Direct Measurement of Turgor and Osmotic Potential in Individual Epidermal Cells

The pressure probe, which is routinely used to measure the turgor potential (Psi(p)) of individual epidermal cells in Tradescantia virginiana (L.), has also been used to sample small volumes of vacuolar fluid from these same cells (as low as 0.02 nl) for measurement of cellular solute (osmotic) potential (Psi(s)) in a micro freezing point osmometer. The water potential components Psi(p) and Psi(o) have been used to calculate the total water potential of individual epidermal cells (Psi(cell)) which has then been directly compared to the total leaf water potential (Psi(leaf)) measured psychrometrically. The relation of Psi(leaf) and Psi(cell) to leaf transpiration indicates that in T. virginiana, a relatively straightforward relation exists between the level of water flow through the leaf tissue, and the DeltaPsi within the leaf, between two points along the water flow pathway. Substantial agreement was found between the two independent, in situ methods of measuring Psi when extrapolated to zero transpiration conditions. These results are discussed with respect to the thermodynamics of water transport in plant tissues.