Cover materials excluding near infrared radiation: effect on greenhouse climate and plant processes

Only about half of the energy that enters a greenhouse as sun radiation is in the wavelength range that is useful for photosynthesis (PAR, Photosynthetically Active Radiation). Nearly all the remaining energy fraction is in the Near InfraRed range (NIR) and warms the greenhouse and crop and does contribute to transpiration, none of which is necessarily always desirable. Materials or additives for greenhouse covers that reflect or absorb a part of the NIR radiation have recently become commercially available. Besides lowering greenhouse temperature (which is the primary aim), a NIR-excluding cover has quite a few side-effects, that may become relevant in the passive or semi-passive greenhouses typical of high-energy climates. For instance, the ratio of assimilation to transpiration (the water use efficiency) should increase. By lowering the ventilation requirement, such a cover may hinder in-flow of carbon dioxide, thereby limiting the photosynthesis rate. A previous desk study has shown that in cooler climates such as The Netherlands, a permanent NIR filter may increase year-round energy requirement up to 10%. In this work we present the first results of a rose crop glasshouse experiment with an internal movable, NIR-reflecting screen. We analyse the effect on greenhouse temperature, carbon dioxide management and humidity and on crop transpiration. We then discuss the most appropriate application of a NIR-selective filter, in view of the prevailing climate and the available ambient management option