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.

The influence of shade and clouds on soil water potential: The buffered behavior of hydraulic lift

“…Water storage in the plant may provide a substantial fraction of the daytime transpirational demand (e.g. Hunt and Nobel, 1987a), buffer daily fluctuations in plant water potential (Hunt and Nobel, 1987b), and, thus, be important for plants in arid environments. Inclusion of such water storage in the model, however, should simply introduce a time lag into predicted soil water potential changes (e.g., Hunt and Nobel, 1987a).…”

“…Hunt and Nobel, 1987a), buffer daily fluctuations in plant water potential (Hunt and Nobel, 1987b), and, thus, be important for plants in arid environments. Inclusion of such water storage in the model, however, should simply introduce a time lag into predicted soil water potential changes (e.g., Hunt and Nobel, 1987a). Moreover, indirect evidence suggests that, for Artemisia tridentata, water storage within the plant is insignificant (Caldwell et al, 1991).…”

Time constant for water transport in loblolly pine trees estimated from time series of evaporative demand and stem sapflow

“…Although the effect of hydraulic capacitance and resistance in causing time lags between tree canopies and stems has been a subject of several studies over the last 2 decades (Edwards et al 1986;Hunt and Nobel 1987;Hunt et al 1991;Landsberg et al 1976;Powell and Thorpe 1977;Running 1980 a, b;), there remains a critical need to measure time lags in water movement through plants for the purpose of estimating canopy transpiration and conductance from stem sapflow data (Diawara et al 1991;Granier and Loustau 1994). The time constant for transport between canopy and stem may be defined as the time taken for stem water flow to reach 67% of its steady state value when subjected to a step change in canopy transpiration.…”

Interactions Between Seedlings of Agave Deserti and the Nurse Plant Hilaria Rigida

“…10 m and transpiration rates are close to zero (Hunt and Nobel 1987c), maximum Oroot was equated to the pre-dawn Osoil at 0.10 m. Hourly values of uproot were then cal-culated based on maximum and minimum 1nroot, and assuming that the same pattern of changes occurred as for 1stem, which was measured hourly. For the root water uptake model, the minimum 4 root was therefore set at 1.01 MPa below 41soil at a soil depth of 0.10 m (the mean root depth).…”