BOUGAINVILLEA INFLORESCENCE MERISTEM DEVELOPMENT: COMPARATIVE ACTION OF GA3 IN VIVO AND IN VITRO

Although gibberellic acid (GA3) promotes stem elongation and inhibits flowering in Bougainvillea, no inhibition of floret initiation was observed when excised meristems were cultured in vitro in media containing GA. These results are predicted by a nutrient diversion hypothesis in which GA-induced inhibition of flower initiation in intact plants is considered an indirect result of growth promotion of competing vegetative sinks with consequent diversion of nutrients away from the inflorescence meristem. The failure to observe GA-induced inhibition of floret initiation in vitro was probably the result of an excess ofmetabolites forgrowth and differentiation of the excised meristems in the media. GA INHIBITS FLOWERING in Fuchsia, Euphorbia (Sachs and Hackett, 1969), Citrus (Guardiola, Monerri, and Agusti, 1982) and many deciduous fruit tree species (Kinet, Sachs, and Bernier, 1985). GA3 applied to the shoot tip of Bougainvillea 'San Diego Red' inhibits flowering and increases shoot elongation in SD (Hackett and Sachs, 1967). Sachs and Hackett (1983) suggested that this GA-induced inhibition of flowering is the indirect result of GAinduced promotion of competing sinks, specifically the rib meristems active in stem and leaf elongation. Nutrients could be diverted away from the inflorescence meristem to such vegetative sinks, and thereby flowering would be inhibited. If this is the case, GA3 should not inhibit development of isolated inflorescence meristems but only development of inflorescences on the intact plant. In the present paper a more detailed study of GA3-induced inhibition of Bougainvillea inflorescence meristem development in vivo was made. The response of the isolated inflorescence meristems to GA3 in vitro was determined, using the system described in a previous paper (Steffen, Sachs, and Hackett, 1988), and compared to that of intact plants. MATERIALS AND METHODS-Development of inflorescence meristems in vivo Plants ofBougainvillea 'San Diego Red' were grown from rooted cuttings in the greenhouse (27 C day, 21 C night) under LD conditions (natural phoI Received for publication 23 February 1987; revision accepted 3 December 1987. 2 Current address: Department of Horticultural Science and Landscape Architecture, University of Minnesota, St. Paul, MN 55108. toperiod extended with incandescent lamps to give 16 hr) to insure that plants were vegetative. All axillary buds were removed except one to ensure the development of a single stemmed plant. When the shoots had elongated and had about 10 fully expanded leaves (2 to 3 months from the beginning of the rooting period) uniform plants were placed in a growth chamber in continuous light (600 E m-2 sec-I to induce flowering (Sachs and Hackett, 1969). When the oldest inflorescences on the plants had reached anthesis, 10 Al of 150 mg 1-1 GA3 (aqueous solution containing approximately 0. 1% (v/v) Tween 20) was applied to the shoot tips. Samples were taken from GA-treated and control plants for the determination ofthe stage of inflorescence meristem development (Steffen et al., 1988) at 1, 2, 3, 4, 6, and 17 days after the application of GA3. Three to 6 plants per treatment were examined on the days indicated. The mean stage of development was plotted as a function of node number, with node one being the first node below the terminal meristem. Development of inflorescence meristems in vitro-Plants were grown as above in LD, and inflorescence meristems (stage 5-6, prefloret) were excised and placed in culture as previously described (Steffen et al., 1988). For these experiments a medium containing 3% fructose was used to induce floret formation (Steffen et al., 1988) and either 0, 0.01, 0.1, 0.5, or 1.0 mg 1-1 GA3 was added. All media were filter sterilized. Thirty meristems per treatment were cultured on a shaker under continuous fluorescent lighting (20 uE m-2 sec-'). At 2 wk after the start of culture the meristems were dissected and their stage of development de-