Osmometer

Abstract: Osmotic pressures (II) of aqueous solutions of polyethylene glycols (PEGs) of average relative molecular weight (M(r)) between 200 and 10,000 were measured using vapor pressure deficit osmometry. The relationships between molarity and II were described with high precision by second order polynomials for each of the PEGs studied. In contrast to previous reports, equivalent weights of different polymers in solution did not generate the same II; low M(r) PEGs generated a higher II than the higher M(r) PEGs. The effect of PEGs upon II represents an interaction between concentration and M(r).

Abstract: 1. Across plant species, drought tolerance and distributions with respect to water availability are strongly correlated with two physiological traits, the leaf water potential at wilting, that is, turgor loss point (ptlp), and the cell solute potential at full hydration, that is, osmotic potential (po). We present methods to determine these parameters 30 times more rapidly than the standard pressurevolume (pv) curve approach, making feasible community-scale studies of plant drought tolerance. 2. We optimized existing methods for measurements of pi o using vapour-pressure osmometry of freeze-thawed leaf discs from 30 species growing in two precipitation regimes, and developed the first regression relationships to accurately estimate pressurevolume curve values of both pi o and pi tlp from osmometer values. 3. The pi o determined with the osmometer (pi osm) was an excellent predictor of the pi o determined from the pv curve (pi pv,r2 = 0.80). Although the correlation of pi osm and pi pv enabled prediction, the relationship departed from the 1 : 1 line. The discrepancy between the methods could be quantitatively accounted for by known sources of error in osmometer measurements, that is, dilution by the apoplastic water, and solute dissolution from destroyed cell walls. An even stronger prediction of pi pv could be made using pi osm, leaf density (rho), and their interaction (r2 = 0.85, all P l 2 x 10-10). 4. The pi osm could also be used to predict pi tlp (r2 = 0.86). Indeed, pi osm was a better predictor of pi tlp than leaf mass per unit area (LMA; r2 = 0.54), leaf thickness (T; r2 = 0.12), rho (r2 = 0.63), and leaf dry matter content (LDMC; r2 = 0.60), which have been previously proposed as drought tolerance indicators. Models combining posm with LMA, T, rho, or LDMC or other pv curve parameters (i.e. elasticity and apoplastic fraction) did not significantly improve prediction of pi tlp. 5. This osmometer method enables accurate measurements of drought tolerance traits across a wide range of leaf types and for plants with diverse habitat preferences, with a fraction of effort of previous methods. We expect it to have wide application for predicting species responses to climate variability and for assessing ecological and evolutionary variation in drought tolerance in natural populations and agricultural cultivars.

Abstract: The hematocrit method as a technique for determining red cell volume under anisotonic conditions has been reexamined and has been shown, with appropriate corrections for trapped plasma, to provide a true measure of cell volume. Cell volume changes in response to equilibration in anisotonic media were found to be much less than those predicted for an ideal osmometer; this anomalous behavior cannot be explained by solute leakage or by the changing osmotic coefficient of hemoglobin, but is quantitatively accounted for by the hypothesis that 20 per cent of intracellular water is bound to hemoglobin and is unavailable for participation in osmotic shifts.