Water costs and water revenues for established and rain-induced roots of Agave deserti

1 Production of new roots leads to increased water uptake and increased respiration. Water uptake can be considered as the revenue from root production, and root respiration divided by the water-use-efficiency as the water cost. Using an analogy to economics, the optimal root mass occurs when the marginal cost of root production equals the marginal revenue of root production. 2 Agave deserti Engelm. has two types of roots, established and rain-induced; the latter were shed three times as the soil dried during year considered. Per unit dry mass, respiration required for growth was 90-fold higher and respiration required for maintaining living tissue was 40-fold higher for rain roots compared with established roots. Conductance to water flow per unit dry mass was 11-fold higher for rain roots compared with established roots. 3 Previous excavations showed that A. deserti can have an established-root length of 44 +/- 8 m (mean +/- SD) and a rain-root length of 13 +/- 3m. Optimal root lengths predicted for this plant were 51 m of established roots and 31 m of rain roots, which were within 1 SD of the measured lengths for established roots but greater than 6 SD for rain roots. Thus, water may be the currency for the production of established roots but not for rain roots.