ENVIRONMENTAL-CONTROL OF POTENTIAL YIELD OF SUNFLOWER IN THE SUBTROPICS

A simple framework was used to analyse the determinants of potential y ield of sunflower (Helianthus annuus L.) in a subtropical environment. The aim was to investigate the stability of the determinants crop dur ation, canopy light interception, radiation use efficiency (RUE), and harvest index (HI) at 2 sowing times and with 3 genotypes differing in crop maturity and stature. Crop growth, phenology, light interception , yield, prevailing temperature, and radiation were recorded and measu red throughout the crop cycle. Significant differences in grain yield were found between the 2 sowings, but not among genotypes within each sowing. Mean yields (0% moisture) were 6 . 02 and 2 . 17 t/ha for the first sowing, on 13 September (S1), and the second sowing, on 5 March (S2), respectively. Exceptionally high yields in S1 were due to high b iomass assimilation associated with the high radiation environment, hi gh light interception owing to a greater leaf area index, and high RUE (1 . 47-1 . 62 g/MJ) across genotypes. It is proposed that the high R UE was caused by high levels of available nitrogen maintained during c rop growth by frequent applications of fertiliser and sewage effluent as irrigation. In addition to differences in the radiation environment , the assimilate partitioned to grain was reduced in S2 associated wit h a reduction in the duration of grain-filling. Harvest index was 0 . 40 in S1 and 0 . 25 in S2. It is hypothesised that low minimum tempera tures experienced in S2 reduced assimilate production and partitioning , causing premature maturation.