Root water uptake, leaf water storage and gas exchange of a desert succulent: Implications for root system redundancy

A technique used for hydroponics was adapted to measure instantaneous root water uptake from the soil for a leaf succulent CAM species, Agave deserti. Comparisons were made to previously modelled water fluxes for A. deserti and to Encelia farinosa, a non-succulent C-3 species. Net CO2 uptake and transpiration for A. deserti under well-watered conditions occurred primarily at night whereas root water uptake was relatively constant over 24 h. Leaf thickness decreased when transpiration commenced and then increased when recharge from the stem and soil occurred, consistent with previous models. A drought of 90 d eliminated net CO2 uptake and transpiration and reduced the water content of leaves by 62%. Rewetting the entire root system for 7 d led to a full recovery of leaf water storage but only 56% of maximal net CO2 uptake. Root water uptake was maximal immediately after rewetting, which replenished root water content, and decreased to a steady rate by 14 d. When only the distal 50% of the root system was rewetted, the time for net CO2 uptake and leaf water storage to recover increased, but by 30 d gas exchange and leaf water storage were similar to 100 % rewetting. Rewetting 10 or 20 % of the root system resulted in much less water uptake; these plants did not recover leaf water storage or gas exchange by 30 d after rewetting. A redundancy in the root system of A. deserti apparently exists for daily water uptake requirements under wet conditions but the entire root system is required for rapid recovery from drought. (C) 1999 Annals of Botany Company.