ADAPTATION TO CLIMATE IN COMMON BEAN (PHASEOLUS-VULGARIS L.) - PHOTOTHERMAL FLOWERING RESPONSES IN THE EASTERN, SOUTHERN AND GREAT-LAKES REGIONS OF AFRICA

The photothermal flowering responses of 25 diverse genotypes of common bean (Phaceolus vulgaris L.) were examined in 25 African Bean Yield A daptation Nurseries (AFBYAN) in the Eastern, Southern and Great Lakes regions of Africa during 1988 and 1991. The trials were located at lat itudes between 0.6 and 29.3 degrees and at altitudes from 780 to 2200 m asl. In those 13 trials where daily records of maximum and minimum t emperature were available, mean preflowering temperatures for individu al genotypes ranged from 17.9 to 24.6 degrees C and mean preflowering photoperiods varied from 12.7 to 14.7 h d(-1). The time from sowing to first flowering (f) for the 25 genotypes varied from 26 to 42 d in th e most-inductive regime to as late as 47 to 80 d in the least-inductiv e circumstances. The stepwise linear regression on daily mean temperat ure during the pre-flowering period explained most (52-86%) of the var iation in the rate of progress from sowing towards flowering of 21 gen otypes. In contrast, in four genotypes (GLPx 92, Ikinimba, G 13671 and G 2816) the fitted values of days to flowering using temperature alon e were much earlier than the times observed in the two trials at Maser u in Lesotho (the highest latitude and coolest location). These differ ences may well reflect photoperiodic effects but from the photothermal combinations encountered this could not be confirmed and so remains t o be proven. The overall mean absolute difference between the observed and fitted time to flowering was just 2.6 d. The estimated optimum te mperatures ranged from 20.4 to 23.3 degrees C, at which the minimum ti mes taken to flower were between 28 and 44 d. The derived base and cei ling temperatures ranged from 7.1 to 13.2 degrees C and from 29.1 to 4 0.2 degrees C respectively. Not surprisingly, the use of long-term mon thly temperatures (for those trials from which daily temperature recor ds were not available) gave poor agreement between predicted and obser ved flowering times. The significance and implications of these findin gs are discussed in relation to those from other studies on the photot hermal flowering responses in common bean and the breeding and testing of common beans in Africa.