Abstract:

The potential yield of a soybean crop is very specific to the genotype and the environment in which it is grown. For a given genotype, the interaction of density with arrangements of plants will play a key role in determining the competition for available resources especially solar radiation, water and nutrients, and thus maximum yield. Inter-plant competition begins when the immediate supply of a single necessary factor falls below the combined demands of all plants. Variation in yield among fields or years will be related to such inter-plant competition and borne out in one or more of the seed yield components of, plant per unit area, pods per plant, seeds per pod or weight per seed (seed size). This paper discusses the yield component of seed size in regulating differences in seed yield with changes in density and row spacing, and differences in seed yield between different years. Several field studies are reviewed where seed size was shown to be similar between nodes on a plant and in pods with varying seed number. However, the basal seed in two and three-seeded pods for many varieties tested was approximately 10% smaller than the middle or terminal seeds. Cotyledon cell number in basal seeds was 10% less than in middle or terminal seeds and all had similar cotyledon cell size. To test this further, we changed seed size through artificially source-sink manipulation, and by light enrichment and shading. Differences in seed size were also related to differences in cotyledon cell number. This relationship of seed size to cell number is intriguing since with induced changes in seed size, seeds at all nodes were of similar size even though lower nodes began filling 15 to 20 days before upper nodes.

Key words: Pod number, Seed size, Pod and seed position, Seed growth rate, Cotyledon cell number