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 caus e 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 lo w light intensities, followed by periods of supplementary lighting of different duration and with different spectral composition. Daily wate r consumption showed genetic variation. Supplementary lighting, both w ith respect to its duration and its spectral composition, had hardly a ny influence on water usage or nocturnal stomatal conductance, but inc reased stomatal opening during the day. The diurnal rhythm of stomatal movement, measured with a computer-modulated high-intensity light sou rce, appeared to be determined by the integrated irradiance in the pre vious light period. High irradiance caused a faster stomatal opening d uring the next light period. Similar to this response, stomatal closin g rates were also stimulated by high light intensity during the previo us day. The observed effects were most pronounced for the cultivar rep orted to be the least sensitive to post-harvest damage by excessive tr anspiration. It is concluded that supplementary lighting does not have a negative impact on the water use efficiency of cut roses. In additi on, the stomatal dynamics suggest that flower quality would benefit fr om being transferred to darkness following a period of several hours o f high light intensity.