The Difference in the Development of Hollowness in Roots of 'Gensuke' Radish between the Early and Late Sowing of Seeds.

Abstract

The Japanese radish cv. Gensuke, grown in the summer, frequently develops a lengthwise hollow cavity in the central part of the root. To study the influence of the sowing date on the anatomy of developing roots, seeds of 'Gensuke' were sown on 14 July (early sowing) and 20 August (late sowing). The maximum soil temperature was monitored at a 5-day interval during the growing season.1. In the early sown plot, the temperature for the first 50 days after sowing was above 25°C, attaining 37°C 8 days after sowing. In the late sown plot, the maximum soil temperatures were 5° to 8°C lower than those of the early sowing during the growing period.2. Root growth was conspicuously suppressed in the late sown plot as compared with that of the early sown plot; this contrast was clearly noticeable after the middle of the growing period.3. Roots with a hollow cavity were more prevalent in plants whose seeds were sown early compared with those which were sown late.4. The anatomy of the roots collected from the early and late sown plots revealed that the vessels in the central region of the pith diverged into two sectors against medually ray cells. The gap between two polarized sectors, widened rapidly in roots from the early sown plot creating a large intercellular air space; roots from the late sown plot formed gaps slowly. A few bag-shaped, large parenchymatous cells formed on the inner surface of the hollow root and projected into the air space. On the contrary, the gaps in roots from the late sown plot filled with large cells except in the most distal portion of the root. These observations lead us to believe that the gap between two vessel sectors in the central region of the pith separates radially in roots of the early sown seed more so than in the pith region of roots in the late sown plot. The formation and protrusion of cells on the cavity surface of the roots from the early sown plot are suppressed by high soil temperature during the middle of the growing season. The intercellular air spaces then coalesce into a large longitudinal gap resulting in hollowness.