EFFECT OF SOWING DATE ON THE GROWTH AND SEED YIELD OF COMMON BEAN (PHASEOLUS-VULGARIS L) IN HIGHLAND ENVIRONMENTS

Evaluations of common bean cultivars in the highlands of Mexico indica ted that land races from that region experience less reduction in seed yield and seed size in late sowings than do lines from other regions. Introduced materials are of interest as sources of increased disease and pest resistance and tolerance to edaphic constraints, however. To quantify effects of sowing date and determine possible underlying caus es, germplasm of diverse origins was evaluated at two sites in the hig hlands using multiple sowing dates. In all trials, seed yield, seed we ight, harvest index and canopy dry weight decreased with late sowings. Large effects of sowing date, cultivar and their interaction were fou nd for the four traits. The possible importance of phenology per se an d of weather conditions was first examined using regression analyses. Variation in seed yield, seed weight, harvest index and canopy dry wei ght was more closely associated with time to maturity than with time t o flowering. Of three weather variables examined, minimum temperature during seed filling revealed the strongest relations with the four tra its. Few interactions of any parameter with line were significant, ind icating that the cultivars did not have a strong differential response to a specific weather condition such as night temperature. Simulation analyses comparing photoperiod-sensitive and day-neutral cultivars in dicated that radiation and temperature explained part of the yield red uction with late sowings. Daylength also had an influence; however, ev en in the day-neutral cultivar. Given that phenology had a strong effe ct on yield and that its inheritance is better understood than that of other physiological traits, priority should be given to understanding the genetic basis of the response of cultivar phenology to sowing dat e in the region.