Evaluation of a non-destructive method for measuring turgor pressure in Helianthus

The portable instrument described by Heathcote, Etherington, and Woodward (1979) for the non-destructive measurement of turgor pressure was evaluated in Helianthus annuus and Helianthus paradoxus. A good correlation was obtained between turgor pressure measured with the instrument and turgor pressure estimated by the pressure-volume technique for individual leaves allowed to dry after excision; however, variation in both the intercept and slope of the relationship occurred between leaves. Consequently, there was no correlation between the output of the instrument for individual leaves and the turgor pressure of the same leaves estimated by conventional methods. Moreover, for a given leaf, the instrument had only a limited ability to detect temporal variation in turgor pressure when compared with turgor pressure calculated from measured values of leaf water potential and leaf osmotic potential. The instrument's output was influenced by its proximity to major veins and by leaf thickness. We conclude that variability in leaf thickness and the presence of large veins limits its usefulness for measurement of turgor pressure in Helianthus.