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Physiological Changes in Plant Hydraulics Induced by Partial Root Removal of Irrigated Grapevine (Vitis vinifera cv. Syrah)

¹ Department of Viticulture and Enology, University of California, Davis, CA 95616; ² Agroscope RAC Changins, Station Fédérale de Recherches Agronomiques, Centre Viticole du Caudoz, Pully, Switzerland.

^* Corresponding author [email: drsmart{at}ucdavis.edu; fax: 530 752-0382]

The influence of partial root removal on hydraulic conductivity of the root system, vine water status, stomatal conductance to water vapor (g_s), net photosynthetic carbon assimilation (A), and recharging of hydraulic capacitance overnight for 5-year-old Syrah grapevines growing on 101-14 Mgt rootstock was examined. The study was conducted in a drip-irrigated vineyard on deep sandy clay loam soils under the conditions of high evapotranspiration demand. Severing approximately 25 to 35% of the total cross-sectional area of framework roots emerging from the trunk resulted in an immediate (one week) decline in predawn leaf water potential (_PD), xylem water potential (_stem), and leaf water potential (_leaf) and was paralleled by reductions in g_s and A. In addition, there was an immediate decline in leaf-specific hydraulic conductance (k_l), indicating that the root system was well balanced with shoot water demands before root severance. These responses were sustained throughout the growing season, although some convergence in g_s, A, and k_l was observed toward the end of the season. Severing roots from the drip-irrigation zone had a slightly greater influence on _PD, _stem, _leaf, g_s, and A, suggesting there was greater hydraulic resistance to water transport and diminished ability to refill xylem or to recharge hydraulic capacitance from nonirrigated soils. Root severance resulted in adjustments to leaf area per vine in 2004, but no such adjustment to leaf area was observed in 2005, and vine water status among the three treatments was similar throughout the 2005 season. Our results indicated that root severance could change water relations under field conditions even for well-watered vines in deep soils with a high water-holding capacity but that grapevines probably have limited quantitative stem and root hydraulic capacitance.

Key words: grapevine, root pruning, root capacitance, gas exchanges, stomatal conductance, water storage, leaf-specific hydraulic conductance

Abbreviations: A: net photosynthetic carbon assimilation; A_max: maximum net photosynthetic rate at ambient CO₂ concentration; E: transpiration rate; C_a: external CO₂ partial pressure; C_i: intercellular CO₂ partial pressure; g_s: stomatal conductance to water vapor; g_max: maximum stomatal conductance to water vapor; k_l: leaf-specific hydraulic conductance (whole plants); PAW: plant available water; r_{m min}: minimum mesophyll resistance to CO₂; intrinsic WUE: intrinsic water use efficiency (A/g_s); _PD: predawn leaf water potential; _leaf: midday leaf water potential; _stem: midday stem water potential; VPD: leaf-air vapor pressure deficit

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