Effects of light intensity and amount of supplemental LED lighting on photosynthesis and fruit growth of tomato plants under artificial conditions

Abstract

We assessed the effects of light intensity (photosynthetic photon flux [PPF in μmol·m^-2·s^-1]) and number of irradiated leaves on photosynthesis and the fruit growth of individual tomato plants to develop supplemental LED lighting techniques for greenhouse tomato production. In Experiment (Exp.) 1, three PPF levels (P200, P500, and P1000) were applied to a post-anthesis tomato plant for three weeks, each plant pruned to have one leaf and one truss with three flowers. The fruit set and leaf and fruit dry-weight increased with increasing PPF; however, after P500 and P1000 treatments, the leaves showed signs of stress and accompanying disorders. Thus, to increase the fruit set ratio and growth rate of tomato fruits and plants, the total amount of irradiation received by each plant should be increased by increasing the number of irradiated leaves, rather than raising the PPF per leaf. For prolonged cultivation, P200 was the optimal PPF per leaf under the tested treatments. Exp. 2 used standard tomato plants with no leaves or trusses removed. We used an assimilation chamber to examine the effect of the number of leaves receiving P200 irradiation on the photosynthetic rate (Pn) per plant (above ground part). The Pn per plant in treatments where one and two leaves were irradiated by supplemental LED lighting were, respectively, 12 and 28% higher than that in the control (only top lighting). Therefore, fruit growth and yields in tomato cultivation may be raised via acceleration of photosynthesis by increasing the number of leaves that receive P200 irradiation rather than by increasing PPF.