Nitrate Absorption and Assimilation in Ryegrass as Influenced by Calcium and Magnesium

The absorption and assimilation patterns of (15)NO(3) (-) supplied as the Ca(2+) and Mg(2+) salts to intact ryegrass (Lolium perenne) seedlings were compared. No statistically significant effect of ambient cation on the amounts of (15)NO(3) (-) absorbed was observed in the initial six hours, but during the subsequent six hours, absorption from Ca((15)NO(3))(2) exceeded that from Mg ((15)NO(3))(2).Lower rates of (15)NO(3) (-) assimilation were found in roots exposed to Mg((15)NO(3))(2) than in those exposed to Ca((15)NO(3))(2). It was proposed that Mg(2+) initiated a restriction in (15)NO(3) (-) reduction in roots, probably as a consequence of a Mg(2+)-induced physiological Ca(2+) deficiency. Lower (15)N translocation rates were also observed from Mg((15)NO(3))(2). These effects of Mg(2+) in depressing (15)NO(3) (-) assimilation and translocation occurred prior to an effect on (15)NO(3) (-) uptake.In shoots, larger amounts of reduced (15)N products occurred with Ca((15)NO(3))(2) than with Mg((15)NO(3))(2). It was concluded that this was due to enhanced translocation of (15)NO(3) (-) (and possibly its reduced products) in presence of Ca(2+) rather than to specific cation effects on (15)NO(3) (-) assimilation in the shoots.