EFFECTS OF LIGHT ON GROWTH AND FLOWERING OF ROSA-HYBRIDA MERCEDES

Growth and flowering of shoots of 'Mercedes' rose was investigated as a function of the level and spectral quality of photosynthetic photon flux (PPP). Experiments were performed with single-shoot plants decapi tated above the two most basal leaves with five leaflets. The developm ent of the two lateral shoots emerging from the axillary buds of these leaves was studied for 4 to 6 weeks. To discriminate between the effe cts of irradiance and light quality, plants were grown in growth chamb ers in which PPF and its spectral composition could be controlled. At a photoperiod of 12 hours, the length, weight, and flowering of the sh oots strongly increased with irradiance. The growth and number of flow ering shoots were always higher for the uppermost than for the second shoot. At the highest PPF (270 mu mol . m(-2). s(-1)), flowering occur red in 89% and 33% of the uppermost and second shoots, respectively. A t an irradiance level of 90 mu mol . m(-2). s(-1), these percentages w ere 6% and 0%. Although length and dry weight of both types of shoots were significantly increased by reducing the amount of blue light at c onstant PPF, flower development was not affected. In a second experime nt, plants grown in while tight (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 mor e flowering shoots than far-red. The red far-red reversibility of this flowering response indicates the involvement of the photoreceptor phy tochrome.