Effects of NAA on the Occurrence of Hollow Root in the Japanese Radish (Raphanus sativus L. cv. Gensuke)

Abstract

Hollowing in the root of Japanese radish (Raphanus sativus L.), a physiological disorder characterized by the lengthwise cleavage of the central stele, has been frequently associated with summer-sown crops. In this study, the involvement of auxin in the occurrence of hollow root in 'Gensuke' radish was investigated by spraying the leaves with a synthetic auxin, NAA, (α-naphthaleneacetic acid) of various concentrations at different growth periods.
Daily foliar applications of NAA for 15 days during four different periods revealed that 10 mg-liter^-1 NAA applied between 16 and 30 days after sowing markedly suppressed root thickening and promoted the development of hollow roots. Treatments with NAA after this period produced little or no effect, especially at 0.1 and 1 mg-liter^-1. Investigation of the effects of NAA on root morphology and the development of hollowing in roots revealed that spraying with NAA from the time seedlings emerge to 30 days after sowing caused roots to form densely arranged xylem vessels, indicating a decrease in the meristematic activity of xylem parenchymatous cells.
The evidence leads us to conclude that the high occurrence of hollow root in plants treated with 10 mg-liter^-1 NAA results from the lowering of meristematic activity in xylem paren-chymatous cells among vessels because the cells lignify. This leads to small roots with densely arranged vessels and suppression of proliferation of "large parenchymatous cells" which usually project from the internal surface of the air space. Consequently, the intercellular air spaces coalesce into a large hollow cavity as the root continues to thicken.