Sunflower Seed Weight and Oil Concentration under Different Post‐Flowering Source‐Sink Ratios

Sunflower (Helianthus annuus L.) seed weight and oil concentration are commonly related to post-flowering source of assimilates (e.g., leaf area index duration, LAD). A predictive variable including both the source of assimilates and the sinks (i.e., seed number) would better account for seed weight and seed oil concentration variability of crops with contrasting seed number and canopy size. We established quantitative relationships between oil weight per seed components and post-flowering source-sink ratio. Field experiments were conducted in Argentina from 1998 to 2001. Four hybrids were cultivated under contrasting plant populations and nutrient supplies. A wide range of LAD (913-3130 m 2 °Cd m -2 ), seed number (4270-8880 seeds m -2 ), and seed weight (41-62 mg) was recorded. In contrast, seed oil concentration was not modified (about 530 mg g -1 ). Post-flowering source-sink ratio (LAD per seed) better accounted (r 2 = 0.69) for seed-weight prediction than LAD (r 2 = 0.42). Maximum seed weight (60 mg) was attained with source-sink ratios ≥0.33 m 2 °Cd seed -1 . Results from our data set pooled together with others of different agro-ecological regions reveal that sunflower crops are normally growing under limiting post-flowering source-sink ratios and a 47% reduction of seed weight occurs when post-flowering source-sink ratio is dramatically (100%) reduced. Seed weight is only 23% increased at saturated source-sink ratios. In contrast, for the wide range of post-flowering source-sink ratios analyzed, seed oil concentration did not vary.