Quantitative knowledge of both photosynthesis and respiration is required to understand plant growth and resulting crop yield. However, especially the nature of the energy demanding processes that are dependent on dark respiration in full-grown tissues is largely unknown. The main objective of the present study was to establish the identity and energy requirements of the most important of these (maintenance) processes, and to gain insight in methods of determining the rates and specific costs of these processes. Developing such methods is more important than obtaining data on the rates of maintenance processes for certain crops, as these rates are likely to vary as a function of e.g. the environmental conditions, developmental stage and species.Leaf respiration rates of 15 potato cultivars (Solanum tuberosum L.) differed significantly (chapter 2). To examine whether growth and maintenance requirements differed, two cultivars were compared. After synchronizing their development, leaf protein content, shoot and leaf respiration, photosynthetic light response curves and the growth parameters (i.e. RGR, LAR, SLA, and LWR) were similar, thus excluding potential differences in growth and maintenance respiration. It was concluded that it is important to study the physiological cause of respiratory differences, before starting to select genotypes for low respiration rate.Protein turnover is generally regarded as an important maintenance process. The component of dark respiration rate associated with overall protein turnover of tissues was quantified in vivo by the use of an inhibitor of cytosolic protein synthesis (chapter 3). The in vivo effect of this inhibitor was assessed by monitoring the inhibition of the induction of the ethyleneforming enzyme activity. The respiratory costs of protein turnover were maximally 17 - 35% of total respiration. The maximum degradation constants (i.e. K d -values) derived for growing and full-grown leaves were up to 2.42 x 10 -6and 1. 12 x 10 -6s -1, respectively.Nocturnal carbohydrate export is another process requiring respiratory energy. The potential contribution of the energy requirements associated with nocturnal carbohydrate export to i) the fraction of dark respiration affected by leaf nitrogen concentration and ii) the dark respiration of mature source leaves, was explored (chapter 4). The estimate of the specific energy cost involved in carbohydrate export (0.70 mol C0 2 [mol sucrose]-1), agrees well with both literature data (0.47 to 1.26) and the theoretically calculated range for starch-storing species (0.40 to 1.20). Maximally 42 to '107'% of the effect of the leaf organic nitrogen concentration on the dark respiration of primary bean leaves, is ascribed to the energy costs associated with nocturnal export of carbohydrates. Total energy costs associated with export account on average for 29% of the dark respiration rate for various starch-storing species.The respiratory energy requirements of maintaining ion gradients were quantified on plant roots (chapter 5). Combining the anion efflux rate (35 neq [g dry weight] -1s -1) with literature data on the specific costs of ion transport, suggests that energy costs associated with re-uptake of ions may account for up to 66% of the total respiratory costs involved in (an)ion influx. A value of 34% of the total respiratory costs involved in (an)ion influx was obtained if the net uptake rate was based on the relative growth rate observed for potato, and assuming phosphate and sulfate to be both 10% of nitrate in- and efflux. Comparison of relative values of the respiration of root and shoot is not useful, as in both tissues other processes add to the total.Estimating the respiratory energy requirements of maintaining ion gradients is complicated by lack of knowledge on efflux kinetics. Therefore, efflux kinetics was studied, using a dynamic simulation model (chapter 6). Simulations showed that the overall efflux kinetics observed in the medium may differ significantly, even if actual efflux rates (and thus costs involved in maintaining ion gradients) in the simulations were equal. Similarly, the relative contribution of ions originally located in the apoplast, cytoplasm and vacuole of different cell layers to these efflux kinetics and the observed cumulative efflux originating from the symplast were different. All these differences were due to the presence or absence of an endodermis, different pathways involved in net uptake and different number of cell layers involved in efflux.Integration of the available knowledge on maintenance, growth and uptake processes enabled to explain the respiration of potato roots. The costs calculated for protein turnover could explain total maintenance requirements (10.2 to 14.8 nmol O 2 [g DW] -1s -1). It was deduced that overall costs for maintaining solute gradients (i.e. re-uptake balancing efflux) account for up to 33% of the overall costs of nitrate influx (i.e. 1/U is up to a factor 1.5 higher if efflux takes place). This agrees well with the results of chapter 5.