This instrument has 80 light sensors placed at one-centimeter intervals along a linear probe. A bubble-level was used to maintain the probe level for all AMB and CAN readings. All recorded AMB and CAN values were the mean of five replicates taken within 10-15 sec. A sun-exposed leaf was detached and immediately inserted into the pressure chamber with the petiole protruding. The chamber was pressurized with CO2 and the amount of pressure required to visibly force sap out through the end of the petiole was recorded.
Higher pressures indicate greater water bonding and greater water stress in the vine.
Vines under chronic water stress generally have smaller leaves and shorter shoots. LWP measurements were made on the center three vines from each sampling group. Three leaves were sampled per vine. Measurements were made under clear skies on 25-26 August 1997 within one hour of solar noon. Measurements were made under cloudy skies and a manila folder was placed in the approximate solar path to provide further shielding from diffuse illumination.