EFFECTS OF WEIGHT AND PLANTING DENSITY OF TUBERS DERIVED FROM TRUE POTATO SEED ON GROWTH AND YIELD OF POTATO CROPS IN EGYPT .1. SPROUT GROWTH, FIELD EMERGENCE AND HAULM DEVELOPMENT

In field experiments conducted in Egypt the performance of seed tubers derived from true potato seed (seedling tubers) was investigated under conditions of decreasing (autumn, i.e., September through January) and increasing (spring, i.e., January through May) daylength and temperature. Field emergence and haulm development were compared in crops from seedling tubers ranging in weight from 0.5 to 35 g at different planting densities. The number of eyes per tuber increased with the seedling-tuber size, but the number of eyes per g tuber was more than 7 times higher in small (1-5 g) seedling tubers than in large (> 20 g) ones. Despite these large differences in the number of eyes, sprout weight per g tuber weight after storage in a non-cooled store was similar in tubers from different size grades. Field emergence was generally faster with large than with small tubers, but the final emergence was about 90% regardless of seedling-tuber size. The number of above-ground stems increased with increasing seedling-tuber size. However, on a per weight basis, 1-5-g tubers were about five times more effective in producing stems than tubers > 20 g. Initial foliage development was much faster froin large than from small tubers. Plants grown from small tubers at low planting density developed more and larger axillary branches and tended to senesce later than plants from large tubers. Ground-cover duration increased with stem density, but at similar stem densities (obtained by the varying planting density) the ground cover was lower in crops from small than large tubers. It is concluded that the higher efficiency per unit weight of small seedling tubers to produce stems is offset by the lower capacity of these stems to produce foliage, which would result in a lower capacity for photosynthesis.