Relationship between the development of hollowing and the separation of vessel sectors in the central region of the root of Japanese radish ( Raphanus sativus L.)

Abstract

The present investigations were carried out to clarify the relationship between the separation of two vessel sectors, especially in the later growth period, and the development of hollowing in the root and the effect of varying the cultural conditions. Roots with hollow cavities were more prevalent in plants whose seeds were sown early in the summer compared with those sown later. The gap between the bifurcated vessel sectors became larger in roots sown earlier. A larger number of hollow roots was observed in the sparsely planted plot than in the high planting density plot, and the gap between the bifurcated vessel sectors became larger in roots at low planting density. Late defoliation, after rapid thickening growth, produced a large number of hollow roots, while early defoliation prevented the root from hollowing due to slower thickening growth. The gap between the bifurcated vessel sectors became wider the later the time of defoliation. A larger number of hollow roots occurred in proportion to increasing amounts of basal fertilizer and larger amounts of basal fertilizer application resulted in a wider space between the two vessel sectors. These facts strongly indicate that the development of hollowing may pass through two indispensable steps. First is the obstruction of cell formation inside the intercellular air space during the middle of the growth period, caused by high soil temperatures. Second is the rapid divergence of bifurcated vessel sectors during the later growth period as a result of enhanced thickening growth. If the first condition is present, the second is the determining factor for the development of hollowing.