Measurement of tissue osmotic pressure.

Osmotic pressure measured by a modified pressure-volume method was compared with that of the mixed sap expressed from frozen and thawed tissue. The error in the latter technique averaged 11 and 16% (too dilute) for greenhouse and field leaves of Zea mays L. at all growth stages. These errors were not consistently calculated by a model of simple mixing between the matric and osmotic fractions, both of which decreased with plant age. Some other alternatives to the pressure-volume method are discussed which are based on a more rapid estimate of the zero turgor point.