Varietal Differences in the Occurrence of Hollowing and Lignification of Parenchymatous Cells in the Roots of Japanese Radish

Abstract

To clarify the anatomical, physiological and varietal differences in the hollowing (internal cavity formation) in roots of the Japanese radish (Raphanus sativus L.), several cultivars were grown under different soil temperatures. Vessel arrangement in the roots, and lignin formation on the wall of parenchymatous cells facing the intercellular spaces in the central region with respect to hollowing were investigated.1. When grown under high soil temperatures, roots of 'Gensuke', 'Kiributo', and'Genko' which have dense vessel arrangement in the central region developed caveties. Roots of 'Kenseiaokubi' (syn. 'Kensei'), 'Kaishinsobuto' (syn. 'Kaishin'), and 'Seieiaokubi' (syn. 'Seiei') have sparsely arranged vessels and do not possess the propensity to develop a central cavity.2. When grown under moderate to low soil temperatures, those cultivars, i. e. 'Gensuke' and 'Wakayama' which tend to form cavities under high soil temperatures produced fewer vessels and did not form cavities. Contrarily, roots of 'Tokinashi' and 'Suikomininengo' (syn. 'Ninengo') produced sparsely arranged vessels and did not exhibit this disorder at any soil temperature.3. Under high temperatures, lignin was deposited in the walls of parenchymatous cells, and vessels were differentiated in the central region of roots of 'Gensuke' and 'Yamada'. Subsequently, the intercellular spaces in these cultivars enlarged and coalesced, forming a hollow cavity. In roots of 'Seiei' and 'Ninengo', vessels were sparsely distributed and no lignin deposits were observed on cells of the central region.From these results we assume varietal difference exist with respect to susceptibility to this disorder by roots when they are exposed to high soil temperatures. In susceptible cultivars with a dense vessel arrangement, the deposition of lignin is brought about by a reduction in meristematic activity of parenchymatous cells attributed to high soil temperature. This prevents the large thin-walled parenchymatous cells from protruding into the intercellular spaces, causing the spaces to coalesce into a large hollow cavity. Contrarily, in cultivars in which vessels are dispersed on account of active proliferation of parenchymatous cells, even under high soil temperatures, lignin is seldomly deposited in cells facing small intercellular spaces. Because the intercellular spaces are continually filled by newly formed parenchymatous cells, they do not coalesce to form a hollow gap.