PITHINESS AND GROWTH OF RADISH TUBERS AS AFFECTED BY IRRADIANCE AND PLANT-DENSITY

Pithiness in radish tubers (Raphanus sativus L.) is a syndrome where x ylem parenchyma cells die and are filled with air. Features associated with rapid tuber growth such as large cells, great distances between strands of vascular tissue and rupture of xylem vessels have been sugg ested to induce pithiness. The objective of this study was to analyse the extent to which pithiness in radish was related to tuber growth. G rowth rates were varied by altering irradiance and plant density. In a ddition, the relationship between pithiness and tuber growth (length, diameter, fresh mass) was investigated in batches of plants grown unde r identical conditions. The growth rates of shoot and tuber strongly i ncreased with increasing inter-plant distance and irradiance. Our resu lts suggest a 'true' effect of assimilate supply on dry matter partiti oning, as not only the ratio between shoot and root dry mass, but also the slope of this relationship decreased with increasing irradiance a nd inter-plant distance. Within a treatment the ratio between tuber gr owth and shoot growth remained constant with plant age. Increase in le ngth of the tuber did not appear to be an important factor in inducing pithiness. Within a batch of plants, a positive correlation between t uber diameter and pithiness was observed only for tubers smaller than 14 mm. An increase in both irradiance and inter-plant distance increas ed tuber size (diameter, mass) and pithiness, bur the effects on pithi ness could not be ascribed only to different tuber sizes. Pithiness st rongly increased with plant age and it was concluded that pithiness oc curred, to a great extent, according to genetic programming. The data showed that for treatments which affect tuber growth, conclusions abou t their effects on pithiness depend on whether comparisons were made a t the same tuber size or the same rime, explaining apparent contradict ions in the literature. (C) 1997 Annals of Botany Company.