The experiment aimed at investigating the pattern of photoassimilate distribution and grain filling as well as their possible relation to drought resistance in wheat cultivars of different breeding backgrounds. Six improved cultivars of wheat, namely Omid, Roushan and Sardari, improved landraces (IL), as well as Azadi, Ghods and Falat(obtained from hybridization between landraces and foreign genotypes were studied under field conditions, The experimental design was a split plot based on a randomized complete block with four replications, moisture regimes being the main plots and cultivars considered as sub plots. Water stress was imposed from late jointing and continued until maturity. Measurements were taken at three stages of anthesis, 20 days after anthesis (daa) and maturity. A decreasing pattern of leaf dry weight from anthesis to maturity was observed. Water stress (WS) caused decrease in leaf weight at 20 daa, however, the percentage of dry matter allocated to this plant organ was not affected. Stem weight increased from anthesis to 20daa starting to decrease thereafter. WS reduced both absolute stem dry weight and the percentage of assimilates allocated to it. Spikes, accounting for 20 percent of total dry weight at anthesis, contributed to 60 percent of dry weight at maturity. WS did not affect spike dry weight until 20 daa. Significant differences were observed between cultivars in terms of organs (leaf stem and spike) weight and the percentages of assimilate allocated to them. However, a clear relation between these parameters and grain yield or drought resistance was not observed. Developmental changes in grain dry weight as well as in grain filling period and rate were was detected in Falat, Omid and Sardari. WS reduced grain-filling rate but did not affect grain-filling duration. Analysis of grain growth indicated that grain size just before the start of grain filling processes and the rate of filling at the latest stages are two important factors determining final grain weight. Grain Growth curves for old cultivar of Omid and for the new cultivar of Falat exhibited the lowest and the highest standings respectively.
)