Osmotic potential and turgor maintenance in Spartina alterniflora Loisel.

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Abstract
The dependence of leaf water potential (Ψ), osmotic potential (π) and turgor pressure (P) on relative water content (RWC) was determined for leaves of tall and short growth forms of Spartina alterniflora Loisel. from a site on Canary Creek marsh in Lewes, Delaware. Tall plants (ca. 1.5 m) occured along a drainage ditch where interstitial water salinity was approximately 20‰, and short plants (ca. 0.2 m) were 13 m away near a pan and exposed to 80‰ salinity during the most stressful period. Leaves were collected at dawn and pressure-volume measurements were made as they desiccated in the laboratory. Pressure equilibrium was used to measure Ψ, RWC was determined from weight loss and dry weight, π was determined from the pressure volume curve, and P was calculated as the difference between Ψ and π. Physical properties of the bulk leaf tissue that have a role in regulating water balance of the two growth forms were estimated: relative water content of apoplastic water (RWCa) relative water content at zero turgor (RWC0), the bulk modulus of elasticity (E), and water capacity (C w). There were no detectable temporal trends in any of the parameters measured from Nune through September and no significant differences between the two growth forms when compared on the basis of RWCa, RWC0, E, and C w. There was a clear difference between the two growth forms with respect to π; at RWC0, π was-4.5±0.40 MPa for short form plants and-3.3±0.40 MPa for tall form. Turgor pressure of plants in the field (P′) was lower in leaves from short form than for the tall form plants with average difference of about 0.4 MPa. In July, P′ in short form leaves dropped to zero by mid-morning as expected for leaves experiencing water stress. These results show that S. alterniflora is capable of reducing osmotic potential in response to increased salinity and that turgor pressure was lower in short growth form than in tall forms.