PHYSIOL. PLANT. SS: 136-142. Copenhagen 1982 The inhibitory effect of gibberellic acid on flowering in Citrus I. L. Guardiola, C. Monerri and M. Agusti Guardiola, J. L.. Monerri, C. and Agusti, M. 1982. The inhibitory effect of gibberellic acid on flowering in Citrus. - Physiol, Plant, 55: 136-142. The application of gibbereilic acid (GA3) at any time from early November until bud sprouting, resulted in a significant inhibition of flowering in the sweet orange [C. sinensis (L.) Osbeck] and the Satsuma (C. unshiu Marc.) and Clementine (C re- ticulata Blanco) mandarins. Two response peaks were evident: the first occurred when the application was timed to the translocation of an unknown flowering signal from the leaves to the buds. The second occurred during bud sprouting, at the time the flower primordia were differentiating. From the pattern of flowering, it appears that the mechanism of inhibition was similar irrespective of the timing of GA3 appli- cation. There was an initial reduction in bud sprouting affecting selectively those buds originating leafless inflorescences. An additional inhibition resulted in a reduction in the number of leafy inflorescences with an increase in the number of vegetative shoots, suggesting the reversion of a floral to a vegetative apex. The inhibited buds sprouted readily in vitro but invariably vegetative shoots were formed, A continuous influence of the sustaining branch is necessary to keep the flowering commitment of the buds; irreversible commitment occurs when the petal primordia are well dif- ferentiated. Additional key words — Bud dornnancy, bud sprouting. Citrus reticulata, Citrus sinen- sis. Citrus unshiu. J. L. Guardiola et ai., Catedra de Fisiologia Vegetal, Escuela Tecnica Superior de Ingenieros Agronomos. Univ. Politecnica, Valencia, Spain. Introduction As in many fruit trees, flowering in Citrus is inhibited by the application of exogenous GA3. Whilst a practical use of this property is being made to regulate flowering in sweet orange (Moss and Bevingtoo 1977), the mechanism of inhibition is not fully understood. An in- terference with flower initiation has been suggested (Monselise and Halevy 1964, Moss 1970), since GA3 is most effective during the winter rest period, at the time when some flowering factor is translocated from the leaves to the buds (Furr and Armstrong 1956, Ayaion and Monselise 1960, Sanchez-Capuchino and Casanova 1973), or during a period of w,ater stress, which readily induces flowering in lemon (Nir et al. 1972). However, later applications of GA3, timed to bud sprouting when flower differentiation is detectable under the micros- cope, are also effective (Nir et al. 1972, Goldschmidt and Monselise 1972, Guardiola et al. 1980). This re- sponse may involve the reversion of a flower bud to a vegetative apex, and the reduction in flowering achieved is related to the increase in shoot length. In contrast, the earlier GA3 applications do not influence vegetative growth. Thus, there is a possibility that GA3 may inhibit flow- ering by acting at two different stages of flower forma- tion. This has been tested in the present study by com- paring the response of three Citrus species to GA3 ap- plications from early November to bud sprouting. Spe- cial attention was paid to the anatomical changes occurring in the buds and to their behaviour when ex- cised from the tree in order to gain more insight into the mechanism of action of GA3 at the bud level. Abbreviations — 2,4-D, 2,4-dichlorophenoxyacetic acid; GA3, gibberellic acid. Received 27 October, 1981; revised 16 February, 1982 136 0031-9317/82/060136-07 J03.00/0 © 1982 Physiologia Plantarum Physiol. Plant, 55, 1982