Effect of Defoliation on Yield and Dry Matter Remobilization under Various Moisture and Nitrogen Regimes in Bread Wheat

The experiment was aimed at investigating the effects of defoliation
(i. e. amanipulation of source size) at different times and with different intensities on yield, grain protein content and remobilization of dry matter from shoot to grain under various levels of soil moisture and nitrogen in winter wheat. Two separate field experiments were conducted simultaneously under water stress vs irrigated conditions at research farm of College of Agriculture, University of Tehran, Karaj, during 2004-2005 growing season. The experimental design was a factorial one of RCB of 3 replications. A combination of four levels of defoliation (D0= control or intact plants, D1= excision of all leaf blades except flag leaf at booting stage, D2= same as D1 but excision at anthesis stage and D3= removal of all leaf blades at booting stage) along with two levels of nitrogen (none vs 250 kg ha-1 of urea fertilizer applied at jointing stage) were randomly assigned to plot units. Water stress was induced by withholding irrigation from late booting stage until plant maturity. Variance analysis of each experiment revealed that defoliation did not affect grain yield in either of the moisture regimes. Extent and efficiency of dry matter remobilization were promoted by defoliation, predominantly in water stress conditions. However, a clear relationship, between this parameter and grain yield, was not observed. Protein content decreased by defoliation only in drought stress conditions. The effect of nitrogen was significant on grain protein content and on remobilization in stress conditions, increasing both. Results of combined analysis showed the significant effect of defoliation on grain yield. The effect of defoliation depended on the environment, was significant for grain weight and as well for grain protein and remobilization. The effect of nitrogen depended on the environment, was significant for grain protein content, and as well for remobilization. Nevertheless, the highest grain protein percent was observed in intact plants with the application of nitrogen fertilizer in water stress conditions.