Garret Stahl, Russ Smithyman, Markus Keller, and Bhaskar Bondada* 
*Washington State University, 2710 Crimson Way, Richland, WA 99354 (bbondada@wsu.edu)

Physiological detection methods for grapevine water status are used to schedule irrigation, but many of these processes are often laborious and time-intensive. Stomatal conductance to water vapor, estimated with a hand-held leaf porometer, provides rapid estimates of vine water status and is thus an ideal tool in irrigation scheduling. Stomatal conductance (mmol/m2 s) and midday leaf water potential (MPa) of varieties Merlot and Chardonnay were recorded from bloom until fruit maturity. During this time, vines were subjected to varying levels of water stress. Midday leaf water potential and stomatal conductance differed between varieties and as the level of water stress increased, both stomatal conductance and midday leaf water potential decreased. A linear relationship between the two measurements existed, indicating that stomatal conductance responds in a similar manner to midday leaf water potential in these varieties and thus could be used as an irrigation scheduling tool. The abaxial density of stomata (stomata/mm2) also differed between varieties and in Chardonnay, was influenced by degree of water stress. The density of stomata in combination with the phenomenon stomatal patchiness should be considered when using stomatal conductance to schedule irrigation. Stomatal patchiness, or non-uni-form stomatal closure, leads to large fluctuations of stomatal conductance occurring in discrete patches across the leaf surface. Multiple readings per leaf may be required to accurately estimate stomatal conductance.

Funding Support: Northwest Center for Small Fruit Research