The physical implications of the various parameters obtained from a pressure-bomb study are explored and related to their possible ecological significance. Our analysis suggests that the original bulk osmotic pressure, the bulk osmotic pressure at incipient plasmolysis, and cell wall elasticity are closely associated with the extent to which a leaf can osmoregulate or conserve water within a certain range of water potential change in the environment and might therefore have certain adaptive value. The pressure–volume relation could be applied to predict changes in leaf water potential at various degrees of water loss in the field. The values of these various parameters were obtained from a pressure-bomb study on single leaves from a wide variety of species. The use of data from single leaves as compared with whole shoots is discussed.

The Scholander–Hammel pressure bomb has been used to measure ontogenetic and seasonal changes in π0 (the osmotic pressure of the symplasm at zero water potential), πp (the osmotic pressure of the symplasm at ‘incipient plasmolysis’), εmax (the bulk elastic modulus near maximum turgor), and a number of other water relations parameters in single leaves of Acer saccharum and several species of Populus and in shoots of Tsuga canadensis and Picea abies. In newly emerged leaves of Acer, Populus, and Picea, π0, πp, and εmax are all small but rise rapidly with leaf development. These parameters stabilize at a maximum value or slowly increase with progress in season. In Acer, εmax declines shortly before senescence. In developing leaves, the water content reaches a maximum before the soluble solutes; this accounts for the low values of π0 and πp.In Tsuga π0 cycles through an annual maximum in winter and a minimum in summer. These changes may correlate with frost hardiness.

Pressure–volume curves were constructed from well defined models and hypothetical shoots in which reasonable values of osmotic pressure and cell wall elastic moduli were specified for cell types of different relative volumes. The pressure–volume curves so obtained closely resembled those of real shoots and leaves. Comparing the bulk parameters obtained from analysis of the constructed pressure–volume curves with the values defined in the models allows us to examine the sources of error in their evaluation. The graphical values of the original bulk osmotic pressure and of the total volume of osmotic (symplasmic) water agreed very well with those defined; however, the osmotic pressure at incipient plasmolysis and the bulk elastic moduli estimated from the graph were generally lower than their actual values originally used in the models. We show that the apparent linear dependence of the bulk elastic modulus of sitka spruce reported by Hellkvist et al. (1974) may not reflect a similar linearity for the elastic moduli of individual cells.

The tempo of water efflux from single Fagus grandifolia leaves has been measured. The resistance to pressure-driven water efflux has been measured for normal leaves, Rs, and for leaves in which extracellular mesophyll spaces are infiltrated with water, Rs*. The ratio Rs*/Rs is about 0.4. The resistance to water flow through the xylem, Rx, was also measured and found to be a small part of Rs, i.e., Rx/Rs = 0.08. The activation energies for water efflux from normal and infiltrated leaves are both about 26 ± 4 kJ/mol.After an analysis of our data, we conclude that the membrane hydraulic conductivity of F. grandifolia leaves is roughly 10−6 cm s−1 bar−1 (1 bar = 100 kPa) and that water travels the shortest path between the cell sap and the nearest xylem vessel, flowing in and out of mesophyll cells through the areas in contact between adjacent cells along the pathway.

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