Along-vein necrosis as indicator symptom on water spinach caused by nickel in water culture

Water spinach (Ipomoea aquatica Forsk, cv. Bamboo-Leaf) grown in aerated nutrient solution in greenhouse was treated with various levels of nickel, copper, zinc, chromium, cadmium, manganese, arsenic, aluminum, and lead. Injury symptoms caused by these metals were compared with each other. Among these metals, only nickel produced specific symptoms of along-vein necrosis on leaves and stems. Leaf yellowing or chlorosis also appeared on the plants treated with nickel, zinc, chromium, cadmium, and manganese, but not on those treated with copper, arsenic, aluminum, and lead. The copper caused only stunting and small leaf symptoms, while the arsenic and aluminum caused only wilting symptoms. Some of these nine elements produced browning or necrotic symptoms on roots, but the symptoms were not specific. All of tested metals except Pd resulted in visible symptoms on leaves or roots for the tested concentrations. The toxicity threshold concentrations for Ni, Cu, Zn, Cr, Cd, Mn, As, and Al were 2.5, 5, 5, 1.25, 1, 20, 2, and 50 ppm, respectively. These results indicate that water spinach has potential for use as an indicator plant of Ni in the environment.