LIGHT INTERCEPTION AND PHOTOSYNTHETIC EFFICIENCY IN SOME GLASSHOUSE CROPS

Productivity of glasshouse crops is strongly limited by light receipt, and efficient interception and use of light in photosynthesis is correspondingly important. Mature row crop canopies of cucumber and tomato intercepted about 76% of the light incident on their upper surfaces; about 18% was lost through gaps between the rows. Light transmitted through the entire depth of the canopy was reflected back by white plastic on the ground, so that the lower surface of the canopy received approximately 13% of the light incident on the upper surface. The light flux incident on the sides of these canopies (c. 2 m tall and 6 m x 16 m in area) amounted to some 20-30% of that incident on the upper surface. About 32% of daylight falling on the glasshouse (c. 9 m x 18 m in area) was intercepted by the glasshouse structure and glazing; of the 68% entering the house, some fell on headlands occupying 35% of the glasshouse area. The loss of light to headlands, and the gain from canopy side-lighting, would be relatively smaller for larger glasshouses. At near-ambient CO2 concentrations, net photosynthetic rates of the cucumber canopy were comparable to those of closed canopies of other glasshouse and field crops which have maximum light conversion efficiencies of 5-8-mu-g CO2 J-1 at 50-200 W m-2 incident light flux density. Efficiency decreases only slightly with stronger light. Glasshouse crops with CO2 enrichment to 1200 vpm achieve conversion efficiencies of 7-10-mu-g CO2 J-1. Efficiencies of utilization of intercepted light, on an energy basis, reach 6-10% in various field and glasshouse crops with near-ambient CO2, and reached an exceptional 11% for the cucumber canopy. Glasshouse crops with CO2 enrichment achieve maximum efficiency of light energy utilization between 12% and 13%.