation in lettuce leaves. The highest shoot fresh weight (FW) and nitrate content detected in lettuce subjected to L24/D12 may be related to the vigorous growth of roots. Specific L/D cycles seemed to strengthen root growth and water absorption of lettuce.

The openness level of RC in PSII (Ψo), ETo/CS, and PIabs were all the highest in lettuce treated with L24/D12, implying that slightly extending the L/D cycle period might promote the energy flowing to the final electron transfer chain. In general, irradiation modes with an extended L/D cycle period had the potential to improve energy use efficiency and biomass of lettuce in PFAL. No obvious stress or injury was detected in lettuce subjected to prolonged L/D cycles in terms of plant growth and production. From the perspective of shoot FW, the optimal treatment in this study was L24/D12, while L120/D60 was the recommended treatment regarding energy use efficiency and nutritional quality.

Chen, Xl., Li, Yl., Wang, Lc. et al. Responses of butter leaf lettuce to mixed red and blue light with extended light/dark cycle period. Sci Rep 12, 6924 (2022).  

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