Krista Shellie* and Brad King
*U.S. Department of Agriculture, Agricultural Research Service, 29603 U of I Lane, Parma, ID 83660 (krista.shellie@ars.usda.gov)

Enhancement of irrigation water use efficiency and water productivity in arid winegrape production regions is hindered by a lack of automated, real-time methods to monitor and interpret vine water status. A normalized water stress index calculated from real-time vine canopy temperature measurements has been shown in winegrape to correspond well with irrigation amounts and events under arid conditions; however, little is understood about how to interpret water stress index values and the relationship of index values with other commonly used indicators of vine water status. The objective of this study was to relate canopy temperature-based water stress index values for the winegrape cultivars Syrah and Malbec with seasonal measurements of water productivity and carbon isotope composition and to compare daily index values with daily measurements of midday leaf water potential, leaf gas exchange, and leaf chlorophyll fluorescence. Own-rooted vines of each cultivar were grown in replicated plots under arid conditions in an experimental vineyard in Parma, Idaho, and over two growing seasons were supplied with either 35 or 70% of their estimated water demand. The daily water stress index decreased rapidly in response to irrigation events and the amounts of decrease correlated with the amounts of irrigation. In both cultivars, vines irrigated at 35% had higher water stress index values, higher water productivity, and less negative ¹³C/¹²C in the sugar of mature berries than vines irrigated at 70%. The relationship between the daily stress index and midday leaf water potential was stronger for Syrah than for Malbec. Findings from this study facilitate interpretation of normalized water stress index values and show that canopy temperature is a sensitive indicator of vine water status. Measurement of canopy temperature and calculation of the water stress index is amenable to automation for use as a decision-support tool in a precision irrigation system.

Funding Support: ARS project no. 2072-21000-047-00D, Idaho State Department of Agriculture Specialty Crop Block Grant