Effect of cytokinin and auxin application on vessel differentiation and development of hollow cavity in the roots of Japanese radish (Raphanus sativus L.)

The present study was carried out to clarify the effect of the exogenous supply of 1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU), 6-benzylaminopurine (BA) or Na-salt of alpha-naphthaleneacetic acid (NAA) on the occurrence of hollow cavity (HC) and vessel arrangement in roots of Japanese radish, 'Gensuke'. Relative to untreated controls, fewer roots exhibited HC when plants were sprayed with 10 mg center dot L-1 CPPU or BA solutions of > 1 mg center dot L-1 during the middle of the growth stage. Conversely, HC occurred more frequently relative to controls when plants were sprayed with 5 mg center dot L-1 NAA during the first half of the growth stage. On the other hand, the application of CPPU during the late growth stage or NAA during the second half of the growth stage produced little or no effect in terms of the occurrence of HC. Vessels in the roots were more sparsely arranged relative to controls when 10 mg center dot L-1 CPPU was applied during the middle of the growth stage. Application of 5 mg center dot L-1 NAA during the first half of the growth stage resulted in a high density of vessels relative to controls. From these observations, it was concluded that the exogenous application of cytokinin during the middle of the growth stage or auxin during the second half of the growth stage strongly affects vessel arrangement in the roots and this morphological change is closely correlated with the occurrence of HC.