How to utilize far-red photons effectively: substitution or supplementation with photosynthetically active radiation? A case study of greenhouse lettuce.

Abstract

The addition of photosynthetically active radiation (PAR, 400-700 nm) with a specific quantity of far-red photons (FR, 700-750 nm) has been demonstrated to positively influence biomass accumulation and nutritional quality in greenhouse lettuce. However, current relevant studies seldom consider comprehensive and systematic comparisons of the efficacy of different approaches: substitution versus supplementation. The present work aimed to compare the two aforementioned strategies, evaluate how they impact plant growth, development and metabolic processes, and analyse the light use efficiency. In this study, loose-leaf lettuce (cv. 'Dasusheng') grown in a glass Venlo-type greenhouse was exposed to six supplementary light treatments, including white-red (WR) light-emitting diodes (LEDs), FR LEDs, and WR plus FR LEDs [WR130 + FR30 (the number was the photon flux density provided by WR or FR LEDs, respectively), WR130 + FR50, WR100 + FR30, and WR80 + FR50]. Lettuce that was grown only under natural light (NL) conditions was considered the control. According to the results of the present study, supplementary light increased biomass accumulation, and the contents of ascorbic acid, total soluble sugar, and starch relative to the control. Lettuce plants treated with WR130 + FR50 treatment presented the highest shoot and root fresh/dry weights, the highest total chlorophyll content, and the best nutritional quality, whereas the lettuce weight did not differ between the WR130 + FR30 and WR100 + FR30 treatments. Compared with that of NL, the stacking of thylakoids increased most intensely in response to the WR130 + FR50 and WR100 + FR30 treatments. Biomass accumulation, nutritional quality, stomatal area, chloroplast area, and expression of photosynthesis-related genes (LHCb, PsbA, rbcL, and rbcS) in lettuce plants, as well as light use efficiency, presented increasing-to-decreasing trends as the FR fraction increased. In conclusion, partially substituting PAR with FR photons coincidentally aligns with the supplementation of FR photons, and a supplementary FR fraction of 0.50 to 0.56 is suitable for greenhouse-grown lettuce under weak light conditions because of the increased photochemical efficiency, biomass accumulation, and carbohydrate content.