EFFECTS OF WEIGHT AND PLANTING DENSITY OF TUBERS DERIVED FROM TRUE POTATO SEED ON GROWTH AND YIELD OF POTATO CROPS IN EGYPT .2. TUBER YIELD AND TUBER SIZE

In field experiments conducted in Egypt the performance of seed tubers derived from true potato seed (seedling tubers) was investigated during two seasons with contrasting daylengths and temperatures. Tuber yield and mean weights per tuber were compared in crops from seedling tubers of different size grades (0.5-1 g to > 20 g) and at different planting densities (5.3 to 21.2 seedling tubers per m2). At a planting density of 5.3 seedling tubers per M2 tuber yield significantly increased with seedling-tuber size. This was closely correlated with higher stem densities and ground-cover duration in crops from large seedling tubers. Differences in tuber yield from small and large seedling tubers were particularly distinct, when foliage development was decreased by low temperatures and short daylengths, or when high temperatures at the end of the growing period reduced tuber bulking. Tuber yields from small seedling tubers were increased at higher planting densities. However, tuber yield per stem decreased with increasing planting density for small seedling tubers. Furthermore, at a certain stem density, tuber yields were always lower in plants grown from small seedling tubers than from large ones, indicating a lower potential for tuber bulking on a per stem basis in the case of small seedling tubers. Mean weight per tuber was lower in plants from small seedling tubers. The number of tubers per stem was negatively correlated with stem density and this relationship was independent of seedling-tuber size. Tuber yield per stem was also negatively correlated with stem density, but at a certain stem density tuber yield per stem was always lower in plants from small than from large seedling tubers. This indicates that the low mean weights per tuber in plants from small seedling tubers were not due to an unusually high number of tubers per stem, but were caused by the low tuber bulking per stem. It is concluded that under the prevailing environmental conditions in Egypt, which- are characterized by a relatively short period of optimal growing conditions, the potential of seedling tubers to produce tubers of marketable weight, even on a per stem basis, decreases with decreasing seedling-tuber size.