PHENOLOGICAL PLASTICITY AS AN ADAPTATION BY COMMON BEAN TO RAIN-FED ENVIRONMENTS

Under rainfed conditions in the semi-arid highlands of Mexico, time to first Bower and to maturity of some cultivars of common bean (Phaseol us vulgaris L.) is strongly reduced with late plantings. Consideration of rainfall patterns in the region suggested that this response might be an adaptation to growing seasons of variable length, where length is determined by time of onset of summer rains and by risk of water de ficit or low temperatures at the end of the season. Analysis of weathe r patterns at two sites in the highlands suggested that when rains com menced in early June, there was a high probability of adequate precipi tation during the season. When rains commenced in mid- to late-July, t otal precipitation was lower. For a third site, total precipitation sh owed no relation with date of onset of rains. Evaluations of 20 bean g enotypes planted at five dates each in 2 yr at Pabellon, Aguascaliente s, (20 degrees 11' N latitude, elevation 1910 m) demonstrated large di fferences in plasticity of the pre-flowering and reproductive periods. Cultivars Garbancillo Zarco and Tlaxcala-475 were late to Bower and m ature under early plantings, but were similar to other genotypes in la te plantings. Other genotypes of highland origin showed little or no p lasticity, suggesting that this trait is not universal among cultivars currently grown there. Furthermore, cultivars and breeding lines from other regions showed no plasticity. Marked phenological plasticity oc curred only in genotypes from the highlands which were highly photoper iod sensitive, but several highly sensitive genotypes shelved no or li ttle plasticity. Similar patterns were noted in a survey of 81 genotyp es that included a wider range of highland germplasm.