The effects of growth-regulating substances and environmental conditions on the composition of Begonia franconis
Liebm. inflorescences were analysed. The inflorescences are generally composed of two male flowers and one terminal female flower.
Auxins, gibberellins, and cytokinins, added to the branch apices, as well as low light intensity, promoted male flowering by increasing the number of male flowers. Removal of branches as well as application of cytokinins induced ramification of the inflorescences by outgrowth of the normally dormant axillary bud in the bract of male flowers. A transition of male flowers into female flowers was not observed.
A model of the sex-regulating mechanism in relation to hormones and environmental conditions is put forward. On the principle that female development requires a high nutritional level it is suggested that the level of assimilates is important in regulating sex expression, hormones influencing the sex of flower buds through their regulation of the flow of assimilates.
The development of flower buds of Begonia franconis
Liebm. was studied in vitro
. Inflorescences with two male and one female bud primordium were inoculated on chemically defined media to analyse the requirements for optimum growth.
Omission of agar increased growth of the buds, on a liquid medium the buds reached a normal and complete development. Growth required both nitrate and ammonium. A cytokinin was also necessary for bud growth, the optimum cytokinin concentration for the female bud being 10 to 30 times higher than that for the male buds.
IAA and ethephon had no effect on bud size, but ABA decreased growth if applied together with cytokinin. Although GA 3
had no effect, GA 4+7
promoted the length of the male perianth.
The regulation of sex expression in Begonia franconis
Liebm. was studied by analyzing the effects of growth-regulating substances on the initiation of floral organs in inflorescence primordia in vitro.
In the absence of growth-regulating substances or with IAA ABA, and Ethephon, no differentiation of flower buds occurred. With 10 -6
M BA all floral organs were initiated and the flower buds reached anthesis. Gibberellins promoted organ initiation in the buds. However, removal of the first bud of the inflorescence primordium strongly reduced organ initiation in the remaining buds by gibberellins. The growth-regulating substances did not change the sex of the buds as determined by their position in the inflorescence. However, female differentiation was inhibited at low sucrose levels that increased the number of male flowers.
It is suggested that sexual differentiation is regulated endogenously by the central region of the inflorescence primordium, the carbohydrate level being a limiting factor for female differentiation.
On stem segments of Begonia franconis
Liebm. in vitro,
only vegetative buds could be induced. On inflorescence pedicels, on the contrary, the formation of adventitious flower buds was abundant. Both cytokinin and auxin were required, the latter during the first 10 days. After the promotion of callus formation, the auxin became inhibitory to flower bud formation. Gibberellin strongly repressed flower bud development.
None of the growth regulators tested affected the ratio of male to female flower buds. Pistillate buds never differentiated directly on the callus, but originated at the bracts on the pedicels of the staminate ones. A high sucrose level was required for female differentiation, particularly.
It is concluded that the sex of Begonia franconis
Liebm. flower buds is determined by the nutritional state of the tissue as a limiting factor for female differentiation.