D. L. de Almeida

Phosphorus efficiency of wild and cultivated genotypes of common bean (Phaseolus vulgaris L.) under biological nitrogen fixation

Abstract Production of common bean (Phaseolus vulgaris L.) in the tropics is often limited by low soil P availability, and P deficiency is therefore a major nutritional constraint to N2 fixation of legumes. Two experiments were conducted in 4 kg pots with a sandy loam soil, to evaluate the efficiency of P absorption and utilization of wild and cultivated genotypes of common bean under biological nitrogen fixation. In Experiment 1, 20 wild and 7 cultivated genotypes were grown at two soil P levels (20 and 80 mg P kg−1 soil); in Experiment 2, 4 wild and 27 cultivated genotypes were grown at the same P levels. The higher P supply increased the number of leaves, leaf area, and dry weight of all plant parts, particularly nodule dry weight. The cultivated genotypes exhibited fewer leaves but greater leaf dry weight, and also lower specific leaf area and leaf area ratio, thereby producing larger and thicker leaves probably with higher photosynthetic activity. The cultivated genotypes presented greater root dry weight and root-to-shoot ratio, suggesting that bean selection could have resulted in plants which invest preferentially 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 within each group. The wild genotypes presented a lower efficiency of P utilization, 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 genotypic variation observed suggests these materials as a source of genetic diversity for nutritional breeding programs.

Cyperus rotundus control by solarization

ABSTRACT Soil solarization by means of polyethylene cover sheets was tested for the control of Cyperus rotundus weed infestation. Following solarization, which lasted for a period of approximately three months, the field was cultivated with carrots and dwarf green beans. Solarization was shown to provide a highly significant reduction of the weed population and to increase carrot yield. As for green beans, soil treatment was less effective and yield was not affected. In the solarized plots no weeding was needed during the vegetable crops cycle whereas the untreated plots required hand weeding within the first month from sowing. Soil solarization significantly reduced ant and earthworm numbers but had no effect on millipede population. Soil microbial biomass carbon (SMBC) was affected by solarization. However, actinomycete and bacterial populations, including nitrogen- fixing bacteria, showed no obvious pattern related to soil solarization.