A Comparison of Petiole Hydraulics and Aquaporin Expression in an Anisohydric and Isohydric Cultivar of Grapevine in Response to Water-Stress Induced Cavitation.

Megan C Shelden,Brent N Kaiser,Rebecca Vandeleur,Stephen D Tyerman

doi:10.3389/fpls.2017.01893

Abstract

We report physiological, anatomical and molecular differences in two economically important grapevine (Vitis vinifera L.) cultivars cv. Grenache (near-isohydric) and Chardonnay (anisohydric) in their response to water-stress induced cavitation. The aim of the study was to compare organ vulnerability (petiole and stem) to cavitation by measuring ultrasonic acoustic emissions (UAE) and percent loss of conductance of potted grapevines subject to the onset of water-stress. Leaf (ψL) and stem water potential (ψS), stomatal conductance (gs), transpiration (E), petiole hydraulics (KPet), and xylem diameter were also measured. Chardonnay displayed hydraulic segmentation based on UAE, with cavitation occurring at a less negative ψL in the petiole than in the stem. Vulnerability segmentation was not observed in Grenache, with both petioles and stems equally vulnerable to cavitation. Leaf water potential that induced 50% of maximum UAE was significantly different between petioles and stems in Chardonnay (ψ50Petiole = -1.14 and ψ50Stem = -2.24 MPa) but not in Grenache (ψ50Petiole = -0.73 and ψ50Stem = -0.78 MPa). Grenache stems appeared more susceptible to water-stress induced cavitation than Chardonnay stems. Grenache displayed (on average) a higher KPet likely due to the presence of larger xylem vessels. A close relationship between petiole hydraulic properties and vine water status was observed in Chardonnay but not in Grenache. Transcriptional analysis of aquaporins in the petioles and leaves (VvPIP1;1, VvPIP2;1, VvPIP2;2 VvPIP2;3, VvTIP1;1, and VvTIP2;1) showed differential regulation diurnally and in response to water-stress. VvPIP2;1 showed strong diurnal regulation in the petioles and leaves of both cultivars with expression highest predawn. Expression of VvPIP2;1 and VvPIP2;2 responded to ψL and ψS in both cultivars indicating the expression of these two genes are closely linked to vine water status. Expression of several aquaporin genes correlated with gas exchange measurements, however, these genes differed between cultivars. In summary, the data shows two contrasting responses in petiole hydraulics and aquaporin expression between the near-isohydric cultivar, Grenache and anisohydric cultivar, Chardonnay.

Highlights

Grapevines respond to water deficit with a variety of physiological and molecular mechanisms including modifications to the liquid pathways of water movement through the root and shoot, and vapor movement through stomata (Lovisolo et al, 2010)
The objectives of this study were firstly, to compare organ vulnerability to cavitation by measuring xylem cavitation in the petiole and stem, petiole hydraulic conductivity (Kpetiole) and xylem anatomical differences between the two cultivars in response to moderate water-stress; secondly, to determine if aquaporin expression was altered in the petioles and leaves both diurnally and in response to water-stress induced cavitation
Psychrometers recorded water potential every 15 min, oscillations were observed throughout the day (Supplementary Figure S2) indicating the continuously changing water status of the leaf most likely due to localized changes in water availability and stomatal conductance (During and Loveys, 1996)

Summary

Introduction

Grapevines respond to water deficit with a variety of physiological and molecular mechanisms including modifications to the liquid pathways of water movement through the root and shoot, and vapor movement through stomata (Lovisolo et al, 2010). Vulnerability to xylem cavitation is dependent on the hydraulic architecture of plants, a feature that varies between and within many plant species (Tyree et al, 1994; Schultz, 2003; Alsina et al, 2007). This variation is influenced by the segmented structure of dicotyledonous plants that permits hydraulic segmentation of different plant organs. Leaf shedding is known to occur in grapevines under extreme episodes of drought (Keller, 2005)

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