Yield and soil water uptake of sunflower sown in spring and summer

Sunflower (Helianthus annuus L.) is both tolerant to water deficits and capable of bearing high yield in response to irrigation inputs. In both situations, knowledge of the seasonal distribution of water uptake and the quantification of water extracted in deeper soil layers are required for irrigation scheduling. In a 2-yr field study in southern Italy, we determined water consumption in relation to seed yield of 'Romsun HS90' sunflower with early (spring; mid-April) and late (summer; mid-June) sowing dates under five irrigation regimes: a rainfed control; periodic replacement of calculated crop evapotranspiration (a meteorological approach); and one, two, or three irrigation applications at selected growth stages assumed critical for yield expression (a phenological approach). Spring-sown crops provided significantly higher yields (3.15 vs. 1.75 t ha-1). Irrigation was necessary to achieve high production, especially for the summer crop. Three waterings gave best results in 1989 only. The treatment providing two waterings always showed similar yield and more efficient use of water with respect to the treatment irrigated according to the meteorological approach and performed better than the single irrigation. The ratio between actual and reference crop evapotranspiration was 20% higher in the period between R1 and R6 for the summer crop and 30% higher for the spring crop, with respect to the adopted crop coefficients (Kc). All the yield components were positively affected hy irrigation; the most important one was found to be the diameter of calathide. Oil concentration in the achenes was always lower in the summer crop and increased when adequate irrigation was provided, especially in the grain-filling stage.