Response of sunflower to strategies of irrigation. I. Growth, yield and the efficiency of water-use

The progressive growth and water-use of twelve treatments of sunflower subjected to a range of irrigation sequences were measured throughout their growing season. The treatments included late-start and early=stop sequences, strategic irrigation and controls that were either irrigated weekly to maintain potential evapotranspiration or were reliant on rainfall. In the 2 years of experimentation, growing-season rainfall and Class-A pan evaporation were 121, 92 and 804, 798 mm respectively. The two control treatments set the boundaries to yield which ranged between 1.1 and 4.5 t/ha all at 48.3% oil content. The efficiency of water-use in the production of seed (EEs) ranged from 4.0 kg/ha mm−1 for the rainfed control to 8.0 kg/ha mm−1 which was achieved by a treatment in which a 2-weekly watering schedule was maintained from budding to maturity. Other strategically irrigated treatments also had water-use efficiencies in excess of that of the weekly irrigated control. Analysis of the seasonal progression of water-use shows that the major contributor to higher water-use efficiency in the sub-optimally irrigated treatments was a reduction in soil evaporation in which the responsiveness of leaf area to water supply played an important role. The efficiency of use of transpired water in the production of biomass (ETb) was constant across all but one treatment in which weekly irrigation was commenced at budding. In this treatment there was a significant (P<0.05) increase in ETb relative to the other crops (24.3 vs. 20.8 kg/ha mm−1). Early vegetative growth was readily manipulable by delayed irrigation which conserved water for the seed filling period and improved yield by an increase in harvest index.