Respostas ecofisiológicas do feijão-caupi sob diferentes disponibilidades de água.

Abstract

The cowpea (Vigna unguiculata (L.) Walp.) is a legume grown by small and medium producers in the North and Northeast of Brazil. In Pará, the northeastern state is the productive pole of cowpea, but the crop still presents low productivity, resulting from several factors such as low soil fertility and climatic adversity, mainly water deficiency. Thus the present work had as objective to evaluate the physiological parameters and the productivity of the cowpea under different layers of water in the soil. The experiment was conducted in the municipality of Castanhal, located in the northeastern region of the state of Pará, in the months of September to November of the years 2015 and 2016 in an area of approximately 0,5 hectare, located in the dependencies of the Experimental Farm of the Federal Rural University Of Amazonia (UFRA), the cultivar used was BR3-Tracuateua, which is recommended for the region. A randomized complete block design with six replicates and four treatments was used. From the reproductive phase (R5) there was a difference in the availability of water in the soil, T1 treatment consisted in the replacement of 100% of the water lost by the evapotranspiration of the crop (ETc), T2 in 50%, T3 in 25% and T4 without irrigation. The determinations of the photosynthesis rates (A), stomatal conductance (gs), leaf transpiration (E), CO2 internal concentration (Ci) and Leaf temperature (Tf) were performed by means of a portable infrared gas analyzer, and the potential (Ψwf) was determined using a Scholander type pressure pump, in four collections with weekly intervals, 7, 14, 21 and 28 days under water restriction, at the phenological stages R5, R6, R7 and R9 respectively. Cowpea were sensitive to the availability of water in the soil, presenting a significant difference between treatments for physiological variables and productivity, with T1 treatment always showing better responses in function of the others. T4 treatment showed reductions of 85% and 86% in A, 76% and 74% in gs, 69% and 67% reductions in E and an increase of 4.09ºC and 4.18ºC in Tf for the years 2015 and 2016, respectively. When a Ψwf of -0.88 MPa was reached, important changes of the studied variables occurred, defining this value as a threshold for the water stress of the crop in the climatic conditions of the northeast of Para. The productivity showed that the different levels of water in the soil had a direct influence on its result, with T4 reducing by 57% for both years, demonstrating that the ideal water supply results in the best growth and development of the crop, and consequently in the increase of productivity.