Bulk elastic moduli and solute potentials in leaves of freshwater, coastal and marine hydrophytes. Are marine plants more rigid?

Bulk modulus of elasticity (epsilon), depicting the flexibility of plant tissues, is recognized as an important component in maintaining internal water balance. Elevated epsilon and comparatively low osmotic potential (Psi(pi)) may work in concert to effectively maintain vital cellular water content. This concept, termed the 'cell water conservation hypothesis', may foster tolerance for lower soil-water potentials in plants while minimizing cell dehydration and shrinkage. Therefore, the accumulation of solutes in marine plants, causing decreases in Psi(pi), play an important role in plant-water relations and likely works with higher epsilon to achieve favourable cell volumes. While it is generally held that plants residing in marine systems have higher leaf tissue epsilon, to our knowledge no study has specifically addressed this notion in aquatic and wetland plants residing in marine and freshwater systems. Therefore, we compared epsilon and Psi(pi) in leaf tissues of 38 freshwater, coastal and marine plant species using data collected in our laboratory, with additional values from the literature. Overall, 8 of the 10 highest epsilon values were observed in marine plants, and 20 of the lowest 25 epsilon values were recorded in freshwater plants. As expected, marine plants often had lower Psi(pi), wherein the majority of marine plants were below -1.0 MPa and the majority of freshwater plants were above -1.0 MPa. While there were no differences among habitat type and symplastic water content (theta(sym)), we did observe higher theta(sym) in shrubs when compared with graminoids, and believe that the comparatively low theta(sym) observed in aquatic grasses may be attributed to their tendency to develop aerenchyma that hold apoplastic water. These results, with few exceptions, support the premise that leaf tissues of plants acclimated to marine environments tend to have higher epsilon and lower Psi(pi), and agree with the general tenets of the cell water conservation hypothesis.