Pithiness and growth of radish tubers as affected by irradiance and plant density

Pithiness in radish tubers (Raphanus sativus L.) is a syndrome where xylem parenchyma cells die and are filled with air. Features associated with rapid tuber growth such as large cells, great distances between strands of vasolar tissue and rupture of xylem vessels have been suggested to induce pithiness. The objective of this study was to analyse the extent to which pithiness in radish was related to tuber growth. Growth rates were varied by altering irradiance and plant density. In addition, the relationship between pithiness and tuber growth (length, diameter, fresh mass) was investigated in batches of plants grown under identical conditions. The growth rates of shoot and tuber strongly increased with increasing inter-plant distance and irradiance. Our results suggest a 'true' effect of assimilate supply on dry matter partitioning, as not only the ratio between shoot and root dry mass, but also the slope of this relationship decreased with increasing irradiance and inter-plant distance. Within a treatment the ratio between tuber growth and shoot growth remained constant with plant age. Increase in length of the tuber did not appear to be an important factor in inducing pithiness. Within a batch of plants, a positive correlation between tuber diameter and pithiness was observed only for tubers smaller than 14 mm. An increase in both irradiance and inter-plant distance increased tuber size (diameter, mass) and pithiness, but the effect on pithiness could not be ascribed only to different tuber sizes. Pithiness strongly increased with plant age and it was concluded that pithiness occurred, to a great extent, according to genetic programming. The data showed that for treatments which affect tuber growth, conclusions about their effects on pithiness depend on whether comparisons were made at the same tuber size or the same time, explaining apparent contradictions in the literature.