Response of eight tropical plants to enhanced ammonia deposition under field conditions prevalent with SO2 and NH3

The impact of SO₂ on the deposition of ammonia and the response of eight tropical tree species to excess deposition of ammonia was investigated. This was achieved by studying physiological aspects like total sugars, protein, nitrate reductace (NR) activity, organic/inorganic nitrogen ratio, specific leaf area and foliar injury in plants growing under field conditions prevalent with SO₂ and NH₃. Analysis of water soluble substances present on foliar surfaces of the trees indicated enhanced NH₄ ⁺ deposition and thereby result in enhanced foliar protein contents. Though the enhanced nitrogen was almost the same in different plants, the plants exhibited differential metabolic disturbances. Critical analysis of the results indicated three distinct types of plant response. Plants like Azadirachta indica, Acacia auriculiformis and Bambusa arundinaceae maintained enhanced total sugars and NR activity and incorporated excess NH₄ ⁺ into proteins, thus enabling the plant to compensate/alleviate SO₂ induced injury. Ficus benghalensis and Ficus religiosa maintained unaltered total sugars and NR activity and could partly incorporate NH₄ ⁺ into proteins, thus modifying the SO₂ impact to some extent. Dalbergia sissoo, Eucalyptus rostrata and Mangifera indica could not incorporate the excess NH₄ ⁺, mainly due to declined total sugars. The results indicate the ability of a plant to undergo species specific metabolic changes in order to cope with the excess nitrogen deposition, which may ultimately result in increasing or decreasing tolerance to SO₂.