To evaluate the effects of drought stress and timing of nitrogen (N) application on some physiological characters, grain yield and contribution of pre-anthesis photoassimilates to grain of bread wheat (triticum aestivum var Mahdavi), a field experiment was conducted at Research Station of Agricultural Collage, Karaj in 2000-2001. A split plot design based on RCB with four replicates was employed. Soil moisture regime (SMR) considered as main plot, included control (C) and drought stress (S) conditions and timing of N application (TN), as sub plots, included; no nitrogen (N0), 120 Kg/ha nitrogen which was applied as 20% + 80% (N1) and 40% + 60% (N2) at sowing and flowering respectively. The drought stress condition reduced significantly grain yield (GY), biological yield (BY), 1000 seeds weight, harvest index (HI) total leaf area index (TLAI), flag leaf and second leaf area (FSLA) and crop growth rate (CGR). TN affected GY, BY, TLAI and CGR, but did not affect the 1000 seeds weight, HI, and FSLA. The Highest and lowest amount of GY, BY, CGR and TLAI were obtained in N2 and N0 treatments respectively. The interaction between SMR and NT on GY, BY and CGR was significant so that N2 treatment showed the highest yield and CGR in control condition and the lowest ones under stress condition. Correlation between CGR and GY was positive and significant in C and S conditions. The contribution of pre-anthesis photoassimilates to grain (CPPG) was significantly increased under drought stress condition. The effect of NT on CPPG was significant. Minimum CPPG belonged to the N2 treatment. Interaction between SMR and NT on CPPG was significant. Correlation between CGR and grain yield was positively significant in control condition. The grain yield was negatively correlated with CPPG.
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