Water use efficiency of castor bean under semi-arid conditions of Brazil

Castor bean is one of the main and promising agricultural crops for the production of biodiesel, castor oil and castor bean cake, which have high added value, especially the castor oil. The objective of the present study was to evaluate the water use efficiency (WUE) of castor bean cultivars by assessing the soil water balance components (storage and water storage variation; internal drainage; capillary rise; irrigation depth; rainfall and actual evapotranspiration), according to the phenological stages. The experimental design was in randomized blocks, in 8 x 6 factorial scheme (eight cultivars: BRS Paraguacu; EBDA MPA11; EBDA MPB01; IAC 2028; IAC 226; BRS Nordestina; IAC Guarani, AG IMA 110-204 and BRS Energia and six periods of evaluation), with three repetitions and 15 plants in each experimental plot. Daily monitoring of soil water content was performed with a capacitance probe (Frequency Domain Reflectometry-FDR, PR2/6 model), at depths of 0.1, 0.2, 0.3, 0.4 and 0.6 m, with 0.6 m being the lower limit of the soil control volume for castor bean. The total soil water potential gradient was determined with tensiometers installed at depths of 0.5 and 0.7 m, and the flux densities were estimated using the Darcy-Buckingham equation. Supplemental irrigation was efficient in maintaining water availability during the crop cycle. Soil water storage was lower for the most productive castor bean cultivar (EBDA MPA 11), with an inverse relationship between yield and water storage, due to its water requirement. The highest and lowest values of internal drainage and capillary rise were found for BRS Nordestina and IAC 226, respectively. EBDA MPA 11 had the best water use efficiency among the castor bean cultivars, differing only from IAC Guarani and EBDA MPB 01.