No abstract

Hydraulics of plants that have different strategies of stomatal regulation under water stress are relatively poorly understood. We explore how root and shoot hydraulics, stomatal conductance (g s), leaf and root aquaporin (AQP) expression, and abscisic acid (ABA) concentration in leaf xylem sap ([ABA] xylemsap) may be coordinated under mild water stress and exogenous ABA applications in two Vitis vinifera L. cultivars traditionally classified as near-isohydric (Grenache) and near-anisohydric (Syrah). Under water stress, Grenache exhibited stronger adjustments of plant and root hydraulic conductances and greater stomatal sensitivity to [ABA] xylemsap than Syrah resulting in greater conservation of soil moisture but not necessarily more isohydric behavior. Correlations between leaf (leaf) and predawn (PD) water potentials between cultivars suggested a "hydrodynamic" behavior rather than a particular iso-anisohydric classification. A significant decrease of leaf in well-watered ABA-fed vines supported a role of ABA in the soil-leaf hydraulic pathway to regulate g s. Correlations between leaf and root AQPs expression levels under water deficit could explain the response of leaf (K leaf) and root (Lp r) hydraulic conductances in both cultivars. Additional studies under a wider range of soil water deficits are required to explore the possible differential regulation of g s and plant hydraulics in different cultivars and experimental conditions.

The role of some aquaporins as CO permeable channels has been controversial. Low CO permeability of plant membranes has been criticized because of unstirred layers and other limitations. Here we measured both water and CO permeability (P , P ) using stopped flow on plasma membrane vesicles (pmv) isolated from Pisum sativum (pea) and Arabidopsis thaliana leaves. We excluded the chemical limitation of carbonic anhydrase (CA) in the vesicle acidification technique for P using different temperatures and CA concentrations. Unstirred layers were excluded based on small vesicle size and the positive correlation between vesicle diameter and P . We observed high aquaporin activity (P 0.06 to 0.22 cm s ) for pea pmv based on all the criteria for their function using inhibitors and temperature dependence. Inhibitors of P did not alter P . P ranged from 0.001 to 0.012 cm s (mean 0.0079 + 0.0007 cm s ) with activation energy of 30.2 kJ mol . Intrinsic variation between pmv batches from normally grown or stressed plants revealed a weak (R = 0.27) positive linear correlation between P and P . Despite the low P , aquaporins may facilitate CO transport across plasma membranes, but probably via a different pathway than for water.