A Novel Cropping Method for Production of High Functioning Crops by Utilizing On-site Solar Energy

Abstract

The utilization efficiency of solar energy through photosynthesis by crops is only a few percent at most. Therefore, most solar energy in the field (on-site) remains unused. Plants are, however, able to use certain types of monochromatic light as an environmental signal, and they exhibit various physiological responses to even small amounts of the light. In particular, the significant induction of secondary metabolite production by blue light is well documented. Here, we propose a novel cropping method where unused solar energy is captured on-site by solar cells and converted to monochromatic light by using LEDs. The crops are then irradiated to stimulate enhanced production of secondary metabolites, thus improving crop function without the need for an external energy source. In this study, the effect of blue light on the production of functional secondary metabolites was evaluated in perilla (Perilla frutescens var. crispa f. purpurea), a traditional medicinal crop in Japan. Perilla leaves contain several functional substances such as perillaldehyde, rosmarinic acid, and anthocyanins, which have notable stomachic, antipyretic, detoxification, sedation, and diuretic efficacy. Plants were grown under natural daylight supplemented with blue light irradiation (460 nm, approx. 10μmol/m2/s, 24 h,) for 108 days before harvest. Control plants were grown without supplemental lighting. The content ratio of perillaldehyde in plants grown with blue light was 60% higher at harvest than in the controls. Moreover, in the vegetative growth stage, the perillaldehyde content increased significantly after only three days of blue light irradiation. These findings suggest that the present approach will benefit agricultural practices in developing countries through more efficient usage of on-site solar energy.