Plant and Soil 215: 123–134, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 123 Relatively large nitrate efflux can account for the high specific respiratory costs for nitrate transport in slow-growing grass species Ingeborg Scheurwater 1,∗ , David T. Clarkson 2 , Judith V. Purves 2 , Geraldine Van Rijt 1 , Leslie R. Saker 2 , Rob Welschen 1 and Hans Lambers 1,3 1 Department of Plant Ecology and Evolutionary Biology, Utrecht University, P.O. Box 80084, 3508 TB Utrecht, The Netherlands; 2 Department of Agricultural Sciences, University of Bristol, AFRC Institute of Arable Crops Research, Long Ashton Research Station, Bristol BS18 9SF, UK and 3 Department of Plant Sciences, Faculty of Agriculture, The University of Western Australia, Nedlands WA 6907, Australia Received 4 May 1998. Accepted in revised form 5 March 1999 Key words: diurnal variation, 15 N, net nitrate uptake rate, nitrate efflux, nitrate influx, specific respiratory costs Abstract In this paper we address the question why slow-growing grass species appear to take up nitrate with greater respiratory costs than do fast-growing grasses when all plants are grown with free access to nutrients. Specific costs for nitrate transport, expressed as moles of ATP per net mole of nitrate taken up, were 1.5 to 4 times higher in slow-growing grasses than in fast-growing ones (Scheurwater et al., 1998, Plant, Cell & Environ. 21, 995–1005). The net rate of nitrate uptake is determined by two opposing nitrate fluxes across the plasma membrane: influx and efflux. To test whether differences in specific costs for nitrate transport are due to differences in the ratio of nitrate influx to net rate of nitrate uptake, nitrate influx and the net rate of nitrate uptake were measured in the roots of two fast-growing (Dactylis glomerata L. and Holcus lanatus L.) and two slow-growing (Deschampsia flexuosa L. and Festuca ovina L.) grass species at four points during the diurnal cycle, using 15 NO 3 − . Efflux was calculated by subtraction of net uptake from influx; it was assumed that efflux of nitrogen represents the flux of nitrate. Transfer of the plants to the solution containing the labelled nitrate did not significantly affect nitrate uptake in the present grass species. The net rate of nitrate uptake was highest during the middle of the light period in all species. Diurnal variation in the net rate of nitrate uptake was mostly due to variation in nitrate influx. Variation in nitrate efflux did not occur in all species, but efflux per net mole of nitrate taken up was higher during darkness than in the light in the slow-growing grasses. The two fast-growing species, however, did not show diurnal variation in the ratio of efflux to net nitrate uptake. Integrated over 24 hours, the slow-growing grasses clearly exhibited higher ratios of influx to net uptake than the fast-growing grass species. Our results indicate that the higher ratio of nitrate influx to net nitrate uptake can account for higher specific costs for nitrate transport in slow-growing grass species compared with those in their fast-growing counterparts, possibly in combination with greater activity of the non- phosphorylating alternative respiratory path. Therefore, under our experimental conditions with plants grown at a non-limiting nitrate supply, nitrate uptake is less efficient (from the point of ATP consumption) in slow-growing grasses than in fast-growing grass species. Introduction When herbaceous plants are grown in hydroponics with free access to nutrients they show inherent differ- ences in maximum relative growth rate (RGR) and rate ∗ FAX No: +31 30 251 8366. E-mail: f.i.scheurwater@bio.uu.nl of nitrate uptake (Poorter and Remkes, 1990; Poorter et al., 1991; Garnier, 1992; Van der Werf et al., 1992; Atkin et al., 1996). The net rate of nitrate uptake and the RGR are often closely and positively correlated (Poorter et al., 1991; Garnier, 1991; Van der Werf et al., 1992).