Distribution of nitrate reductase activity in nodulated lucerne plants

Nitrogen fixing plants of lucerne (Medicago sativa L. cv. Aragón) were grown in a glasshouse for three months in the absence of nitrate, and then supplied with 5 mM KNO₃ for a week. In control (non-nitrate fed) plants, nitrate reductase activity (NRA EC 1.6.6.1) was detected only in nodules. After nitrate supply, root NRA showed a transient increase. Shoot NRA increased with time, paralleling changes in nitrate distribution; stem NRA represented nearly 50% of total NRA in plant tissues. Total nitrogen, expressed on a dry weight basis, tended to decrease in shoots upon nitrate supply. Bacteroid NRA (EC 1.7.99.4) showed a great variation depending on Rhizobium meliloti strains, ranging from 5 to 40% of total plant NRA. However, different Rhizobium strains did not give different results in terms of plant growth parameters, nitrate or organic nitrogen content.