End-of-production light treatments as a tool for controlling chemical composition of herbs and lettuce

Growing plants in indoor environments (“Vertical farms”/”Plant factories”) provides good opportunities of controlling all climatic factors, including light intensity, daily light integral, and light quality. Thus, it is possible to control the quality of the produce to a great extent. The present study examines the possibilities to use “end-of-production” treatments with narrow band light to control the quality of leafy vegetables. Two different experiments were performed, comprising different types of leafy vegetables which were grown in sole multi-wavelength LED light, or in a greenhouse with high-pressure sodium light, for the first weeks of the production cycle. At the end of the production cycle, plants were subjected to narrow-band light at different peak wavelengths for the last four days before harvest. Four different narrow-band light treatments (peak wavelengths 456, 520, 596, and 663 nm) were included in the study. The leafy vegetables were analysed with respect to their content of chlorophylls, carotenoids, and total phenolic content, in addition to biometric data (fresh- and dry weight). The results indicated the potential of end-of-production treatments to modify the concentration of secondary metabolites in leafy vegetables, with especially blue and yellow light treatments having the potential of increasing the concentration of phenolic compounds and lutein/ϐ-carotene. However, for some of the treatments and cultivars, fresh- and/or dry weight was significantly reduced by the end-of-production treatment.