Common reed produces starch granules at the shoot base in response to salt stress

Common reed (Phragmites australis) is a well known salt-tolerant plant and it is suggested that reeds recover Na⁺ in the xylem sap of the shoot base (basal part of the shoot), store it temporarily in the shoot base, release it into the phloem sap, and then retranslocate it to the roots. To investigate whether Na⁺ is retained in the shoot base of reeds, confocal laser scanning microscope (CLSM) observations were conducted using an intracellular Na⁺-specific fluorescent probe. The CLSM observations revealed that reeds produced a large number of the starch granules at the shoot base when salt-stressed, and that the fluorescence indicating the location of intracellular free Na⁺ was observed in the same position as the starch granules. The Na content of starch granules was considerably greater than that of the shoot base, whereas the potassium (K) contents of the granules was only slightly greater than that of the shoot base. Reeds produced Na⁺-binding starch granules in the parenchyma cells of the shoot base when salt-stressed; these starch granules may decrease intracellular free Na⁺. It is proposed that the site-specific production of Na⁺-binding starch granules constitutes a novel salt tolerance mechanism.