Can extreme light diffusion still increase crop growth in greenhouses?

Research has largely proved the benefits of diffuse light above direct light for many horticultural crop species. The diffusiveness of a greenhouse cover can be characterized by the hortiscatter parameter (NEN2675) from 0 (no diffusiveness) to 1 (perfect Lambertian diffuser). However, the effect of hortiscatter larger than 60% on the crop was not studied yet. To investigate the effects of a very high hortiscatter on crop light interception, photosynthesis and growth, an experiment was performed by Wageningen UR Greenhouse Horticulture with a lenticular material which was able to generate extreme light diffusion (hortiscatter = 0.9) without light loss. Such a level of light diffusion was compared with medium diffusion (hortiscatter = 0.65) and no diffusion (hortiscatter ~0). Young tomato plants were placed on elevated growing tables and grown until the stage of first truss flowering, at a high density to simulate the high LAI values of a grown-up crop. The materials allowing for the differences in light diffusion were placed on metal frames above the plants. Each material was used on two tables. The resulting PAR sum was not equal for all tables due to position effects in the greenhouse as well as differences in light transmission of the covering materials used to achieve the hortiscatter levels. When both diffusion factor and PAR sum are considered in the statistical analysis, the plants grown under the very high diffusing material showed 25% more leaf area, 14% higher fresh and 12.5% higher dry weight than plants from the other two treatments. The photosynthesis as well as the light use efficiency, expressed as grams of dry matter produced per mol PAR, increased with increasing hortiscatter. In a follow-up a larger trial in separate compartments would need to verify these results in real growing conditions and evaluate the effects on fruit production during a longer experiment.