Determining the Optimal Amount of Irrigation Water for Borage at Different Growth Stages Using a Central Composite Design

Despite Gilan Province being a rainy region in Iran, inappropriate distribution of rainfall during the growing season has a negative impact on the growth of crops, including borage. This study was conducted to determine the proper irrigation regime for borage at different stages of growth. For this purpose, fifteen field experiments, based on 15 combinations obtained from the Central Composite Design, were conducted using high and low levels of treatments including water requirement (without irrigation (I1) and full water supply (I2)), two growth stages (stem elongation (D1), stem elongation and flowering (D2)), and two years with different rainfall (drought (Y1) and wet season (Y2)). The experiments were performed in Giulan Agricultural and Natural Resources Research and Education Center located in Rasht. Irrigation and stage of growth had the most and the least effects on borage yield and growth parameters, respectively. Dry season increased the grain yield, oil yield and oil percentage, while other parameters reached their maximum value in wet year. The value of 1000-seed weight for D2 was equal to 18.9 g, and the values of dried flower yield, fresh flower yield, seed yield, and economic yield are equal to 468, 858, 486 and 983 kg.ha-1, respectively. Oil yield and oil percentage for D2 were equal to 160 kg.ha-1 and 33.5%, respectively. Considering the optimal levels for all parameters based on the interpolation of factor values in the central composite design, if the percentage and yield of oil is less important than the other parameters, it is necessary to provide 88% of irrigation water for the stem and flowering stages in the wet season. In order to achieve maximum yield and oil content, full supply of irrigation needs in dry season and in the stem stage should be considered.