Under Pressure: Does Osmotic Potential of the Culture Medium Affect Growth of Seagrasses?

Seagrasses have large carbohydrate reserves in their rhizomes, which are used to survive over winter and other periods of reduced light. Seagrass seedlings also have carbohydrate reserves in an enlarged hypocotyl, which assist during establishment. Axenic seedling cultures of the seagrass Posidonia coriacea were used to test whether the osmotic potential of the culture media affects growth. Sucrose, glucose and mannitol were added to the culture media (individually and in combination) at O to 90 mM to determine whether they have a nutritive, osmotic or combined effect on growth. To ensure osmotic potentials of the culture media were similar, carbohydrate concentrations were compared in millimoles rather than g 1-1 . Osmotic potential was calculated using the formula [ps] -CiRT, where [ps] is osmotic potential, C is sugar concentration (mM), i is ionisation constant, R is gas constant and T is the temperature. Osmolality of the culture media was also determined using a one-ten Fiske osmometer. Glucose and sucrose were found to have a nutritive effect on growth as biomass and leaf number increased with their increasing concentrations. Mannitol had. a non-dose dependent effect on growth, suggesting an osmotic rather than nutritive role. Growth in the no carbohydrates treatments was significantly reduced compared with treatments containing glucose and/or sucrose. These findings suggest that P. coriacea can use carbohydrate reserves from the hypocotyl but will also assimilate carbohydrates from the culture medium.