Nitrate reductase activity in roots and shoots of aquatic macrophytes

Aquatic macrophytes grow in an environment where nitrogen can be available at quantitatively significant concentrations around both roots and shoots. Because of the generally higher concentration of NO 3 − in the bulk water compared to the sediment, and because of the energetic advantage of reducing NO 3 − in shoots compared to roots, aquatic plants were expected primarily to reduce NO 3 − in shoots. To test this hypothesis, nitrate reductase activity (NRA) of roots and shoots was measured in vitro on 18 aquatic macrophytes, representing 12 species. The plants were collected at seven locations with bulk water [NO 3 −] ranging from 0 to 550 μM and interstitial water [NO 3 −] ranging from 2 to 400 μM. All plants exhibited low NRA (