Ammonium-inhibited growth of suspension-cultured rice cells as affected by medium pH

The possible involvements of a decrease in medium pH and accumulation of ammonium in ammonium-inhibited growth of rice cells were investigated. Ammonium, applied at concentrations ranging from 20 to 50 mM, markedly inhibited cell growth and decreased medium pH. The accumulation of ammonium in rice cells was observed only when ammonium concentration was 40 mM or higher. Ammonium-inhibited growth was alleviated when medium pH was buffered with MES [2-(N-morpholino)-ethanesulfonic acid. However, no difference in ammonium level was observed between buffered and unbuffered ammonium-fed rice cells. Succinic acid, alpha-ketoglutaric acid, glutamic acid and glutamine were found to be effective in reversing ammonium-inhibited growth of rice cells and reducing a fall in pH in the ammonium-fed medium. Succinic acid, alpha-ketoglutaric acid and glutamic acid decreased the level of ammonium in ammonium-fed rice cells. However, glutamine was unable to decrease the ammonium level in ammonium-fed rice cells. The current results suggest that a decrease in medium pH is a factor responsible for growth inhibition of ammonium-fed rice cells.