Water Relations of Seagrasses

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Abstract
The stationary volumetric elastic modulus (εs) of the leaf cells of three seagrasses (Halophila ovalis (R.Br.) Hook, Zostera capricorni Aschers, and Posidonia australis Hook f.) was evaluated from estimates of εs plus intracellular osmotic pressure (εs + IIi) and IIi. The estimates of (εs + IIi) were made using a linear displacement transducer to measure very small changes in thickness of leaf tissue produced by changes in external osmotic pressure (IIo). εs increases with increasing turgor pressure in each of the species and the maximum values of εs are: 22 megapascals for H. ovalis, 17 megapascals for Z. capricorni, and 51 megapascals for P. australis. There is a hysteresis in thickness changes versus changes in IIo which indicates a hysteresis in the relationship between volume and turgor pressure. The hysteresis results in εs being different for swelling and for shrinking cells over the same range of IIo and this may be important in other aspects of plant-water relations. A new design of an apparatus employing a linear displacement transducer for measuring very small changes in tissue thickness is described. The new design has the advantages of virtually frictionless movement and a precision of 0.05 micrometer.