Estimating age-dependent costs and benefits of roots with contrasting lifespan: comparing apples and oranges

The relation between root age and root function is poorly understood, despi te its importance to root longevity. The effect of root age on respiration rates and P-32-uptake kinetics was de termined for roots excavated from mature apple and citrus trees (median roo t life spans of 30 vs 300 d). To evaluate whether root longevity maximizes the efficiency of nutrient capture, daily and lifetime efficiencies were ca lculated by dividing simulated P-uptake benefits (solute transport model) b y 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 ro ot physiology in determining the age at which a root reaches its maximum ef ficiency. Further testing of the efficiency of nutrient capture as a predictor of roo t life span requires measurement of both soil properties and age-specific p hysiology of roots including their mycorrhizal fungi.