Estimating age-dependent costs and benefits of roots with contrasting life span: Comparing apples and oranges

The relation between root age and root function is poorly understood, despite its importance to root longevity. The effect of root age on respiration rates and ³²P-uptake kinetics was determined for roots excavated from mature apple and citrus trees (median root life spans of 30 vs 300 d). To evaluate whether root longevity maximizes the efficiency of nutrient capture, daily and lifetime efficiencies were calculated by dividing simulated P-uptake benefits (solute transport model) by age-specific respiration costs. We found that: respiration rates and P uptake capacity change with root age in a species-specific way; and soil characteristics that determine the rate of nutrient depletion in the rhizosphere are as important as changes in root physiology in determining the age at which a root reaches its maximum efficiency. Further testing of the efficiency of nutrient capture as a predictor of root life span requires measurement of both soil properties and age-specific physiology of roots including their mycorrhizal fungi.