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

Abstract

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 C3 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.