Effect of salinity on growth, mineral composition, photosynthesis and water relations of two vegetable crops; New Zealand Spinach (Tetragonia tetragonioides) and water Spinach (Ipomoea aquatica)

New Zealand spinach (Tetragonia tetragonioides L.) is widely cultivated throughout the world for use as a vegetable, ground cover and medicinal plant; however its salt tolerance has not been clarified. This study examined the difference in the salt tolerance mechanisms between New Zealand spinach and water spinach (Ipomoea aquatica L.). Both plants were exposed to salt stress by daily irrigation with 0, 50, 100 and 200 mM NaCl solution for 14 days. The growth of water spinach was markedly and gradually reduced with increasing salinity, whereas that of New Zealand spinach was increased with elevating salinity, indicating that New Zealand spinach is halophilic. The leaf water potential (LWP) and osmotic potential (OP) were also gradually decreased with increasing salinity; the reduction of the LWP and OP was higher in New Zealand spinach than water spinach. New Zealand spinach accumulated more Na+ ions in the leaves. The photosynthetic rate (Pn) and transpiration rates (Tr) of both species decreased with increasing salinity, but both the Pn and Tr in New Zealand spinach were maintained at a higher level than in water spinach.