Morphological and Physiological Responses of Cucumber Seedlings to Supplemental LED Light under Extremely Low Irradiance

In order to inhibit spindling growth and improve quality of cucumber seedlings under low irradiance, effects of supplemental light-emitting diodes (LED) light (SL) on morphological and physiological characteristics of cucumber seedlings at different growth stages under extremely low irradiance (ELI) were investigated. Supplementary monochromatic, dichromatic and trichromatic LED light on cucumber seedlings were conducted in experiment one, and supplements of combination ratios and intensity of blue and red LED light (RB) were conducted in experiment two. The morphological and physiological parameters of cucumber seedlings were promoted effectively by supplemental monochromatic red light or dichromatic containing red light (RB and RG) under ELI as early as one-leaf seedling stage, as demonstrated by suppressed length of hypocotyl and first internode, increased stem diameter and biomass, higher net photosynthetic rate (P_n) and soluble sugar content. Monochromatic or additional green light was not beneficial to cucumber seedlings under the ELI. The length of shoot and hypocotyl decreased, while stem diameter and leaf area increased as early as one-leaf seedling stage by RB SL. Root activities, root–shoot ratio, activities of catalase (CAT) and peroxidase (POD), as well as palisade–spongy ratio in the leaf of cucumber seedlings were promoted effectively by increasing blue light proportion (1R1B/1R2B). Increasing light intensity (50/75) enhanced soluble sugar accumulation in leaves. There were synergistic effects of RB ratio and light intensity on increasing stem diameter, leaf area, seedling index and decreasing hypocotyl cell area of the vertical section. In conclusion, 1R2B-75 may be the optimal SL to inhibit spindling growth of cucumber seedlings under ELI condition.