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We compared stomatal ammonia compensation point (χ s ) and its metabolic regulation in tobacco (Nicotiana tabacum) leaves of a quick-leaf-senescence phenotype ZY90 and a slow-leaf-senescence phenotype NC89. Compared with NC89, ZY90 had significantly higher χ s values between 40 and 60 d after leaf sprouting in spite of its lower nitrogen content. During the same time, a steeper decline in glutamine synthetase activity was detected in ZY90 leaves, simultaneously with a steep increase in χ s . These results suggested that the quick leaf senescence phenotype exhibited high NH 3 emission potential due to efficient nitrogen recycling and remobilization, and glutamine synthetase played a key role in regulating χ s in ZY90.Additional key words: glutamine synthetase, nitrogen content, quick-and slow-leaf-senescence phenotypes, tobacco.