YIELD AND SOIL-WATER UPTAKE OF SUNFLOWER SOWN IN SPRING AND SUMMER

Sunflower (Helianthus annuus L.) is both tolerant to water deficits an d capable of bearing high yield in response to irrigation inputs. In b oth situations, knowledge of the seasonal distribution of water uptake and the quantification of water extracted in deeper soil layers are r equired for irrigation scheduling. In a 2-yr field study in southern I taly, we determined water consumption in relation to seed yield of 'Ro msun HS90' sunflower with early (spring; mid-April) and late (summer; mid-June) solving dates under five irrigation regimes: a rainfed contr ol; periodic replacement of calculated crop evapotranspiration (a mete orological approach); and one, two, or three irrigation applications a t selected growth stages assumed critical for yield expression (a phen ological approach). Spring-sown crops provided significantly higher yi elds (3.15 vs. 1.75 t ha(-1)). Irrigation was necessary to achieve hig h production, especially for the summer crop. Three waterings gave bes t results in 1989 only. The treatment providing two waterings always s howed similar yield and more efficient use of water with respect to th e treatment irrigated according to the meteorological approach and per formed better than the single irrigation. The ratio between actual and reference crop evapotranspiration was 20% higher in the period betwee n R1 and R6 for the summer crop and 30% higher for the spring crop, wi th respect to the adopted crop coefficients (K-c). All the yield compo nents were positively affected by irrigation; the most important one w as found to be the diameter of calathide. Oil concentration in the ach enes was always loner in the summer crop and increased when adequate i rrigation was provided, especially in the grain-filling stage.