Abstract
Bulk modulus of elasticity (ɛ), depicting the flexibility of plant tissues, is recognized as an important component in maintaining internal water balance. Elevated ɛ and comparatively low osmotic potential (Ψπ) 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 Ψπ, play an important role in plant-water relations and likely works with higher ɛ to achieve favourable cell volumes. While it is generally held that plants residing in marine systems have higher leaf tissue ɛ, to our knowledge no study has specifically addressed this notion in aquatic and wetland plants residing in marine and freshwater systems. Therefore, we compared ɛ and Ψπ 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 ɛ values were observed in marine plants, and 20 of the lowest 25 ɛ values were recorded in freshwater plants. As expected, marine plants often had lower Ψπ, 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 (θsym), we did observe higher θsym in shrubs when compared with graminoids, and believe that the comparatively low θsym 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 ɛ and lower Ψπ, and agree with the general tenets of the cell water conservation hypothesis.
Highlights
The flexibility of plant tissues, as characterized by bulk modulus of elasticity (1), is an important component of plant–water relations (Steudle et al 1977; Joly and Zaerr 1987)
Cell wall elasticity and osmotic potentials (Cp) are closely linked to changes in cell volume (e.g. DV/DC 1⁄4 V/ (1 + comparatively low osmotic potential (Cp)); Tyerman 1982), and it is possible that rigid tissues can limit dehydration as both low Cp and high 1 together are effective means of maintaining vital cellular water content (Cheung et al 1975; Bartlett et al 2012)
We argue that for S. robustus, S. patens and H. wrightii to survive in these highly dynamic euryhaline environments, like estuarine macroalgae, maintaining flexible cell walls could be advantageous provided cytoplasmic ion content remains low and/or Cp remains high
Summary
The flexibility of plant tissues, as characterized by bulk modulus of elasticity (1), is an important component of plant–water relations (Steudle et al 1977; Joly and Zaerr 1987). Cell wall elasticity and osmotic potentials (Cp) are closely linked to changes in cell volume (e.g. DV/DC 1⁄4 V/ (1 + Cp); Tyerman 1982), and it is possible that rigid tissues can limit dehydration as both low Cp and high 1 together are effective means of maintaining vital cellular water content (Cheung et al 1975; Bartlett et al 2012) This concept, termed the ‘cell water conservation hypothesis’, likely plays an important role in marine and coastal plants that accumulate ions or other solutes as an osmoregulatory response to elevated environmental salts. One study found that three of five freshwater perennials receiving drought had decreased 1 and/or increased Cp (Touchette et al 2007)
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