Relationship Between Thermoinduction and Photoinduction of Flowering and Dormancy in Hordeum bulbosum L., a Perennial Grass

Induction of axillary bud dormancy (as manifested by initiation of the bulb which is associated with the dormant buds) and of reproductive development in H. bulbosum are closely linked processes. Both are potentiated by thermoinduction (vernalization of the seed) and express themselves as a result of subsequent photoinduction in long (16-h) photoperiods. Partial photoinduction of vernalized plants, which sufficed for bulb initiation, was insufficient for full expression of flowering: the reproductive development of the shoot apex was arrested and reverted to vegetative development. This was terminated by formation of a second bulb and a normal inflorescence, both typical of non- induced plants. Intercalation of as many as 30 short (8-h) photoperiods after the end of thermoinduction did not diminish the developmental response of the shoot apex to subsequent photo- induction. On the contrary, intercalation of short photoperiods after vernalization increased the effectiveness of subsequent photoinduction, with respect to the bulb-forming response and to the reproductive development of the shoot apex. Both photoperiodic regimes in this sequence were effective in promoting bulb initiation, However, effectiveness of short photoperiods was much smaller than that of long photoperiods and was progressively decreasing as their number increased. The position of the bulb internode showed remarkable parallelism with the number of leaves which had emerged by the start of photoinduction, but the intensity of photoinduction had no effect. No parallelism was found with the total number of leaves produced by the apex (i.e. including leaf primordia). The significance of these results is discussed.