Differences in NH₃ compensation point among tobacco (Nicotiana Tabacum L.) cultivars and its relationship with N metabolization

To investigate NH3 compensation point (s) and its differences in leaves of three flue-cured tobacco genotypes and the related N metabolization, foliar apoplastic NH4+ concentration and pH were measured by apoplast extraction technique, the s (25℃) was calculated on the basis of measurements, and the activities of related enzymes and nitrogen parameters were also assayed. In the leaves of the three cultivars, an increase in s was observed as the onset of senescence. Compared with the leaves of slow-senescing cultivar, the leaves of the rapid-senescing tobacco cultivar with lower N status had a stronger decrease in glutamine synthetase activity (GS, EC 6.3.1.2) and a more substantial increase in s. These results indicate that the rapid-senescing cultivar may have a high potential for NH3 emission from senescing leaves into the atmosphere, and the difference in s values is related with the N metabolism of the cultivars.