Response of sunflower (Helianthus annuus L.) to sowing date and plan density under Mediterranean conditions

The productivity of rainfed sunflower under Mediterranean conditions depends strongly on the water availability and the efficiency of its utilisation by the crop. Amongst other factors, sowing date and plant density may interact decisively with water supply. Two field trials were conducted in the South of Portugal on a Vertisol to evaluate the effect of these two factors on the productivity of several sunflower cultivars with different growth cycle. The study of the effect of the sowing date was carried out over 4 years and comprised four cultivars and four sowing dates with the last one corresponding to the traditional sowing date. The second trial included 3 sowing densities (2.5, 5.0 and 7.5 seeds m-2) and three cultivars, and was performed over 2 years. An early sowing date led to a simultaneous increase of leaf area duration (LAD) and water uptake during the critical period between flower bud appearance and flowering. The earlier sowing dates also increased the number of seeds per area without decreasing its weight, resulting in higher crop yields. Although significant, the difference in crop yields between densities was low in absolute terms. Crop yield was highest with the medium plant density (P2: 3.5 plants m-2). Whereas LAD between flower bud appearance to flowering and flowering to maturity increased from the lowest (P1: 1.7 plants m-2) to the highest density (P3: 4.6 plants m-2), the water uptake did not vary significantly between densities during these two periods and decreased even slightly for the highest plant density. The lowest plant density increased significantly the mean seed weight in relationship to the other two plant densities, but that was not enough to compensate the significant decrease in seeds m-2. The higher mean seed weight of P2 was sufficient to compensate the slightly lower number of seeds m-2 in relation to P3.