Abstract
Supplemental red light irradiation at the beginning of the dark period or blue light irradiation at the end of the dark period at a low level (0.6% of the irradiance of the light period) compared to no supplemental irradiation (control) promoted biomass production and was accompanied by an increase in the leaf area in spinach. Photosynthetic rate and stomatal conductance measured under white light in the leaves of plants grown under supplemental lighting during the dark period were the same as those of the control plants. Microscopic analysis of the transverse and longitudinal directions of the leaves revealed that red light irradiation at the beginning of the dark period increased leaf cell size, while leaf cell size was not affected by blue light irradiation at the end of the dark period. These results suggest that red light irradiation at the beginning of the dark period accelerates leaf expansion by an increase in leaf cell size and blue light irradiation at the end of the dark period accelerates leaf expansion by an increase in leaf cell number. The increase in leaf area by supplemental lighting leads to an increase in photosynthesis at the whole-plant level, which contributes to the enhancement of biomass production under suboptimal light irradiation.
