A hypothesis of plant susceptibility to atmospheric pollution based on intrinsic nitrogen metabolism: Why acidity really is the problem

Several hypotheses have been put forward to explain the causes of damage to plants by atmospheric pollution. None however, fully explain why some species are more prone to damage than others. In the literature, certain higher plants are more commonly reported as suffering damage from atmospheric pollution. These include climax species such as conifers, some broadleaf species such as oak and beech, and ericaceous species. pioneer species such as highly nitrophillous woody species and ruderal herbaceous species are seldom reported in connection with damage from atmospheric pollutants. A screening of 18 species (woody and herbaceous) for 5 physiological and metabolic parameters and subsequent regression analysis of the data, showed that plants could be grouped according to their tolerance to acidity. There was a marked positive correlation between leaf nitrate reductase (NR) activity and the buffering capacity index (BCI) of the leaf Foliar mistings and soil applications of 3 mof m(-3) KNO3 to 6 assorted plant species also revealed that pioneer species, can induce NR activity when NO3- is applied directly to the foliage or soil. However, climax species have a much lower ability to increase NR activity in response to foliar and soil NO3- inputs. Leaf NO3- assimilation carries with it the direct generation of OH., involves greater uptake of base cations and confers greater physiological buffering capability through the close interaction of carbon and nitrogen acquisition in the leaf. The hypothesis states that leaf nitrate assimilators are physiologically more competent at buffering acidic inputs whether directly as H+, or indirectly through uptake of acidic pollutants.