PAR scattering by greenhouse covers and its effect on plant growth

The forward scattering properties of dry and condensate covered standard glass and ordinary polyethylene (PE) were determined by means of a laboratory measuring unit. This unit consisted mainly of a cold box / hot box system equipped with a radiation source and a goniophotometer. The experiments revealed that scattering by dry glass was negligible, in contrast to dry PE which scattered 47% of the transmitted radiation. When covered with condensate, 81 and 78% of the radiation transmitted by glass and PE, respectively, was scattered. To assess the scattering effects on plant growth, net photosynthesis rates were calculated for a tomato crop by means of a simplified model. Depending on the parameter values, condensate on glass enhanced the tomato plant photosynthesis rate by at most 13 to 27% compared with the dry state. This positive effect was partly counteracted by the lower radiation quantity available to the plants. The occurrence of condensate on ordinary PE, however, reduced the net photosynthesis rate by at least 3 to 6% due to considerably lower available radiation quantities. In all cases, higher plant growth rates were found under dry and wet ordinary PE than under dry glass