Effects of Light on Growth and Flowering of Rosa hybrids `Mercedes'

Growth and flowering of shoots of `Mercedes' rose was investigated as a function of the level and spectral quality of photosynthetic photon flux (PPF). Experiments were performed with single-shoot plants decapitated above the two most basal leaves with five leaflets. The development of the two lateral shoots emerging from the axillary buds of these leaves was studied for 4 to 6 weeks. To discriminate between the effects of irradiance and light quality, plants were grown in growth chambers in which PPF and its spectral composition could be controlled. At a photoperiod of 12 hours, the length, weight, and flowering of the shoots strongly increased with irradiance. The growth and number of flowering shoots were always higher for the uppermost than for the second shoot. At the highest PPF (270 μmol·m -2 ·s -1 ), flowering occurred in 89% and 33% of the uppermost and second shoots, respectively. At an irradiance level of 90 μmol·m -2 ·s -1 , these percentages were 6% and 0%. Although length and dry weight of both types of shoots were significantly increased by reducing the amount of blue light at constant PPF, flower development was not affected. In a second experiment, plants grown in white light (12 h/day) received a short treatment with low-intensity red or far-red `light at the end of each photoperiod. An end-of-day treatment with red light resulted in significantly more flowering shoots than far-red. The red far-red reversibility of this flowering response indicates the involvement of the photoreceptor phytochrome.