Abstract
Hollow root, a physiological disorder, which is manifest by the lengthwise hollowing of the central stele of Japanese radish root, has been frequently associated with the summer-sown crop. The present study sought the relationship between the occurrence of hollowness and the stage and rate of root growth that were manipulated by changing plant density and thinning time.
1. Root growth was relatively slow for the first 21 days after sowing and then became rapid. Large xylary vessels became aligned radially in the central region of the root 7 days after sowing. Continued cell division in the central region caused the vessels to separate into two diametrically opposite groups producing a space between them as the separation progressed. Intercellular air spaces formed among parenchymatous cells in the central region 21 days after sowing. Hollow zones appeared 28 days after sowing as these air spaces coalesced and enlarged, concurrent and proportionate to the increase in root growth.
2. The mean root weight increased significantly when plant density was reduced early in the growing period instead of late. Anatomical observations made at harvest revealed that the space between vessel sectors in the central region of the root, the space among vessels in the outer region and the size of parenchymatous cells were larger in roots from early thinned plots than were those from the later thinned plots. Hollow roots were more prevalent in plots thinned early, whereas they were rare in roots harvested from those plots which were thinned 31, or more days after sowing.
Therefore, it is surmised that the intercellular air spaces that develop in young roots coalesce into longitudinal pockets when vessel groups separate diametrically by rapid thickening of the root. The rapid growth is attributed to active division and enlargement of parenchymatous cells during' the maturation period. Conversely, the separation of vessel groups is suppressed and the hollow air spaces do not develop as root growth is inhibited by high plant density.
