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 leve ls of irrigation water salinity, topped to fixed numbers of leaves, and all owed one fruit. The objectives were to determine whether productivity is li ght limited, to characterize the light environment in the canopy, and to pa rameterize canopy structure. Light distribution outside and inside the greenhouse on a clear day, measur ed with the LAI2000, showed that direct beam radiation is attenuated by sim ilar to 95% by the greenhouse structure, and that part of the direct radiat ion is scattered into other angles. This results in a predominantly diffuse radiation environment in the greenhouse. Photosynthetic photon flux densit y (PPFD) fluxes above and below the canopy, measured with linear PPFD senso rs, showed crop absorption to be 70-77%, or 35-45% of the outdoors global r adiation. Vertical PPFD profiles at the central axis of mature rows showed that the extinction coefficient with respect to cumulative downward leaf ar ea index is similar for the different treatments. Vertical distribution of leaf area, harvested at 20 cm intervals, fit a four parameter beta distribu tion. Saline irrigation was found to increase the vertical variance, but no t other parameters. Final individual fruit weight for plants was directly related to absorption of radiation by the individual plants. A second experiment, in which plant s at high and low density were compared with plants grown at high density b ut with half of the plants sprawled on the ground, indicated that root comp etition is less important than light competition with respect to final frui t weight. These results indicate that Israeli greenhouse melon production i s light limited.