Irrigation Effects on Plant Water Relations and Productivity of Thompson Seedless Grapevines

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Abstract
An optimum irrigation management strategy requires information on the yield loss associated with quantified field water deficits and the ability to assess the adequacy of irrigation intensity during the growing season. This study was conducted to develop a water‐production function for grape (Vitis vinifera L.) and to establish plantbased water‐status measurement techniques and critical values that assess irrigation regime adequacy. Field studies were conducted for 3 yr with ‘Thompson Seedless’ grapevine on Panoche clay loam (fine‐loamy, mixed [calcareous], thermic Typic Torriorthent) and Hanford sandy loam (coarse‐loamy, mixed, nonacid, thermic Typic Xerorthent) in the San Joaquin Valley of California. Differential irrigation quantities were supplied in several increments ranging from 40 to 120% of the expected nonstressed crop evapotranspiration (ETcrop). Midday measurements of leaf water potential (ΨL), water stress index (CWSI), and stomatal conductance (g1) closely associated with imposed water deficits and the resulting grape yields. All plant‐based water‐status‐reflecting measurements were highly correlated. Grape yields followed declining water availability essentially linearly over the range of available data. A 50% reduction in ETcrop resulted in a 26% yield loss. Irrigation intensities that resulted in ΨL > −0.9 MPa, CWSI < 0.2, and gs > 0.008 m s−1 maintained high grape productivity. These results can be used to profit‐maximize irrigation water as a production input.