EVIDENCE FOR AN INHIBITORY ROLE OF ENDOGENOUS AUXIN AND GIBBERELLIN IN FLOWER INITIATION OF THE SHORT-DAY PLANT BEGONIA x CHEIMANTHA EVERETT
The present paper deals with preliminary investigations on the effect auxin and gibberellin in the flowering of Begonia x cheimantha (Lorraine Begonia or Christmas Begonia). As shown by Post (1942), this is a short-day plant (SDP). The critical day-length at 20–25°C was found by Rünger (1957) to be about 12 ½–13 hours. However, at low and intermediate temperature the requirement for short days is not an obligate one, so that at 21° and lower temperatures the plant flowers also in continuous light (Heide 1962). Figure 1 shows that at intermediate temperatures flowering is much earlier in short than in long days, but as the temperature decreases this effect of day-length gradually disappears. Thus, it is seen that in long days (LD) flowering is inhibited by high temperature, whereas in short days (SD) temperature has the opposite effect.
From previous studies on the control of regeneration of adventitious buds and roots in detached leaves it appeared that the conditions which are inductive for flowering (short days and low temperature) also are stimulatory for adventitious bud formation, whereas root formation is stimulated by relatively high temperatures and long days (Heide 1964, 1965). This applies both to intact plants and to detached leaves.
In order to study this relation between environment and regeneration ability of the leaves, studies were also made on the level of endogenous auxin in the leaves of plants grown under various environmental conditions (Heide 1967, 1968).
These studies revealed that with an increasing number of short days the level of an acidic auxin, most probably indole-3-acidic acid (IAA), steadily decreased in the leaves until a constant low level was reached after about 2 weeks (figure 2). This is also the length of short-day treatment required to produce optimum flowering response (Heide 1962), although some flowering result from as little as 2 or 3 SD.
Similarly, there is a decrease in this auxin also with decreasing temperature, particularly at high light intensity (Heide 1967, 1968). It should further be noticed that the level of a gibberellin-active factor also decreased in response to short days and even more so than did the auxin (Heide 1967).
On this background, and since similar effects have been observed in