Why do fast- and slow-growing grass species differ so little in their rateof root respiration, considering the large differences in rate of growth and ion uptake?

Herbaceous plants grown with free access to nutrients exhibit inherent diff erences in maximum relative growth rate (RGR) and rate of nutrient uptake. Measured rates of root respiration are higher in fast-growing species than in slow-growing ones. Fast-growing herbaceous species, however, exhibit low er rates of respiration than would be expected from their high rates of gro wth and nitrate uptake. We investigated why the difference in root O-2 upta ke between fast- and slow-growing species is relatively small. Inhibition o f respiration by the build-up of CO2 in closed cuvettes, diurnal variation in respiration rates or an increasing ratio of respiratory CO2 release to O -2 uptake (RQ) with increasing RGR failed to explain the relatively low roo t respiration rates in fast-growing grasses. Furthermore, differences in al ternative pathway activity can at most only partly explain why the differen ce in root respiration between fast- and slow-growing grasses is relatively small. Although specific respiratory costs for maintenance of biomass are slightly higher in the fast-growing Dactylis glomerata L. than those in the slow-growing Festuca ovina L., they account for 50% of total root respirat ion in both species. The specific respiratory costs for ion uptake in the f ast-growing grass are one-third of those in the slow-growing grass [0.41 ve rsus 1.22 mol O-2 mol (NO3-)(-1)]. We conclude that this is the major cause of the relatively low rates of root respiration in fast-growing grasses.