An experiment on Syrah in the Ecotron design of the AGRO Montpellier/INRA campus vineyard, with the control of different water regimes, allowed to test the physiological meaning and the field of application of different modalities of leaf water potential measurements : predawn and daily potential for leaves in a normal micro-environment (which keep all of their interest and generality), « stem » potential (which must be called differently and reserved to particular conditions) for shadow enveloped leaves. Field observations on Syrah and Merlot confirm the interest of some methods.