THE CARBON COSTS OF ROOT SYSTEMS OF PERENNIAL FRUIT CROPS

The carbon requirements for growth and maintenance of root systems of perennial fruit crops are discussed with reference to the definition and costs of an 'optimum' root system. For mature plants, root growth is shown to be a relatively small sink for carbon, but very sensitive to variations in assimilate supply. The carbon lost due to respiration directly associated with ion uptake is also very small, although it may represent a significant sink at certain times of the year. Root maintenance, however, represents a large sink, perhaps accounting for more than 50% of the carbon utilized within the root system. The carbon losses to root associations (e.g. mycorrhiza) and via leakage are difficult to quantify, but are considered as sinks of intermediate dimensions. Stress effects on root carbon costs are considered, using water (deficit and excess) as an example. Adaptation to stress may involve increased growth of the root system, constituting an immediate carbon cost and subsequently higher maintenance costs for the increased biomass. With increasing likelihood of stress, the 'optimum' root system will have larger dimensions, and hence increased carbon costs. A functional basis for considering the carbon costs of root systems of perennial fruit crops is presented. It enables integration of root system costs in terms of the whole-plant carbon economy.