A mechanical interpretation of pressure chamber measurements-what does the strength of the squeeze tell us?

Argument still continues about what properties of a plant organ the pressure chamber measures. A mechanical (as opposed to a thermodynamic) analysis is made of the system squeezed by the pressurized gas, the non-gaseous part of the leaf. The boundary of the system is defined so that it remains at constant mass, and constant density is assumed, during the squeeze. This is equivalent to assuming constant volume. On those assumptions, it is shown that the liquid is brought to the cut surface by a change of shape of the system. Generic mechanical principles are then used to deduce a priori, a quantitative interpretation of the balance pressure. The formal mechanical interpretation involves two variables, the interfacial tension and the change in surface area, which cannot currently be measured. Instead of these, we used two related variables which can be measured, the mass fraction of water in the leaf (Q) and the maximum mass fraction of water at full saturation (Qx) to deduce an approximate mechanical interpretation. When Q is close to Qx, we deduced that the balance pressure (Pb) required for the shape change should be approximately proportional to the reduction in mass in changing from Qx to Q, a variable called the relative water loss (RWL). The constant of proportionality (κ) is a basic characteristic of the type of leaf used, and the final relation, Pb = κ (RWL) is called Relation A. We then deduce that the constant κ should be an approximately linear function of Qx. The linear function is defined by limiting values, so that when Qx is 1, κ is predicted to be 0 bar, and at the other extreme, when Qx is 0, κ is predicted to be in the range 500-1000 bar. This is called Relation B. Experiments with 32 leaves from 10 species are used to test the mechanical interpretation. The results showed that Relation A was a reasonable approximation for most of the tested leaves. The data for 10 species, were used to estimate Relation B, confirming that as Qx approached 1, κ did approach 0 bar as predicted, and that as Qx approached 0, κ approached ~750 bar, consistent with the a priori prediction of 500-1000 bar. The relations were also successfully tested using independent published data. An estimate of Qx is shown to be of considerable practical value in (a) converting Pb to water status and vice versa; (b) characterizing leaf morphology and composition; and (c) rationalizing quantitatively the functional classes of xerophytes, mesophytes and hygrophytes. The assumption of constant density inside the outer boundary of the non-gaseous material cannot be guaranteed, and when this is violated, our (or any other) interpretation of Pb is unreliable. Investigation of the conditions under which this assumption is invalid should be a high priority.