Investigating Leaf Water Potential Using Freezing Point Depression and Dielectric Permittivity.

Irrigation of many tree and vine crops is done under deficit conditions where knowledge of plant water status is important. Scientists have tried a variety of approaches to make this measurement, from thermocouple psychrometers to 2D pressure jacks, but none have performed well enough to gain general acceptance.  Thus, researchers and agronomists who need these measurements are left with the cumbersome Scholander pressure chamber.  More than 30 years ago, researchers described a freezing-point technique that would quickly and accurately measure leaf water potential using the well-known relationship between the two.  Although results from the work showed promise, the electronics and power required to make the measurement prohibited the instrument from going to the field.  Modern electronics make a field instrument possible.  The prototype device integrates a small cooling element, a thermocouple, and a miniaturized capacitive dielectric sensor that combine to cool the leaf while monitoring its temperature and the state of water.  Results showed a spike in sample temperature along with a change in the dielectric as the leaves froze, indicating water liquid forming ice with the resultant spike in temperature due to the latent heat of freezing. Values of water potential derived from the freezing point data were lower than expected based on pressure chamber results, indicating possible correlations with osmotic rather than to total water potential, but more work is require to confirm these results.