Ammonium ion, ethylene, and NaCl-induced senescence of detached rice leaves

The possibility that NH4+ accumulation is linked to the senescence of detached rice (Oryza sativa) leaves induced by NaCl was investigated. NaCl was effective in promoting senescence and in increasing NH4+ content of detached rice leaves. NaCl-promoted senescence is mainly due to the effect of both Na+ and Cl- ions. NaCl had no or slight effect on relative water content, suggesting that an osmotic effect is unlikely to be a major factor contributing to senescence of these leaves. NaCl-induced NH4+ accumulation was due to enhanced nitrate reduction and decreased glutamine synthetase activity. Exogenous NH4Cl, which caused an accumulation of NH4+ in detached rice leaves, also promoted senescence. It was found that an increase in NH4+ content preceded the occurrence of senescence caused by NaCl. Results also show that NaCl-promoted senescence is unlikely to be due to the lack of glutamate, glutamine, aspartate, and asparagine. The current results suggest that NH4+ accumulation is linked to NaCl-induced rice leaf senescence. Since ethylene is known to be a potent promoter of leaf senescence, we also investigated the role of ethylene in the regulation of NH4+-promoted senescence of detached rice leaves. NaCl or NH4Cl treatment resulted in a decrease of ethylene production. Evidence was presented to show that NH4+ accumulation in detached rice leaves does not change tissue sensitivity to ethylene. Clearly, the possible involvement of ethylene in NH4+-promoted senescence is excluded.