255 Effect of Diffuse Glass on Climate and Plant Environment: First Results from an Experiment on Roses F.L.K. Kempkes, C. Stanghellini, N. García Victoria and M. Bruins Wageningen UR Greenhouse Horticulture Wageningen The Netherlands Keywords: covering material, climate control, greenhouse design, diffuse glass Abstract High energy use in rose cultivation at higher latitudes is caused by the need for artificial light to supplement scarce sun radiation. On the other hand, since too high radiation levels are known to reduce flower quality, shading is widely applied during spring and summer, either through movable screens or seasonal whitewash. In both cases damage to the crop is avoided at the cost of reducing potential assimilation by reduction of (PAR) light. Recent research on cucumber has shown that diffusing cover materials have the potential of improving the uniformity of vertical light distribution in a crop, therefore decreasing the energy load on the uppermost crop layer to the advantage of underlying crop layers. Light diffusion, however, usually implies a loss of overall transmission. This drawback can be avoided by antireflection coatings, so that most recently diffusive glass covers have become available with the same transmissivity of standard glass. The application of such a cover on roses could decrease the need for shading so that a desired radiation sum could be achieved with less need for artificial light. Such an experiment is in progress on a crop of roses (cv Red Naomi) at the research station of Wageningen UR Greenhouse Horticulture in Bleiswijk, NL, in two compartments. One is covered with diffuse glass with anti-reflection coating with a light transmission of 93% perpendicular and 83% hemispherical and haze factor of 72%. The reference compartment is covered with standard glass. This paper describes the effect of a diffusing cover on the greenhouse climate (air- and plant-temperature, humidity and ventilation requirement) and water balance compared to a greenhouse covered with standard glass. It’s showed that the effect on greenhouse climate is limited but flower temperature is reduced in the diffuse compartment which has effect on flower quality. It seems therefore that the material has potential for reducing leaf burning and if the use of shade screens or whitewash can be reduced there is a potential increase of yield with the same energy input. INTRODUCTION Light is not evenly distributed in greenhouses. In particular, tall crops such as cucumber, sweet pepper and tomato have a high leaf area index and intercept a large quantity of light with the upper leaves, while the middle and lower leaves receive much less light and contribute very little to photosynthesis, growth, and in the end, production. As the uppermost leaves may often be light-saturated, it can be argued that a more uniform light distribution would result in higher overall assimilation (Ellsworth and Reich, 1993). At least, if the lowermost leaves have enough photosynthetic capacity to take advantage of the additional light. This was proven by Hovi et al. (2004) who showed that a higher amount of artificial light within a crop−achieved by inter- lighting−significantly increased photosynthesis of the lower leaves of cucumber. Uniformity of light distribution can be realized by diffuse light. From earlier investigations in forests (Farquhar and Roderick, 2003; Gu et al., 2003), apple trees (Lakso and Mussleman, 1976) and grass canopies (Sheehy and Chapas, 1976) it is known that diffuse light is able to penetrate deeper into a plant canopy in comparison to direct light and that photosynthesis in forests is increased by diffuse light. There are also indications that plants have developed mechanisms to use diffuse light more efficiently Proc. IS on GreenSys2011 Eds.: C. Kittas et al. Acta Hort. 952, ISHS 2012