Summary: Stomata of grapevine leaves respond to the evaporative demand ofthe attnosphere and to
changes of soll water. Leaf epidermis and roots are regarded as sensors of air and soll humidity. Besides a
hydraulic communication between soll and leaf, non·hydraulic signals represent a metabolic communication
between roots and Stomata: Stomatal conducrance, and thereby the rate of CO assirnilation, of turgid vine leaves
declined when pan ofthe roots was subjected to progressively drying soil. 2
Under water stress conditions stomata of leaves reduce transpiration and fully account. for putative non·
Stomatal inhibition of CO assirnilation. They enable vine leaves to optimize their CO uptake to water loss ratio
( water use efficieny) und~r sometimes rapidly changing a mbient and internal concfitions. A close correlation
between CO assirnilation and stomatal conduetance indicates a precise funcrioning of Stomatal acrion and
thereby a high water use efficiency; this correlation coefficient which is generally high in grapevines was
demonstrated to increase under water stress conditions. As a screening, the CO assirnilation to stomatal
conduetance ratio and the CO assirnilation to transpiration ratio provide valuable2 inforrnation on the water
economy of grapevine varieties futder drought conditions.