The effects of alfalfa mosaic virus (AMV) on growth, nodule formation and nodule function in the annual burr medic, Medicago polymorpha cv. Circle Valley, were investigated in glasshouse pot experiments. Systemically-infected plants from AMV-infected seed produced 21% less shoot dry weight and accumulated 24% less fixed nitrogen in shoots than healthy plants when harvested 53 d after germination. At day 75, infected plants showed similar shoot dry weight losses (22%), but the quantity of nitrogen fixed fell by only 15%. At day 53, soluble sugar, starch and bacteroid concentrations in nodules were unaffected by AMV infection, but nitrogenase specific activity was decreased by 25% and soluble amino acids by 20%.
Although AMV infection resulted in no differences in the number of nodules formed in the first 11 d after germination or at any harvest thereafter, nodule mass was decreased by 23% for virus-infected plants at day 53. However this difference disappeared by day 75. Growth of AMV-infected plants was decreased probably because of impaired N2 fixation with nodule function affected rather than nodulation. Increased nodule mass relative to plant weight in virus-infected plants, seen at day 75, implied some degree of compensation for the limitation in N2-fixing capacity. ELISAs for AMV antigen indicated that nodules were active sites of virus multiplication.