YIELD-DETERMINING PROCESSES IN RELATION TO CULTIVAR SEED SIZE OF COMMON BEAN

Cultivar seed size and yield potential of common bean (Phaseolus vulga ris L.) are negatively associated. Large-seeded cultivars are predomin ately of Andean background, while small-seeded cultivars are predomina tely of Mesoamerican background. Crop growth analyses were conducted t o examine the association of cultivar seed size with crop growth rate (CGR), seed growth rate (SGR) on a land-area basis, intensity of parti tioning to seed (land-area SGR divided by CGR), and effective seed-fil ling period. Sixteen cultivars, ranging in seed size from 190 to 540 m g seed(-1), were grown at two sites in Colombia at elevations of 1000 m (warm) and 1800 m (cool). The seed size groups did not differ in CGR at either site. At the warmer site, the small-seeded Mesoamerican Lin es had greater mean partitioning to seed (1.29 vs. 0.97 for the Mesoam erican and Andean lines, respectively), greater land-area SGR (11.7 vs . 8.3 g m(-2) d(-1)), and higher yield (2030 vs. 1750 kg ha(-1)) than did the large seeded Andean lines. At the cool site, the seed size gro ups did not differ in any of these variables. At both sites, the Mesoa merican lines flowered approximate to 6 d later and acquired more tota l N than did the Andean lines. It is postulated that the greater land- area SGR and yield of the Mesoamerican lines at the warmer site was du e to their greater accumulation of N, which allowed them to sustain gr eater rates of canopy photosynthesis during seed-filling.