Effect of nitrogen fertilizer levels on grain yield and grain growth related traits of wheat genotypes under post-anthesis heat stress conditions

To study the effect of nitrogen levels on grain yield and grain growth of wheat genotypes under optimum and post-anthesis heat stress conditions, two separate field experiments were conducted in delayed and optimum sowing dates in Ahvaz, Iran in 2006 - 2008 cropping seasons. The experimental site had a mild winter and dry, hot summer. Wheat genotypes in delayed sowing date experienced heat stress in post anthesis. Each experiment was arranged as split plot in randomized complete block design with three replications. The N application rates were (50, 100, and 150 kg.ha-1) assigned to the main-plots. Sub-plots consisted of six bread and durum wheat genotypes. Results indicated that in both optimum and delayed sowing date, grain yield decreased significantly as nitrogen level decreased. The effect of N treatments on 1000-grain weight (TGW), effective grain filling rate (EGFR) and effective grain filling period (EGFP) was not significant. The longest and the shortest EGFP under optimum (26 days) and in delayed sowing date (13 days) belonged to cv. Star, respectively. High temperature during grain filling period, enhanced EGFR (15%) and reduced TGW (31%) and EGFP 40%). Therefore, although the rate of assimilate accumulation increased under stress conditions, but this enhancement could not compensate EGFP reduction and led to 31% reduction in TGW. Results also showed thatgenotypes that headed late (Star and D-84-5) generally had higher reduction in TGW and EGFP than those genotypes headed earlier under post-anthesis heat stress conditions.