No 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.

Limitations to photosynthesis were examined for Encelia farinosa Toney et A.Gray, a common C3 sub-shrub in arid regions of south-westem United States, for plants grown in full sunlight and those shaded to 40% of full sunlight. The initial slopes of CO2 assimilation (A) versus intercellular CO2 concentration curves were similar for sun and shade plants at low photosynthetic photon flux density (PPFD) but higher for sun plants as the PPFD increased, indicating a greater limitation by carboxylation capacity in shade plants. Sun plants had higher electron transport rates but a lower ratio of electron transport capacity to carboxylation capacity (Vmax); the ratio was inversely proportional to mesophyll conductance for both sun and shade plants. Dark respiration decreased with decreasing leaf water potential (Ψ1) in sun plants but remained unchanged in shade plants; day respiration was little affected by PPFD for both sun and shade plants. Stomatal conductance (gs) was similar for sun and shade plants under high soil-moisture conditions but higher in sun plants as Ψ1 decreased; for all data considered together, changes in the leaf-air vapour pressure difference accounted for 71% of the variation in gs. The lower A for shade plants of E. farinosa apparently resulted from a lower Vmax, as well as a lower gs when plants were under water stress.

The leaf transpiration efficiency (A/E, where A is the assimilation rate and E the transpiration rate) is widely used to evaluate plant responses to the environment, yet little attention has been paid to its relationship with vapour pressure deficit (D), the driving force for E. The proposed model is based on the increasingly recognised linear relationship between the ratio of intercellular to ambient CO2 partial pressures (cI/ca) and D. Unlike previous models for A/E, the proposed model does not assume that the leaf and air temperatures are the same or that ci/ca is constant. A/E predicted by the model agreed with that measured for the C3 Encelia farinosa and the C4 Pleuraphis rigida, common species in the north-westem Sonoran Desert, based on gas exchange measured in the field and in environmental chambers. The dependency of cI/ca and A/E on D was additionally evaluated using published data for five other C3 species and two other C4 species. Generally, ci/ca was more sensitive to changes in D for the C4 species than the C3 species. The predictions for A/E by the model were also compared with predictions using a constant ci/ca, either a general cI/ca (0.7 for C3 and 0.3 for C4) or a species-dependent mean cI/ca. Overall, the proposed model performed best for both the C3 and C4 species; using the general cI/ca always resulted in an over-prediction of A/E.