Light distribution and canopy structure in greenhouse muskmelon

Light distribution, canopy structure, and yield were studied in vertically grown greenhouse 'Galia' melon. Plants were at three densities and two levels of irrigation water salinity, topped to fixed numbers of leaves, and allowed one fruit. The objectives were to determine whether productivity is light limited, to characterize the light environment in the canopy, and to parameterize canopy structure. Light distribution outside and inside the greenhouse on a clear day, measured with the LAI2000, showed that direct beam radiation is attenuated by ∼95% by the greenhouse structure, and that part of the direct radiation is scattered into other angles. This results in a predominantly diffuse radiation environment in the greenhouse. Photosynthetic photon flux density (PPFD) fluxes above and below the canopy, measured with linear PPFD sensors, showed crop absorption to be 70-77%, or 35-45% of the outdoors global radiation. Vertical PPFD profiles at the central axis of mature rows showed that the extinction coefficient with respect to cumulative downward leaf area index is similar for the different treatments. Vertical distribution of leaf area, harvested at 20 cm intervals, fit a four parameter beta distribution. Saline irrigation was found to increase the vertical variance, but not other parameters. Final individual fruit weight for plants was directly related to absorption of radiation by the individual plants. A second experiment, in which plants at high and low density were compared with plants grown at high density but with half of the plants sprawled on the ground, indicated that root competition is less important than light competition with respect to final fruit weight. These results indicate that Israeli greenhouse melon production is light limited.