THE ROLE OF AUXIN IN THE PHOTOPERIODIC TUBERIZATION IN BEGONIA EVANSIANA ANDR.

The formation of aerial tubers in Begonia plants, an SD response,was inhibited by IAA, NAA and IBA applied to their leaves duringthe dark periods. The effectiveness of IAA differed accordingto the time of application during the dark periods, and themost sensitive time varied with daylengths employed. In order to inhibit the tuberization under optimal photoperiods(8-hr SDs), IAA had to be applied during the first 2 days orso of the SDs. Under non-optimal photoperiods, however, IAAwas effective even when applied somewhat later. The auxin activity of leaf extracts from the plants subjectedto 8-hr SDs decreased during the first 2 or 3 days to a minimum,and then increased until finally began to decrease, again; undernon-optimal photoperiods, the minimum of auxin activity wasattained more slowly. The paper-chromatographic study suggestedthat the change in auxin activity was mainly due to the changein IAA content. The number of SDs making the auxin content minimal agreed withthe minimum number of SDs required for tuberization. On the basis of the above results, the part played by endogenousauxin in photoperiodic induction is discussed. ¹Present address: Institute for Agricultural Research, TôhokuUniversity, Sendai.²Present address: Biological Institute, Yamaguchi University,Yamaguchi.