the effect of planting date on partitoning coefficient in some species of wheat

in order to determine probable effects of sowing dates on partitioning coefficient of dry matter to different organs (leaf, stem and reproductive organs) in different wheat varieties (koohdasht, zagros and tajan) an experiment with crbd design and four replications was conducted. 5 sowing dates were included (29 december, 2007, 30 january, 2 march, 27 march, 1may, 2008). in order to quantify partitioning coefficients of different organs, dry matter of each organ was measured separately during 10 stages of important phenological stages and related dry matter of each organ was plotted against rest of dry matter. form these plots, phenological inflection points in respect to dry matter partitioning were determined. our results indicated that emergence to booting, booting to anthesis and anthesis to maturity are three distinct periods that partitioning coefficients can be determined. sowing dates did not change these periods, but partitioning coefficients were affected by sowing dates. these coefficients varied among three different phases. leaf coefficients for first, second and third phases varied among 0.3-0.54, 0.17-0.26 and 0.019-0.046 respectively (whit a decreasing trend). partitioning coefficients of reproductive organs for the same periods were 0.0-0.017, 0.13-0 and 0.28-0.5 respectively (whit an increasing trend). partitioning coefficients of stem also varied from 0.36-0.57. our results confirmed that the effects of environmental variables (temperature and photoperiod) should be considered to quantify yield of wheat by using partitioning coefficients.