Growth and flower development in roses as affected by light

Growth and flowering of shoots of rose ‘Mercedes’ were investigated as a function of the level and spectral quality of the photosynthetically active radiation (PAR). 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 over a period of 4 to 6 weeks. In order to discriminate between the effects of irradiance level and light quality, plants were grown in growth chambers in which both the amount of PAR and its spectral composition could be controlled. At a photoperiod of 12 h the length, weight, and flowering of the shoots strongly increased with irradiance. Weight and number of flowering shoots were always higher for the uppermost than for the second shoot. At the highest PAR level (270 μmol m-2s-1) flowering occurred in 89 and 33 ␘f the uppermost and second shoots, respectively. At an irradiance level of 90 μmol m-2s-1 these percentages were 6 and 0&Eth;Although the length of both types of shoots was significantly increased by reducing the amount of blue light at constant PAR, 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