Ammonia-related metabolism of tobacco varieties differing in their susceptibility to Alternaria alternata

Some studies report that ammonia is an important factor of disease development in tobacco plants and various post-harvest fruits. Four tobacco (Nicotiana tabacum L.) varieties resistant or susceptible to Alternaria alternata (Fries) Keissler, a tobacco pathogenic fungus, were used to investigate whether there are differences in ammonia accumulation and the related metabolism of senescing leaves. The results showed that: (a) the leaves of susceptible varieties had significantly higher apoplastic [NH ₄ ⁺ ], pH, and ammonia emission potential (??-values) than resistant varieties during the period from 40 to 60 days of leaf age; (b) leaf tissue [NH ₄ ⁺ ] and total N concentrations in the tobacco varieties were not in line with their susceptibility or resistance to disease; (c) the increases in the apoplastic pH, ??-values, and leaf [NH ₄ ⁺ ] occurred in parallel with a significant decline in glutamine synthetase activity. Compared with the resistant varieties, apoplastic pH values and ?? values were increased more rapidly in the susceptible varieties due to a steeper decline in glutamine synthetase activity and a slower increase in glutamate dehydrogenase activity. In conclusion, NH₃ accumulation or NH3-dependent alkalinization rather than [NH ₄ ⁺ ] and total N appears to be mainly attributed to the enhanced susceptibility of tobacco plants to A. alternata.