ROOT-SYSTEM WATER-UPTAKE AND RESPIRATION FOR AGAVE-DESERTI - OBSERVATIONS AND PREDICTIONS USING A MODEL BASED ON INDIVIDUAL ROOTS

Water uptake and root respiration for the desert succulent Agave deserti Engelm. were predicted in the laboratory and in the field using a model based on root hydraulic conductivity (LP) and the rate of maintenance respiration previously determined in the laboratory for individual roots as non-linear functions of root age, soil temperature (Tsoil), and soil water potential (Ψsoil). Root carbon dioxide loss and shoot transpiration were measured in the laboratory for plants with root systems in solution and in water-saturated soil, where Tsoil was varied from 10°C to 40°C. In the field, soil was also varied (from 11°C to 28°C) while Ψsoil was maintained at -0.01 or ≤-5 MPa, or rsoil was allowed to oscillate naturally while Ψsoil declined from -0.05 to -0.89 MPa over a 4-d period. After each experimental treatment, roots were divided into 15 age classes (1 week to 3 years), and the root xylem water potential was measured. Averaging over all experiments for the entire root systems, the predicted total daily water uptake was 102% ±4% of the observed transpiration, and the predicted total daily root respiration was 96%±5% of that observed (mean ± s.e. mean, for n = 11 d). Thus, the respiratory maintenance cost of root system water acquisition can be accurately predicted based on the behaviour integrated for individual roots. © 1991 Annals of Botany Company.