Aumento dos teores de fósforo e de molibdênio em sementes de feijoeiro (Phaseolus vulgaris L.) via adubação foliar

Abstract

The present study had as objective to evaluate the viability to increase the concentration of phosphorus (P) and molybdenum (Mo) in bean seeds (Phaseolus vulgaris L.) through the foliar fertilization, and the effect of the highert concentration of Mo in seeds in the growth and the nitrogen biological fixation for the plant. Two experiments were carried out for this purpose. The first, under field conditions, had the objective of increasing the concentration of P and Mo in bean seeds by foliar fertilization. The experimental design was a 2x4x3 factorial in a randomized block with four repetitions: in the plots two levels of P applied to the soil were tested (0 and 50 kg P ha-1), in the subplots four treatments of foliar fertilization (without foliar fertilization, foliar fertilization with P, foliar fertilization with Mo, foliar fertilization with P and Mo), and in the subsubplots three cultivars (Carioca, Manteigão and Rio Tibagi). The foliar applications were accomplished at 52 and 71 days after emergence, for each treatment, in the dosages of 5 kg P ha-1 and 120 g Mo ha-1. At grain maturity, in the useful area of 5 m2 within each subplot, the grain yield was measured, and the concentrations of N, P and Mo were determined in a seed sample. The foliar fertilization with P increased the concentration of P in the seeds, in the absence or presence of foliar fertilization with Mo, but this increase was significant only for the lower level of P applied to the soil. The foliar fertilization with P reduced the concentration of Mo in the seeds, in the two levels of P applied to the soil, indicating a possible effect of competition between P and Mo, when leaf absorption and transport to the grains occur. The foliar fertilization with Mo provided seeds with an average concentration of 9.7 mg Mo g-1, whereas in the plants that did not receive foliar fertilization the seed concentration was around 0.2 mg Mo g-1. The foliar fertilization with Mo increased the seed concentration of Mo about 50 times, when compared with the foliar fertilization without Mo. The second experiment, in the greenhouse, had as objective to evaluate the effect of the increased Mo concentration in bean seeds, obtained by the foliar fertilization, in the growth and biological N2 fixation of three bean cultivars. The experiment had a randomized block design with five repetitions, in a 3x2x2 factorial: three bean cultivars (Manteigão, Carioca and Rio Tibagi), two concentrations of Mo in the seed (low and high Mo concentrations), and two dates of harvesting (30 and 45 days after emergence). Two plants were grown per pot with 3 kg of soil. The higher Mo concentration in seeds increased the mass and N content in shoots, in all cultivars, at the two harvests. The higher Mo concentration in seeds increased the nitrogenase activity in cultivars Carioca and Manteigão at the first harvest, and in the cultivar Manteigão at the second harvest. The higher seed Mo concentration reduced the number of nodules in cultivars Carioca, at first harvest, and in the cultivars Carioca and Manteigão at the second harvest, probably due to the earliness of plants originated from seeds with high Mo concentration. It can be concluded that foliar fertilization arises as a technological alternative for producing bean seeds with higher concentrations of P and Mo, therefore higher N2 biological fixation.