In this study yield and some morpho - physiological traits of twenty bread wheat lines were compared in a split plot RCB based design with four replications in Karaj¬Iran. Main factor consisted of two irrigation levels: with and without moisture stress, The sub - factor being composed of twenty wheat lines. Statistical analysis indicated that there were significant differences between these lines, and all traits were negatively affected by drought stress. Grain yield was significantly reduced, due to much reduction in grain weight in each spike itself due to drought stress during the grain filling period. The results indicated that, for higher grain yield where there is stress conditions, spike number per unit area, as well as seed number per spike should be increased. This would compensate for yield reduction coming from lower mean grain weight. In addition, because of moisture limitation for harvest index increase, straw to grain yield should be reduced too. To obtain higher grain yield under normal conditions, seed number per spike, number of spikes per unit area, and thousand grains weight should be increased respectively. Since there is no moisture limitation under no drought conditions, it might be better to select late maturity cultivars to increase vegetative period. Regarding direct and negative effect of plant height on harvest index, the foliage growth should be so increased as no lodging would occur while the harvest index being kept high.
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