Relationship between the Occurrence of Hollowing and Lignification of Parenchymatous Cells in the Root of Japanese Radish cv. Gensuke

Abstract

In the Japanese radish cv. Gensuke, hollowing of the root has generally been regarded as peculiar to summer-sown crops. Hollowing starts as the intercellular air spaces in the stele near the pith begin to coalesce. The intercellular air spaces are formed among large parenchymatous cells, but these cells are unable to divide fast enough to prevent the formation of a central cavity, a phenomenon associated with high soil temperature during root growth. In general, cell division and enlargement become restricted when cells begin to lignify. Taking these facts into consideration, the present investigation was carried out to elucidate how lignification of xylem parenchymatous cells surrounding the intercellular air space is related to the occurrence of hollowing.
Weights of radish leaves and roots were much smaller when plants are grown under high soil temperature than when temperatures are low or moderate. The parenchymatous cells surrounding the intercellular air space show a positive red reaction to phloroglucinol-HCl reagent when the plants are grown under high soil temperature, while cells surrounding the air space are colorless in the plants grown under low and moderate soil temperatures. The rate of plant growth was significantly faster in the early sown plot than in the late sown plot. The average soil temperature in the early sown plot was as much as 6° to 12°C higher than it was in the late sown plot. Large hollow cavities were found to be more numerous in roots from the early sown plot. In roots from the early sown plot large projecting cells were seldom observed inside the air space owing to the active lignification of cells on the surface of the hollow space; but in roots from the late sown plot large unlignified cells projected into the hollow zone. In examining 60-day-old roots, the surface cells of the hollows were found to vary from large, brownish ones to small, whitish parenchymatous ones. The brownish tissues of the hollow stained intensely red with phloroglucinol-HC1, reagent whereas the whitish tissues turned slightly pink with the same reagent.
We conclude from these results that: 1) the intercellular air spaces often coalesce into longitudinal hollow pockets in roots grown under high soil temperature, and 2) that active lignification of the cells lining the hollow space stops cells division and this prevents cells from intruding into the hollow. Conversely, lignification seldom occurs in cells of the central stele when roots are not heat-stressed; and because the parenchymatous cells continue to proliferate rapidly, the intercellular air spaces remain small and do not coalesce into a large hollow cavity.