Do reactive oxygen species ROS induced by NaCl contribute to ammonium accumulation in Spartina alterniflora?

Growth, activity of antioxidant enzymes viz. glutathione reductase GR, superoxide dismutase SOD, catalase CAT, ascorbate peroxidase APX and guaiacol peroxidase GPX, and some metabolic processes related to ammonium metabolism were investigated in a salttolerant Spatina alterniflora.In comparison to 0 mM–NaCl treatment, growth of S. alternifloraplant increased significantly at 200 mM NaCl, but was highly inhibited at 500 mM NaCl. Ammonium concentration in the leaves and roots increased 2.1–3.4 times when plants were treated with 500 mM NaCl. Under 200 mM NaCl, antioxidantenzyme activities increased, however, at 500 mM the antioxidant system was unable to compensate reactive oxygen species induced by NaCl. At this high level of salinity, ammonium production through nitrate reductase NR was inhibited, but no significant changes in the activities of glutamine synthetase GS or glutamate dehydrogenase GDH were found. We conclude that the accumulation of ammonium under high salt stress was not due to inhibition of the assimilatory activities of GS or GDH. Ammonia accumulation under high salinity may result from amino acid and protein catabolism activated by reactive oxygen species ROS andor a lack of carbon skeletons to incorporate ammonium into organic molecules due to a decrease in photosynthetic activity in saltstressed plants.