PHOSPHORUS EFFICIENCY OF WILD AND CULTIVATED GENOTYPES OF COMMON BEAN(PHASEOLUS-VULGARIS L) UNDER BIOLOGICAL NITROGEN-FIXATION

Production of common bean (Phaseolus vulgaris L.) in the tropics is of ten limited by low soil P availability, and P deficiency is therefore a major nutritional constraint to N-2 fixation of legumes. Two experim ents were conducted in 4 kg pots with a sandy loam soil, to evaluate t he efficiency of P absorption and utilization of wild and cultivated g enotypes of common bean under biological nitrogen fixation. In Experim ent 1, 20 wild and 7 cultivated genotypes were grown at two soil P lev els (20 and 80 mg P kg(-1) soil); in Experiment 2, 4 wild and 27 culti vated genotypes were grown at the same P levels. The higher P supply i ncreased the number of leaves. leaf area, and dry weight of all plant parts, particularly nodule dry weight. The cultivated genotypes exhibi ted fewer leaves but greater leaf dry weight, and also lower specific leaf area and leaf area ratio, thereby producing larger and thicker le aves probably with higher photosynthetic activity. The cultivated geno types presented greater root dry weight and root-to-shoot ratio, sugge sting that bean selection could have resulted in plants which invest p referentially in root production, a valuable trait in low P soils. The means of shoot dry weight and P content of wild and cultivated groups were similar in both P levels, although there were distinctions withi n each group. The wild genotypes presented a lower efficiency of P uti lization, as attested by smaller P utilization index and higher shoot P concentration at the lower P level. These results indicate that wild bean genotypes are not more tolerant to low P soils, but the genotypi c variation observed suggests these materials as a source of genetic d iversity for nutritional breeding programs. (C) 1997 Elsevier Science Ltd.