Growth of Rhododendron cultivars as affected by temperature and light

Summary

The purpose of this study was to investigate the adaptation of four Rhododendron cultivars to contrasting light and temperature conditions. Two evergreen rhododendron cultivars and two deciduous azaleas were grown for 112 d under short day (14 h) and long day (20 h) photoperiods combined with temperatures of 15 and 24°C. Additionally, these cultivars were compared for daylength extension at 24°C/long day under two irradiation treatments (incandescent lamps and fluorescent tubular lamps). The number of flushes of growth increased with increasing photoperiod and temperature in both evergreen cultivars and in R. canadense; azalea #89132 made only one flush in all treatments. In the evergreen cultivars the number of leaves per shoot in the first flush did not differ significantly between treatments, indicating that this character was predetermined by conditions during bud development. The number of leaves in later flushes increased with increasing photoperiod and temperature. The elongation growth of most flushes was also enhanced by longer photoperiod and higher temperature. High irradiation during photoperiodic extension further enhanced the growth. Azalea #89132 made more flower buds under high than low irradiation. The two evergreen cultivars differed in their growth habit. ‘Pohjola’s Daughter’ tended to continue growth in long days or at very high temperatures, and is thus predicted to thrive best in a maritime or semi-maritime cool climate. ‘Helsinki University’ responded to short daylength by ceasing growth regardless of temperature, and could be expected to perform successfully also in continental climates at latitudes around 45° N. R. canadense seemed to do best in a cool climate, but azalea #89132 should in time acclimatize in all kinds of climates within the limits of this study.