Introducción a la metodología del estudio de usos y actitudes

4.1 In trod u ction

The degree to which plants are affected by the presence of an atmospheric pollutant such as NH^ is determined by a combination of factors. These include firstly, the frequency, concentration and longevity of exposure and subsequent uptake rates; secondly, the ability to detoxify the pollutant by storage or assimilation; thirdly, acclimation and adaptation to pollutant conditions and finally, the interaction of NHx with other pollutants both in the atmosphere and the plant tissue. Plants have little control over the external concentration and longevity of exposure. The presence of a cuticular barrier offers a protective barrier to some pollutants, the effectiveness of which varies markedly between species. The cuticle can however become damaged where abrasion, insect attack and acidic pollutants dissolve the waxy covering, thus decreasing the integrity of the structure and aiding the entry of pollutants. The presence of stomatal pores provide an easy route by which pollutants, particularly in gaseous form, can enter into a plant, as normal photosynthetic processes require that stomata are open fo r most of the daytime. Wet deposited NH^ can enter the plant through the foliage as described above and also, via deposition to the soil and subsequent uptake by roots. Although variation in mechanical resistance of the leaf cuticle is undoubtedly an important aspect of species susceptibility, it is evident from the literature that this cannot be the full explanation for species damage.

There is a growing awareness that biochemical and physiological properties may also have an im portant role in defining a plant's

tolerance to atmospheric pollution in general. The physiological properties of a plant, which regulate the assimilation and detoxification of NH^ may be of paramount im portance in ensuring the survival of plants in polluted environments.

Investigations in Chapter 3 have identified a variety of physiological variables as indicators o f NH^ pollution. Changes in NR, GS and ME activities plus organic acids, show consistent and reproducible changes in NH^ treated plants. However, no definitive link has been established between physiological responses and overall plant susceptibility to excess NH^. Some authors have suggested links for other pollutants for example Rabe and Kreeb, (1979) studied the changes in several enzyme activities and chlorophyll content in response to SO2 pollution. O f the 7 plant species studied, Rabe and Kreeb concluded that some species were more susceptible than others, based on their previous ‘pollution conditioning’, before exposure to SO2. Similarly, a different level o f enzymatic response was found to occur in plants studied in the previous chapter, based on the intrinsic metabolism of the plant, before exposure to NH^. It would appear therefore, that plant susceptibility to NH^ is likely to be decided by a combination of factors, which reflect differences in plant types, location and overall plant metabolism under natural environmental conditions.

NH^ deposition is not exclusive to specific ecosystems, thus a wide variety of plants are subjected to excessive NH^ levels. Even so, reports of damage from NH^ are often concerned with specific plant types, most notably woody species, for example coniferous trees, climax broad leaf trees and ericaceous species (Soares et cd., 1995). These woody species are notable for their sclerophyllous leaves and thick waxy cuticle. In comparison, herbaceous species and fast growing perennials are not as frequently reported as suffering

comparable damage from excess NH^. In addition, the effects of NH^ on plants are often localised and patchy, often with healthy individuals of one species present alongside damaged individuals of the same species (Nihlgard, 1985; Crawford, 1989).

As N metabolism is central to nearly all plant biochemistry, it is likely that the presence of excess NH^ will affect plant metabolism as a whole. The difficult task is to identify which particular aspects of plant metabolism provide the best indicators of susceptibility. The role of NH^ in N nutrition and the acidification problems associated with uptake and assimilation of NH^, offer two areas which may be significant in determining plant susceptibility to NH^. Wellbum (1994) stated that many plant species growing in nutrient deficient habitats, are often more prone to damage from NH^, than plants growing in nutrient rich habitats. Differences in N metabolism in species found in both habitats will inevitably occur. It is these differences that may ultimately dictate the susceptibility of a plant to excess NH^. This chapter describes how a survey of plant physiological characteristics may provide some indication of plant susceptibility to NH^ pollution .

4.2 E xperim ental d esig n

4.2.1 R ationale

A total of 32 plant species including woody perennials, herbaceous species, ferns and mosses were collected from various field sites in Hertfordshire and Bedfordshire between May and August, 1992 (Table 4.1). The location of the field sites are detailed in Chapter 2.2. The emphasis on the collection was to provide a varied and non-biased selection of plant material encompassing different plant types from different habitats. Individuals o f P. vulgaris (n = 3) were grown under greenhouse conditions as outlined in Chapter 2.3 and included in the