Factors affecting the relationship between tuber size and dry-matter content

Two experiments are reported which examined the effect of various treatments on the relationship between tuber dry-matter content and tuber size using 0.25 inch size grade increments. The data were analyzed by an orthogonal polynomial technique and it was found that the relationship was of a quadratic nature in all cases. Significant treatment differences were found between planting densities, row widths and varieties. At small tuber sizes treatments with high stem densities produced tubers with a higher dry-matter content than tubers from low stem densities. At larger tuber sizes the dry-matter content of tubers from high densities declined most rapidly. The treatment differences at small tuber sizes were generally explained by the rates of water use by the crop and it is suggested that the decline in tuber dry-matter content at large tuber sizes may be a result of a moisture sress induced change from starch to sugar in parts of the tuber. The implications of these findings are discussed. The type of relationship found would indicate that the production of large tubers does not necessarily result in high dry-matter content. It is clearly essential to use very closely graded sizes of tuber when examining treatment effects on factors such as dry-matter content.