**1.5 Influence on yield and N assimilation**

L−tryptophan, considered as a physiological precursor of auxins in higher plants, was applied to soil to evaluate its influence on yield of several crops. Kucharski and Nowak (1994) found that L−tryptophan did not affect the yield of above ground part and roots of field bean. On the other hand, positive effects on corn and cabbage growth were reported (Sarwar and Frankenberger, 1994; Chen et al., 1997).

Amino acids were used to partially replace NO3 <sup>−</sup> or foliar spray in many plants. In most case, the application of amino acids led to decreased nitrate content and increased total nitrogen content in lettuce, Chinese cabbage, onion, pakchoi or other leafy crops (Gunes et al., 1994, 1996; Chen and Gao, 2002; Wang et al., 2004). Some authors suggested that plants probably preferred amino acids as sources of reduced nitrogen, and nitrate uptake was inhibited by amino acids. In fact, there was little evidence or data to support the conclusions. It has not been distinguished that increased total nitrogen came of nitrate or amino acids.
