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Description
Two post-veraison irrigation levels were applied to a Sauvignon blanc vineyard (1 and 3.5 mm/day for the L and H treatments, respectively). Four weeks after veraison, net photosynthesis measurements were taken using a portable gas exchange system. Leaf water potential (LWP) was determined as well. Average midday LWP's were -1.5 and -1.9 MPa in the H and L treatments, respectively. Net photosynthesis decreased and stomatal resistance (rs) increased as LWP decreased where no differences between treatments were detected. This may be due to similar magnitude of osmotic adjustment. The significant increase in nonstomatal resistance from morning to mid-day (as was found in both treatments) well explains the reduction in water use efficiency in response to decreasing LWP'S. A significant increase in rs accompanied by an increase in both LWP and vapor pressure gradient (VPG) indicate a possible direct effect ofVPG on stomatal resistance. Prichard, T. L. (Veihmeyer Hall, University of California, Davis, CA 95616). Volume balance approach to quality winegrape irrigation. Water management has long been recognized as having a significant influence on both yield and quality components of winegrapes. Winegrapes differ from other perennial crops in that water management is commonly used as a tool to influence fruit maturation and must composition. Conventional estimation of perennial crop water use relies on reference evapotranspiration (ET 0) and crop coefficients (K), which varies with growth stage. For winegrapes, this approach over-estimates vine water use required for maximum crop performance. The practice of inducing water deficits requires an innovative approach to estimating vine water use. Proposed is the use of a term accounting for the severity and timing of water deficits called performance coefficient (K ) in addition to the use ofboth ET 0's and K's. K values, Pdetermined experimentally, vary from 1.0 iii early season to 0.30 near harvest. Once sufficient water use is estimated, a simple-to-use, non-computer volume balance method of irrigation scheduling is suggested. This method balances sufficient water use with available soil stored moisture, in-season rainfall and irrigation.
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