Nitrate Assimilation in Seedlings of Zea mays L.

Abstract

<p> The experiments described in this thesis were performed to investigate the: 1. Role of nitrate in the growth of maize seedling and 2. Role of amino acids in the assimilation of nitrate by the growing embryo.</p> <p> Nitrate stimulated the germination of Zea mays L. seeds by 10 to 15 percent. Further growth of the embryo axis, up to 6 days, however, was not affected by nitrate. During the early growth of the seedling, endosperm nitrogen was able to support the requirements of the embryo for 6 to 8 days. After a lag of 2 days, the protein content of the embryo increased linearly up to 6 days at a rate of 597 μ g a day. Some increase was observed between 6 and 8 days also. After 8 days, the protein level of embryo plateaued. Addition of 10 mM nitrate caused an increase in the protein and total nitrogen of the embryo only after 6 days. The endosperm mutant of maize, opaque-2, also responded to the nitrate in the same way.</p> <p> The protein content of the primary leaves increased linearly between 5 and 7 days. After 7 days, there is no increase in the leaf protein. Nitrate increased the protein level of primary leaves by 25 percent after 7 days. It protected against the further loss of protein in mature leaf. These results suggest that the primary role of nitrate in the growth of maize seedlings is to prevent protein loss.</p> <p> Out of 8 amino acids tested individually, only lysine and to a lesser extent arginine, inhibited the induction of nitrate reductase in the maize root tips. Different ammonium salts had no effect on the induction of nitrate reductase. The initial rate of induction in opaque-2 mutant (high lysine) was lower than the wild type, W64A (low lysine). From a comparison of the rate of induction of nitrate reductase between young and mature leaf, it was suggested that the amino acid supply from the endosperm may inhibit the induction of nitrate reductase. In the young maize seedling, this effect of amino acids may be more effective in vivo and in this way the assimilation of exogenous nitrate could be restricted, when the endosperm amino acids are supporting the growth of the embryo.</p>