Aims: Leaf (Ψl) and stem (Ψstem) water potentials were measured on grapevines to determine the effects of shoot location on both methods to assess vine water status.
Methods and results: Cabernet-Sauvignon and Merlot used in this study were grown at two locations in California. Measurements were taken at midday in July (Merlot) and at two times of the day (morning and afternoon), on two dates in August (Cabernet-Sauvignon). Measurements of Ψl and Ψstem, stomatal conductance and transpiration were taken on shoots entirely exposed to direct solar radiation or on shoots totally in the shade at the times of measurement. There were significant differences (P < 0.05) between Ψl and/or Ψstem measured on shoots exposed to direct solar radiation and those in the shade. Both Ψl and Ψstem were significantly greater on the shoots exposed to direct sunlight compared to those in the shade. There was no significant difference between Ψl measured on shaded leaves and Ψstem determined on the fully exposed shoots.
Conclusions: Regardless of method used, water potentials were highly correlated with stomatal conductance measured on leaves in direct sunlight at the same time. All means of measuring grapevine water potential used in this study were highly correlated with one another.
Significance and impact of the study: The data indicate that any of the techniques used in this study would be a sensitive indicator of vine water status and that the Ψ of shaded leaves would be an alternative to the measurement of Ψstem.
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