Charles Obiero* and Markus Keller
*Washington State University, 24106 N Bunn Rd, Prosser, WA, 99350 (charles.obiero@wsu.edu)

Irrigation management in winegrape production is progressive and water-conserv­ing due to the widespread use of regulated deficit irrigation, but little information is available to guide growers on irrigation scheduling for different varieties. This study optimized vineyard irrigation management by winegrape variety. A field trial was conducted in 2021 at a WSU research vineyard, Prosser, to evaluate the responses of 30 winegrape varieties to imposed water deficit. Varieties were fully irrigated through bloom, then the soil was subjected to two dry-down cycles to create a gradual soil water deficit. The first cycle began at fruit set and the second, at veraison, following irrigation to replenish soil moisture to near field capacity. There was an approximately two-fold difference in canopy size among the 30 varieties and the soil dried down faster under varieties with bigger canopies. Contrary to expectations, all varieties were isohydric under mild water stress, but became anisohydric as stress intensified below a common soil moisture threshold. While predawn Ψ_l was similar in the two dry-down cycles, midday Ψ_l was consistently (i.e., across the soil moisture spectrum) 0.4 to 0.6 MPa below predawn Ψ_l during the first dry-down but only 0.2 to 0.4 MPa below predawn Ψ_l during the second dry-down. These findings suggest that varietal differences in canopy size are a poor predictor of the physiological behavior of a variety under water deficit and that varieties with bigger canopies may need more frequent irrigation than do varieties with a smaller canopy to maintain the same plant water status.

Funding Support: Washington State Department of Agriculture Specialty Crop Block Grant Program and Washington State Grape and Wine Research Program