Relações hídricas do cacaueiro: efeito da área foliar, potencial da água do solo e condições de sombreamento

Abstract

Cacao cultivation has expanded to areas with very different environmental characteristics from traditional farming regions, including the use of irrigation and planting in the full sun. This has demanded studies on water relations and water consumption by the plant. The quantification of the transpiration is essential for the implantation and management of the crop, as this is the main factor in the determination of the water demand of the cacao tree. Currently there is a trend of shading removal a few years after planting, however, the relationships between shade levels and water consumption and crop production have not been adequately studied. Two experiments were carried out with the objective of: A) estimating the transpiration of young cocoa plants according to leaf area (AF), reference evapotranspiration (ETo) and soil water potential (Ψa); B) to evaluate the relationship of solar radiation and other meteorological variables with the transpiration and vegetative growth of the cacao tree cultivated under two conditions of banana shading in the semi - arid region of Bahia. The first experiment was conducted in a greenhouse with plants irrigated by drip irrigation. Initially all plants were irrigated daily and a defoliation was performed forming 4 groups with the same number of leaves. Subsequently half of the plants remained with frequent irrigation and the other part had the irrigation suspended. There was no significant difference in mean daily transpiration in plants with average leaf area between 1.11 and 3.71 m2. With soil water potential up to -24.89 kPa, mean transpiration (L day-1) could be estimated linearly as a function of ETo (mm day-1). Under soil drying conditions, daily transpiration presented a linear reduction of 1.5% kPa-1 from a critical potential -24.89 kPa. In the second experiment the sap flow was estimated in adult cocoa plants in two rows with different shading. In one of the rows with four cocoa plants, banana trees were thinned around them to increase cocoa exposure to solar radiation. In the other row with four cocoa plants the banana trees were kept to maintain the shading. Four radiation sensors were installed in each row of plants. There was a reduction of 22 and 38% of the solar radiation in the imposed shading conditions, which affected the transpiration and the vegetative growth of the cocoa plants in a direct and significant way.