Environmental Control of Flowering in Vicia faba L.

There is a heteroblastic change in leaflet number in many stocks of Vicia faba, the rate of change being affected by the temperature and photoperiod under which the plants are grown. In all except the earliest flowering stocks of broad beans, and particularly at high temperatures, flower initiation shows a quantita-tive long-day response. For full development of the initiated inflorescences long days are required. Flower initiation may be accelerated in all except the earliest flowering stocks of V.faba by brief exposures to low temperatures, particularly when the plants are grown in short days at high temperatures. The response to low temperatures is more rapid at I0° C. than at 4° C. but eventually approaches saturation at both temperatures. More prolonged exposure to low temperatures delays flower initia-tion. The response to low temperatures increases with increasing plant age but can occur during embryo development on the mother plant. At temperatures above 14° C, and particularly above 23° C, a reaction inhibitory to flower initiation occurs. This reaction is probably restricted to the diurnal dark periods but is operative at all stages of the life cycle, including embryo development. Its inhibitory effects may be overcome by subsequent cold treatment, and when the low temperature processes have reached saturation subsequent high temperatures are no longer inhibitory. Although nucleosides could accelerate flower initiation, purine and pyrimidene analogues did not, with one exception, reduce the response to low temperature treatment.