PLANTING DENSITY EFFECTS ON DRY-MATTER PARTITIONING AND PRODUCTIVITY OF SUNFLOWER HYBRIDS

Dry matter partitioning and yield formation are key points for successful simulation of seed yield. The response of dry matter partitioning and yield of sunflower (Helianthus annuus L.) to plant population was studied in field experiments performed at Cordoba, Spain in 1990 and 1991. Four hybrids (SW-101, Arbung E353, Sungro 385 and S530) were grown under irrigation at plant populations ranging from 0.5 to 10 plants m-2. The hybrids differed in the durations of the emergence-floral initiation (FI) and FI-anthesis periods but not in the duration of seed filling. The responses of biomass, seed number and yield to planting density were dependent on the hybrid. The leaf partitioning coefficient decreased and the head partitioning coefficient increased with increasing plant population, suggesting that the head has a priority for dry matter partitioning. The number of seeds per head changed with plant population through changes in the number of flowers and in the fertility ratio of the central part of the capitulum. Single-seed mass decreased with increasing planting density while the amount of oil per seed was little affected, showing the dependence of oil accumulation on factors other than carbohydrate supply. Seed yield response to planting density followed a saturation-type curve with the plateau around 4.5 t ha-1, indicating that potential yields under irrigation can be achieved by using short-cycle cultivars if plant population is high enough. Increasing potential yield of the sunflower should focus on the improvement of the harvest index of the long-cycle hybrids.