Accumulation of ammonium ion in cadmium tolerant and sensitive cultivars of <Emphasis Type="Italic">Oryza sativa</Emphasis>

Cd-tolerant and Cd-sensitive rice cultivars were used to study the role of NH4 + accumulation in Cd-induced toxicity. NH4 + accumulation seems to be involved in regulating the toxicity of rice seedlings caused by CdCl2. This conclusion was based on the observations that (a) on treatment with CdCl2, NH4 + content increased rapidly in the leaves of the Cd-sensitive cultivar (cv. Taichung Native 1, TN1) but not in the Cd-tolerant cultivar (cv. Tainumg 67, TNG67), (b) pretreatment with abscisic acid (ABA) enhanced Cd tolerance and reduced Cd-induced NH4 + accumulation in TN1 seedlings, (c) exogenous application of the ABA biosynthesis inhibitor, fluridone, decreased Cd tolerance and increased NH4 + content in leaves of TNG67, (d) exogenous application of phosphinothricin, an inhibitor of glutamine synthetase (GS), which resulted in NH4 + accumulation in the leaves, also induced toxicity similar to Cd in TN1 seedlings. Evidence is presented to show that Cd-induced NH4 + accumulation in TN1 leaves is attributable to a decrease in GS activity. Since Cd-treated TN1 leaves had higher glutamine and glutamate contents than control leaves, it is unlikely that glutamine (or glutamate) depletion is the mechanism which regulates Cd-induced toxicity. Abbreviations: ABA – abscisic acid, DW – dry weight, FW – initial fresh weight, GS – glutamine synthetase, PPT – phosphinothricin, TN1 – Taichung Native 1, TNG67 – Tainung 67