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Intracanopy lighting (IL) is a newly proposed supplemental lighting technique, irradiating lower leaves of plants from light sources placed between plant stands in high-wire grown vegetable production in a greenhouse. Light-emitting diodes (LEDs) are an appropriate light source for IL because of their compactness and low operating temperatures. To take advantage of features of LEDs, which emit light with a narrow band of wavelengths, we investigated short-term effects of spectra on instantaneous photosynthesis and long-term effects on a decrease in photosynthesis during the senescence of irradiated lower leaves of chamber-grown sweet pepper plants. First, the instantaneous photosynthetic rate of each plant’s fifth leaf was measured under white and monochromatic blue, green and red LED lights at a photosynthetic photon flux density (PPFD) of 150 μmol ms-. We confirmed that the rate was low under green light. Second, we grew plants for five weeks in five treatments: four treatments with the fifth leaves irradiated with white, blue, green and red LEDs at a PPFD of 150 μmol ms-, and one treatment without irradiation. After the treatments, photosynthetic rates of the irradiated leaves were measured to investigate the long-term effects. Measurements were performed at PPFDs of 300 and 1500 μmol mswith red and blue LEDs, irrespective of the treatments. The lighting treatments seemed to retard the decreases in photosynthetic rates irrespective of the spectra, but stomatal conductance (gs) of the leaves grown under green light tended to be low compared to those measured under the other colors. In addition to the lower instantaneous photosynthesis, green light might have less enhancing effects on the photosynthesis of the irradiated leaves from the viewpoint of long-term effects. Based on these results, we concluded that green LEDs are not appropriate as an IL light source from the viewpoint of single-leaf photosynthesis.

Effects of supplemental lighting to a lower leaf using light-emitting diodes with different spectra on the leaf photosynthetic rate in sweet pepper