Journal of Experimental Botany, Vol. 53, No. 374, pp. 1635-1642,
July 1, 2002
© 2002 Oxford University Press
The contribution of roots and shoots to whole plant nitrate reduction in fast- and slow-growing grass species
Received 10 December 2001; Accepted 13 March 2002
1 Department of Plant Ecophysiology, Utrecht University, PO Box 80084, 3508 TB Utrecht, The Netherlands
2 Zoology Department, PO Box 56, University of Otago, Dunedin, New Zealand
3 Plant Sciences, Faculty of Agriculture, The University of Western Australia, Crawley WA 6009, Australia
4 Department of Biology, University of York, PO Box 373, York YO1 5YW, UK
Abbreviations: cu, specific respiratory costs for ion transport; cg, specific respiratory costs for growth; NRA, nitrate reductase activity; NRAroot,max, maximum rate of root NO3– reduction; NNUR, net NO3– uptake rate; Proot, proportion of total plant NO3– reduction that occurs in roots; RGR, relative growth rate; RMR, root mass ratio; rt,CO2, total rate of root CO2 evolution with or without NO3– reduction being performed in the roots; rm, specific respiratory costs for cellular maintenance.
The hypothesis was tested that slow-growing grass species perform a greater proportion of total plant NO3– reduction in their roots than do fast-growing grasses. Eight grass species were selected that differed in maximum relative growth rate (RGR) and net NO3– uptake rate (NNUR). Plants were grown with free access to nutrients in hydroponics under controlled-environment conditions. The site of in vivo NO3– reduction was assessed by combining in vivo NO3– reductase activity (NRA) assays with biomass allocation data, and by analysing the NO3– to amino acid ratio of xylem sap. In vivo NRA of roots and shoots increased significantly with increasing NNUR and RGR. The proportion of total plant NO3– reduction that occurs in roots was found to be independent of RGR and NNUR, with the shoot being the predominant site of NO3– reduction in all species. The theoretical maximum proportion of whole plant nitrogen assimilation that could take place in the roots was calculated using information on root respiration rates, RGR, NNUR, and specific respiratory costs associated with growth, maintenance and ion uptake. The calculated maximum proportion that the roots can contribute to total plant NO3– reduction was 0.37 and 0.23 for the fast-growing Dactylis glomerata L. and the slow-growing Festuca ovina L., respectively. These results indicate that slow-growing grass species perform a similar proportion of total plant NO3– reduction in their roots to that exhibited by fast-growing grasses. Shoots appear to be the predominant site of whole plant NO3– reduction in both fast- and slow-growing grasses when plants are grown with free access to nutrients.
Key words: Key words: Nitrate uptake, nitrate reductase, nitrogen, nitrogen assimilation, relative growth rate.
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