We characterize the salinity stress response of Iris hexagona, a freshwater species, by measuring three putative osmolytes, betaine, proline, and dimethylsulphonio-propionate (DMSP) in plants after short (3 days) and long-term (4–5 months) exposure to NaCl. HPLC analyses show that untreated control leaves contained 2.1, 0.2 and 3.2 mg g⁻¹ DW of betaine, proline, and DMSP, respectively, and establish the presence of these compounds in the Iridaceae. Within 4 days of salinity stress (200 mM NaCl) betaine, proline and DMSP increased significantly. Among vegetative tissues, the highest level of proline occurred in roots; betaine and DMSP were highest in leaves. Analyses of generative tissue (flowers and flower stalks) after long-term exposure showed the highest levels ([10 mg g⁻¹ DW) of all examined compounds in petals but only proline and betaine increased with salinity. All three substances showed a basipetal gradient in flower stalks. Although the examined compounds responded to salinity, the osmometry of the sap indicated that they comprise less than 10% of the osmotically active solutes. The temporal and spatial changes in the distribution of the analyzed compounds indicate complex responses to salinity.
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