EFFECTS OF DIFFERENT LIGHT TREATMENTS ON THE NOCTURNAL TRANSPIRATION AND DYNAMICS OF STOMATAL CLOSURE OF 2 ROSE CULTIVARS

In the cultivation of roses, supplementary lighting is thought to cause an increase in nocturnal transpiration and consequent low turgidity of the cut flowers, leading to problems in quality. Two cultivars with diverging tolerances to this phenomenon were subjected to high and low light intensities, followed by periods of supplementary lighting of different duration and with different spectral composition. Daily water consumption showed genetic variation. Supplementary lighting, both with respect to its duration and its spectral composition, had hardly any influence on water usage or nocturnal stomatal conductance, but increased stomatal opening during the day. The diurnal rhythm of stomatal movement, measured with a computer-modulated high-intensity light source, appeared to be determined by the integrated irradiance in the previous light period. High irradiance caused a faster stomatal opening during the next light period. Similar to this response, stomatal closing rates were also stimulated by high light intensity during the previous day. The observed effects were most pronounced for the cultivar reported to be the least sensitive to post-harvest damage by excessive transpiration. It is concluded that supplementary lighting does not have a negative impact on the water use efficiency of cut roses. In addition, the stomatal dynamics suggest that flower quality would benefit from being transferred to darkness following a period of several hours of high light intensity.