An Enlightened View on Protected Cultivation of Shade-Tolerant Pot-Plants: Benefits of Higher Light Levels

Commercial protected cultivation of shade-tolerant pot-plants in The Netherlands has expanded enormously in the last decade. Typically, very low daily light integrals are applied (3-5 mol PAR m-2 day-1), which are achieved by use of heavy screening and application of a layer of chalk (CaCO3) on the outside of the greenhouse to increase reflection of incoming irradiance. Although these low daily light integrals are meant to avoid damage by high light intensities and/or high temperatures, it is clear that they carry a production penalty, since potential crop growth is directly related to the amount of light that can be captured and efficiently used. Additionally, it remains unclear whether current daily light integrals are too conservative, which would create room for optimisation. Recently, a number of experiments have been carried out to examine the possibilities and limitations for cultivation of several shade-tolerant pot-plants at higher daily light integrals. For most species, plants could be grown faster when more light was allowed. Also, a significant reduction in energy use for heating could be achieved if more natural irradiance was allowed to enter the greenhouse. However, use of more light required higher levels of relative humidity to avoid light damage. In this paper, we present a synthesis of experiments, as well as an outlook to further improvements.