Initiation of Adventitious Bud in Root of Sweet Potato, Ipomoea batatas Poiret

Abstract

Shooting from the root-cuttings have been known in many plant species, and this phenomenon is very important in agriculture with regard to the vegetative propagation of crops and the regeneration of weeds. In this paper some observations on adventitious bud-initiation in root of sweet potato were undertaken to clarify the factors controlling the initiation of adventitious bud. It has been said that adventitious buds initiate in the tissue such as phellogen or parenchyma of cortex from parent root and in a part adjacent to the base of lateral root. In the present observation, the meristem regarded as adventitious bud-primordium was observed only in the cortex of the lateral roots which was surrounded by the cortex tissue of parent root (fig. 1A∼C, fig. 2). Many adventitious buds bearing leaf-primordia were observed in the cortex of parent root, and the vascular system of the adventitious bud connected with that of the lateral root, not of the parent root. From these observations, it is considered that the primordium initiated in lateral root develops to adventitious bud and intrudes into the cortex tissue of the parent root and then grows bigger and bigger which ultimately bears several leaf-primordia (fig. 1). In rare cases, the meristematic cell-aggregate was recognized in the cambium layer of parent root adjacent to the base of lateral root, and this meristem was observed always in a longitudinal row with the lateral root along the parent root (fig. 3A). In a scar, several lateral roots of which vascular system individually connected with that of parent root were found in a longitudinal row along the parent root, and some of them were newly formed ones (fig. 3B, C). Therefore, it seemed that these meristematic cell-aggregates developed to the new lateral roots and not to the adventitious buds. It can be concluded that the meristem in the cortex of lateral root develops to adventitious bud and that of the cambium layer of parent root develops to lateral root. This conclusion differs from the opinion that the meristem initiated in root is undetermined one, namely it will become that of a root or a shoot. There are a few different opinions about the stage of development of root when an adventitious bud initiates, and it has also been thought that the stage varies with the varieties of sweet potato. In this observation, the adventitious bud-primordium was observed in the juvenile root such as 1 mm in diameter in all the varieties examined (table 1). Therefore, it is confirmed that an adventitious bud generally initiates in the juvenile atage of root development in sweet potato. It was reported that the damage of a lateral root causes the initiation of adventitious bud from the near part to the base of that lateral root. In an experiment where lateral roots were cut off from parent root, no tendency to promote the initiation of adventitious bud by such damage was recognized, however, the promotion of the development of the adventitious bud already initiated was observed slightly (table 2). These findings suggest that the initiation of adventitious bud in sweet potato is caused, by the endogenous factors existing in the root itself.