Protein turnover is generally regarded as one of the most important maintenance processes in plants in terms of energy requirements. In this study, the contribution of protein turnover to the respiratory costs for maintenance in the roots of two grass species, the fast-growing Dactylis glomerata L. and the slow-growing Festuca ovina L., is evaluated. Plants were grown under controlled-environment conditions in a nutrient solution to which NO3- was added at a relative addition rate of 0.2 and 0.1 mol N mol(-1) N already present in the plant d(-1) for D. glomerata and F. ovina, respectively, so as to obtain a steady exponential growth rate close to the plants' maximum relative growth rate. Pulse-chase labelling with C-14-leucine was used to determine the rate of protein turnover in the grass roots. The rate of turnover of the total protein pool did not differ significantly between the two species. The protein degradation constant in D, glomerata and F. ovina was 0.156 and 0.116 g protein g(-1) protein d(-1), respectively, which corresponds with a total protein half-life of 4d and 6d. Assuming specific respiratory costs for protein turnover of 148 mmol ATP g(-1) protein, the estimated respiratory costs for protein turnover in the roots were 2.8 and 2.4 mmol ATP g(-1) root DM d(-1) in D. glomerata and F. ovina, respectively. Both the fast- and the slow-growing grass spent between 22-30% of their daily ATP production for maintenance on protein turnover, which corresponds to 11-15% of the total root ATP production per day. Note that the data presented in this abstract are based on the assumption that 50% recycling of the C-14-labelled leucine took place in the roots of both grass species.