Further Studies on the Relationship between Growth Regulators and Tuberization of Dahlias

Abscisic acid (ABA) and gibberellin (GA) treatments, applied to budless leaf cuttings of Dahlia under long day conditions, did not increase the total weight of the cuttings but did affect the region where photosynthates accumulate. GA treatment inhibited the growth of tubers and roots, and enhanced thickening at the petiole base. ABA did not affect the weight of roots or petiole, but promoted the growth of tuberous roots. Anatomical examination of the tuberization revealed that it is caused by inter‐fascicular cambial activity producing parenchyma cells inward. No anatomical difference could be observed between the GA‐induced thickening at the petiole base and the natural or ABA‐induced thickening of the roots. In single node cuttings, taken from various sites along the branch, tuberization ability increased with proximity to the base of the branch, where axillary buds are more inhibited. Short‐day conditions which promote tuberization, increased the endogenous level of ABA‐like inhibitors in intact plants. External conditions and growth substances which inhibit growth also inhibited tuberization. Benzyladenine treatments did not affect tuberization. It is suggested that in addition to their respective indirect effects via the enhancement or suppression of top growth, GA and ABA directly control tuberization. ABA levels which increased under short‐day conditions seem to produce a sink in the roots. GA probably inhibits tuberization by interfering with the trans‐location of photosynthates to the root area.