Isopiestic technique: measurement of accurate leaf water potentials.

Sunflower leaf tissue of known potential was obtained by equilibrating an interveinal leaf sample, at constant temperature in air, with a potential determined by sucrose solutions. Equilibration occurred within 17 hours. Except for one determination, all measurements of the water potential of the equilibrated samples with an isopiestic technique were within 0.1 bar of the known potential of the tissue. This finding indicates that thermocouple psychrometers can measure accurate values of water potential when an isopiestic technique is used.