UV radiation as a tool to control growth, morphology and transpiration of poinsettia (Euphorbia pulcherrima) in variable aerial environments

Greenhouse production of poinsettia calls for strict control of morphological parameters, which may be achieved through the use of chemical growth retardants. Use of such chemicals is becoming restricted thus alternative methods for growth control are needed. Here the effects of UV radiation were tested on Euphorbia pulcherrima (Willd ex. Klotzch) in controlled environment under moderate (60%) and high (90%) relative air humidity (RH), to determine the potential to control plant morphology. Vegetative plants ('Christmas Feelings') received UV during the dark period, while two generative cultivars, one strong growing phenotype 'Infinity Red' ('IR') and one more compact phenotype 'Bravo Bright Red' ('BBR'), received UV at the end of the light period (EOD). The morphology of vegetative plants was mainly affected by RH rather than UV radiation. Generative plants were also strongly affected by RH, though both cultivars showed reduced plant diameter, shoot biomass, leaf area, and bract area when exposed to UV, as well as increased leaf chlorophyll content, though responses to UV were stronger in moderate RH compared to high RH. Transpiration of leaves and bracts was mainly affected by RH not UV, and photosynthesis and production time were not affected by either RH or UV. We conclude that UV radiation is a potential tool to grow more compact plants, though its effects are partially determined by the aerial environment.