Enfoques conductista, mentalista y componentes de las actitudes

3.1.1 V isible versus invisible resp onses o f plants to NH, The exposure of plants to atmospheric pollutants, such as NH^, can result in an abundance of responses. Saxe (1991) conveniently separated these into two distinct categories visible responses, where changes caused by the presence of a pollutant, can be visually distinguished from a non-exposed individual; invisible responses, where changes occur at a biochemical and cellular level.

Previous work on visible responses has included firstly, growth responses, for example changes in root / shoot ratios, crop yield, quality and dry matter accumulation; secondly, foliar damage such as leaf fall, leaf chlorosis and necrosis; and thirdly, changes in species survival and composition at the level of the ecosystem, fo r example forest dieback and heathland to grassland transitions. The above studies account for the bulk of research into the effects of NH^ on plants and have been extensively reviewed by Mathy (1988a); Anon (1988a); Anon (1988b); Zottl (1990); Pearson and Stewart (1993) and Fangmeier et cd. (1994). Studies o f visible effects can contribute towards greater understanding of biological responses and biochemical change. They also provide one approach to the biological assessment of critical loads and levels for ecosystems (Anon 1993a; Anon 1993b).

From initial exposure to atmospheric NH^, to eventual changes in leaf integrity, growth dynamics and species composition, a period of

time exists where a multitude of invisible physiological and biochemical changes occur (Figure 3.1). Although combined studies of both visible and invisible perturbations provide a useful and complete approach to studying the impact of atmospheric pollutants on plants (Wild and Schmitt, 1995), recent research has begun to concentrate specifically on invisible changes. This is partly due to a general reduction in some pollutant levels for example, SO2 in developed countries, which can cause visible foliar damage (Treshow and Anderson, 1989). Where levels of pollutants greatly exceed background levels, such as NH^ emissions from livestock in the Netherlands (Voorburg and Monteny, 1990), visible injury does not always occur (van der Eerden and Pérez-Soba, 1992; van der Eerden et al., 1992). More importantly, visible injury cannot always distinguish the type of pollutant, i.e. gaseous or wet deposited, or whether it is a single pollutant or pollutant mixture that is causing the damage. Likewise, it is difficult to distinguish between direct effects from foliar uptake and indirect effects from soil uptake. This situation may be particularly applicable to NH^ pollution.

Given the above points, it is clear that a more sensitive and representative method of determining the effects of NH^ on plants will, in some instances, be required. The use of physiological and biochemical responses has the potential to achieve this, by revealing more subtle reactions to atmospheric pollution, which can occur long before visible evidence is present. W ellbum (1994) describes the use of physiological and biochemical responses as being infinitely superior to the use of visible injury, especially where visible injury may not be solely associated with air pollution. Evaluation of physiological and biochemical responses may lead to the rapid assessment of the potential damage plants may experience in the presence o f excess NH^, as well as being relevant to more detailed assessment o f underlying causes.

Altered growth and development Modified sensitivity to natural stresses eg.drought, frost, pests

Changes in biochemistry eg. enzymes, organic acids, amino

acids, cations

Absorption by foliage and movement into internal tissues

Damage to cell structure eg. membranes

Exposure to Air Pollutants

Agricultural changes eg. altered crop yield, harvest timing,

quality of crop

Natural ecosystem changes eg. altered competition, species diversity, nutrient

cycling Plantation forest

changes eg. loss of foliage, changed annual growth, nutrient cycling Changes in other physiological processes eg. photosynthesis, water relations, synthesis of primary materials

F ig u re 3 .1 Stages in foliar plant responses to atmospheric pollutants. (Adapted from Anon, 1988b)

3.1.2 P re-req uisites for p h ysiological responses to e x c e ss N H ,

In order for a physiological response to be used as an indicator o f NH^ pollution, there are several factors that need to be considered.

pollution will undoubtedly cause a complex chain of reactions, many of which may reflect other stress factors. Physiological responses therefore need to be specific to the pollutant being investigated and easily and quickly measurable, producing unambiguous results over a wide range of pollutant concentrations and plant species. Physiological processes in plants are however often non-uniformly assigned to metabolic processes and reactions (Wild and Schmitt, 1995). This potential ambiguity can be overcome by considering a mixture of physiological parameters at the same time, ideally those with a known or hypothesised involvement with the particular pollutant under study.

3.1.3 Previous w ork on p hysiological responses to NH,

Using physiological responses as indicators of damage to plants in the presence of atmospheric pollutants is by no means a new concept. Rabe and Kreeb (1979), Darrall and Jager (1984), Darrall (1989) and Saxe (1991) have conducted extensive reviews of physiological responses to a range of atmospheric pollutants such as SO;, NOj and O3. As a result, a variety of enzymatic and metabolic responses have been isolated and characterised that are specific to these pollutants. However, little mention was made in these instances to NH^ pollution. Previous work on NH^ pollution has concentrated on effects in association with total N deposition and acid rain (W ellbum, 1994). Research has however been carried out on tissue N content, nutrient and amino acid changes, in response to NH^ pollution. Fangmeier et cd.

(1994) found a general increase in the above parameters in plants treated with NH^. However, the responses were variable and did not

always reflect the concentration of applied or the duration of the experiment. Clough (1993) and Richter et cd., (1995) also commented on the large degree of seasonal variability in N and amino acid contents of plants and the non uniformity of individual amino acid changes, in relation to atmospheric pollution. These factors make it potentially difficult to quantify changes in amino acids and in some cases total N, as a result of excess deposition, especially where similar changes also take place in the presence of NO^ pollution.

It is only recently that enzymatic and metabolite responses specific to NH^ have been considered in more detail. One such enzymatic response is the induction o f GS activity in the presence of excess NH^ (Pérez-Soba et <3/., 1994). GS is the major enzyme of N H / assimilation in higher plants and it s enzymatic response will be considered in more detail in this chapter.

3.1.4 Identifying p hysiological responses to excess NH,

The role plays in the normal N nutrition of a plant offers the possibility of identifying physiological responses that may show a significant change in the presence of excess atmospheric NH^. This investigation will consider aspects o f N metabolism in plants that are directly and indirectly associated with the assimilation of NH^. These include enzyme activities of GS and NR, the main enzymes of N assimilation in plants. Changes in the concentration of organic acids will also be considered, as these are indirectly involved in the metabolic pathways of NR and GS, thus can be related to the uptake of NH^ and subsequent effects. Methods o f pH regulation in response to inputs from NH^ assimilation, which may involve enzyme activities of ME and organic acids, will also be considered.