Avaliação da qualidade de sementes e respostas fisiológicas em Brachiaria brizantha (Hochst Ex. A. Rich) Stapf cv. Marandu e Brachiaria humidicola (Rendle) Schweickerdt, submetidas a diferentes níveis de deficiência hídrica.

Abstract

The gender Brachiaria is the main component of the current pastores cycle in the Brazilian cattlebreeding, because it adapts to the most varied types of soils. That work was driven in Western Amazônia Embrapa, in Belém, Pará (1° 28' S e 48° 27 W), with the objective of evaluating quality seeds and the effects of the hydric stress in the physiologic behavior in two tropical grasslandss Brachiaria hrizantha (Hochst Ex. A. Rich) Stapf cv. Marandu and Brachiaria humidicoía (Rendle) Schweickerdt, submitted at different leveis of water déficit. The soil was taken from a pastage located in a area with defined periods of water déficit, varying from four to five months. The work was executed in two phases, under controlled conditions. The first stage had the purpose of determining the initial quality of the lots of seeds, physical tests constants (weight of 100 seeds, humidity content and curve of hydration seeds), of viability (germination percentage and tetrazoil test) and vigor (emergency percentage, index of germination speed (IVG), index of emergency speed (TVE) and electric conductivity). The second stage of the research was conducted at greenhouse. Through the curve of retention of water in the soü, the factor levei of humidity of the soil was determined, that was maintained through irrigation, determined by the individual weighting of the vases and complementation of the weights with water twice a day. Atthe 0, 7,14 and 21 days of the imposition of the leveis of humidity of the soil (35, 42, 49 and 56 days after the emergency of the plant), the estomatal resistance was evaluated in the following schedules 6:00h, 9:00h, 12:00h, 15:00h and 18:00h with the use of diffusion porometer (AP-4), the rates of liquid photosynthesis, rate transpiration and the temperature leaf, in interval of 12:00h, through IRGA, Ll-6200. The water potential (MPa), it was determined through pressure bomb of Scholander, at same hours of estomatal resistance and the microclimatic variables: temperature of the air, relative humidity of the air and the photosynthetically active radiation was collected during the experimental period. The analysis of the data was accomplished by the SAS, where the treatments were entirely randomized in fatorial delineament 2x3x4 (species vs. levei of humidity of the soil vs. evaluation time), with four repetitions per treatment and treatment averages were compared by the test of Tukey (P<0,05). The answer variables (water potential of leaves, rate of liquid photosynthesis, transpiration rate, temperature of leaves, estomatal resistance, leveis of crude protein and total carbohydrates contents of the leaf and of the stem. The evaluation results of the physiological quality of the seeds demonstrated that the seeds of the B. hrizantha were more vigorous than B. humidicoía, when placed to field. The hidric stress affected significantly ali the studied variables, however the B. hrizantha had better performance, under conditions of water déficit and the leaves water potential of base and the temperature leafwere good indicators for monitoration ofthe water déficit in the studied grassland.