The effect of planting date on duration of phonological phases in wheat cultivars and it's relation with grain yield

In order to study the effect of planting date on duration of phenological periods in wheat cultivars, a randomized compelet block design experiment with four replications was conducted in Gorgan University of Agricultural Science and Natural Resources research station. The factors were eight sowing dates (14 December, 20 January, 18 February, 20 March, 16 April, 21 May, 30 July and 6 August) and seven cultivars (Aria, Atrak, Koohdasht, Shirudi, Tajan, Taro and Zagros). Final grain yield was measured and phenological stages were monitored by 20 and 10 individuals, respectively. The results revealed that the days from planting to emergence and emergence to stem elongation were different significantly. Correlation coefficients of emergence rate against temperature indicated a strong correlation in all cultivars (R²≥0.9^**). In this research, both temperature and photoperiod affected stem elongation rate. Minimum of days to emergence and stem elongation were observed in 30 July (3.17 and 23.33 days) and 6 August (3.07 and 19.93 days). Increasing of the days from emergence to stem elongation in the first sowing date in comparison with last sowing dates caused the coincidence of optimum leaf area production with the highest incoming radiation to canopy. This was one of the most important factors which affected yield variability between different sowing dates (changed from 4773 Kg ha^-1 to 370 Kg ha^-1 when sowing date was delayed from 14 December to 21 May). Also, results revealed that days from emergence to stem elongation is so critical to determine grain yield, as had significant correlations with yield (R²≥0.84^**) and LAI (R²≥0.79^**).