The combined effects of relative air humidity (RH) and UV radiation were tested on three cultivars of Euphorbia pulcherrima (Willd ex. Klotzch) at different ontogenetic stages in controlled environment growth chambers. In addition, the effects of UV radiation alone were tested on a fourth cultivar in a greenhouse compartment with natural background light. Growth chamber plants grown at 60 % or 90 % RH were either exposed to 0.15 W m-2 UV radiation for 40 minutes in the middle of the dark period (vegetative plants) or at the end of the light period (EOD, generative plants) or not exposed to UV.
Vegetative ‘Christmas Feelings’ poinsettia responded strongly to RH. High RH increased plant height, shoot length, the number of leaves per shoot, leaf area, plant diameter and leaf petiole length, while decreasing leaf thickness and internode length. The effects of UV were minor and UV exposure resulted in a decrease in the number of side shoots on the main shoot, a decrease in leaf area and a decrease in petiole length. Generative ‘Infinity Red’ and ‘Bravo Bright Red’ poinsettia did not show an obvious stronger morphological response to either RH (60 % and 90 %) or UV alone, and the interaction effects indicate that the RH at which the plants were grown dictates the magnitude and direction of the UV response. Intraspecific differences were found between the two cultivars and the less compact cultivar ‘Infinity Red’ showed a stronger response to UV compared to the compact cultivar ‘Bravo Bright Red’. The stomatal responses of ‘Infinity Red’ and ‘Bravo Bright Red’ indicate stronger effects of RH than UV on both leaf and bract conductance measured in both light and dark conditions. Stomatal aperture size of ‘Infinity Red’ plants was affected by UV in the light, which caused a significant increase in stomatal aperture, while under dark conditions the effect of RH was much stronger and resulted in larger stomatal apertures under 90 % RH than 60 % RH. Photosynthesis in these cultivars showed no effect of RH or UV.
Generative ‘Christmas Day’ poinsettia grown in a greenhouse compartment at constant RH (70 %) and either exposed to 7.5 minutes of 0.8 W m-2 UV EOD radiation or not exposed to UV indicated significant morphological responses to UV radiation, which resulted in a significant decrease in plant height, plant diameter, shoot length, internode length, leaf area and bract area, and also an increase in both leaf and bract thickness. No significant differences were found in time to flowering. Light response curves of ‘Christmas Day’ poinsettia indicate no effect of UV radiation on photosynthesis, though exposure to UV resulted in higher transpiration and stomatal conductance rates at all light intensities tested.
In summary, the responses to UV radiation in poinsettia were dependent on ontogenetic stage, cultivar, and background climate such as RH and light conditions
