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58 p r a c t i c a l w i n e r y & v i n e ya r d J U ly 2 0 1 4 g r a p e g r o w i n g ever, it can be very subjective and not easy to quantify. • vine : Measurements of vine can be done using pressure chamber. The cost of the instrument is relatively low, and the technique is relatively easy to grasp. Previous studies have shown that stem water potential ( stem ) measured with bagged leaves at solar noon is less vari- able than midday leaf water potential ( leaf ), 17 and it is more consistently related to irrigation treatments. 6 • Shoot growth rate: Tissue expansion is very sensitive to even mild water stress; 8 therefore, decreasing or zero shoot growth rate (vigor) indicates water stress. In practice, shoot growth rate is easy to measure and can be used as a good indicator with repeated measure- ments. However, due to the sensitivity of shoot growth, it would not be applicable for more severe water stress once the growth has stopped. • Leaf gas exchange and sap flow rate: A less sensitive physiological process to water stress is leaf gas exchange, which includes stomatal conductance (g s ) and rates of transpiration (E) and photosyn- thesis (A). 8 Like all plants, vaporized H 2 O diffuses out from (as transpiration) and CO 2 diffuses into (to be utilized in photo- synthesis) grapevine leaves through the pores on their surface (stomata). Sap flow rates essentially measure the transpira- tion rates on a whole-vine basis. In general, water stress decreases sto- matal aperture, and thus g s , E, A and sap 17 16 15 14 13 12 11 10 9 8 117 BB FS V H 146 173 187 201 Day of the year Soil moisture (%) 214 227 249 263 276 Figure 2. Changes in soil moisture (%) are shown throughout the growing season in a vineyard block where vines were irrigated at 100% vine evapotranspiration (et c ). Crop coefficient was calculated based on growing degree days. Phenological stages are indicated above the x- axis as bud break (BB), fruit set (FS), véraison (v) and harvest (H). dashed line indicates the hypothetical changes in soil moisture if 100% of the actual et c is applied to the soil.