Nitrate levels and stages of growth in hypernodulating mutants of Lupinus albus. II. Enzymatic activity and transport of N in the xylem sap

The enzymatic study and transport of N in the xylem sap was carried out wit h a view to observing the influence of different nitrate levels and growth stages of the plant in chemically treated mutants of Lupinus albus. Several stresses induce a reduction in plant growth, resulting in the accumulation of free amino acids, amides or ureides, not only in the shoot, but also in the roots and nodules. Although enzyme activity is decisive in avoiding pr oducts that inhibit nitrogenase by ammonium, little is known about the mech anism by which the xylem carries these products. However, this process may be the key to the function of avoiding the accumulation of amino acids in t he cells of infected nodules. The behaviour of the enzymes nitrate reductas e (NR), phosphoenolpyruvate carboxylase (PEPC), glutamine synthetase (GS) a nd nitrogen compounds derived from fixation, such as N-alpha-amino, N-ureid es and N-amide in mutant genotypes were observed. The NR enzyme activity wa s highly influenced by the application of nitrate showing much higher value s than those in the non-application of nitrate, independently of genotype, being that the NR, the best evaluation period was in the tenth week. The L- 62 genotype characterized with nitrate- resistance, clearly showed that the enzyme PEPC is inhibited by presence of nitrate. The L-135 genotype (nod(- ) fix(-)) showed GS activity extremely low, thus demonstrating that GS is a n enzyme highly correlated with fixation. With regard to the best growth st age for GS, Lupinus albus should be evaluated in the seventh week.