Growth response of cool tolerant soybean to variation in sowing date.

Sowing date trials were conducted in two seasons (1999/2000 and 2000/2001) at the same site at Lincoln University, Canterbury to determine the effect of sowing date on soybean dry matter accumulation, partitioning, and seed growth rate and duration, and their relationship with seed yield. Three sowing dates (15 November and 7 and 29 December) and four cultivars (Northern Conquest, March, Maypole and Alta) were used in 1999/2000 and four sowing dates (2 and 17 October, 1 and 16 November) and two cultivars (Northern Conquest and March) were used in 2000/2001. The maximum total dry matter (TDMmax) was attained around the start of seed filling (R5) in all cultivars for all sowing dates. In both seasons TDMmax was strongly associated with seed yield; both were significantly reduced for the 1999 December sowings. Seed growth rate (ISGR) and seed filling period (SFP) were also affected by sowing date; the highest ISGR (5.1 mg seed -1 day-1) and lowest SFP (33.2 days) occurred for the 15 November sowing in 1999, while the highest ISTG and lowest SFP occurred for the 1 November sowing in 2000. WMAGR and ISGR also had significant linear relationships with seed yield, and SGR was strongly associated with WMAGR. However, seed yield was poorly associated with SFP and the partitioning coefficient. The factors that influence WMAGR and hence TDMmax in soybean also regulate seed yield.