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 NO3- in the bulk water compared to the sediment, and because of the energetic advantage of reducing NO3- in shoots compared to roots, aquatic plants were expected primarily to reduce NO3- 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 [NO3-] ranging from 0 to 550 muM and interstitial water [NO3-] ranging from 2 to 400 muM. All plants exhibited low NRA (<2 mumol NO2- g(-1) DW h(-1)) in both roots and shoots, except for the amphibious species, Cardamine amare, where shoot NRA reached 24 mumol NO2- g(-1) DW h(-1). Higher NRA was however inducible, increasing 2-17 times in five selected species when induced with 500 muM NO3-. In 11 out of 17 plants with measurable NRA, the shoot:root NRA ratio was <1, showing that despite the proposed advantages of shoot NO3- reduction, root reduction is apparently still of importance in aquatic plants. (C) 2003 Elsevier Science B.V. All rights reserved.