Non-Steady-State Water Flow for Three Desert Perennials With Different Capacitances

Abstract

Non-steady-state water flow through plants was modelled using an electric circuit analog incorporating capacitance (change in water volume per unit change in water potential). Predictions of leaf water potential agreed with measurements for a C₄ grass with a small capacitance, Hilaria rigida, and for a C₃ shrub with an intermediate capacitance, Encelia farinosa. Predictions differed from measured stem water potentials for a stem succulent having crassulacean acid metabolism and a large capacitance, Ferocactus acanthodes, presumably because of daily variations in osmotic pressure. As capacitance increased, minimum shoot water potential and the maximum water uptake rate by the roots lagged further behind maximum transpiration rate. Predicted daily water uptake by roots was equal to daily water loss for H. rigida and E. farinosa, but not for F. acanthodes for which capacitance effects were particularly important. Because tissue volumes were large, water flow for F. acanthodes would be expected to reach the steady state only if conditions were constant for about 2 days, hence steady-state flow is not expected in the field. For all three species, capacitance was largely determined by total water volume rather than by the bulk elastic modulus.