Summary
Some 109tonnes of chlorophyll are destroyed each year on land and in the oceans. The fate of these chlorophylls is, however, largely unknown. This review describes the developmental stages at which chlorophyll breakdown occurs in aquatic and terrestrial biological systems, and the destruction arising from herbivory, disease, pollution and other physical hazards. At the cellular level, an attempt is made to separate the breakdown of chlorophyll during senescence from the many other events associated with cell destruction and death.

A consideration of the more important chemical and biophysical properties of chlorophylls and their derivatives is provided, together with data on their spectral properties. The biosynthetic and biodegradative pathways of chlorophyll metabolism are, so far as is possible, described with some predictions as to the likely fate of the missing tonnes. Two types of degradation are recognized; the first involves up to five defined enzymes concerned with the early stages, the second covers the less well defined enzymic and non-enzymic destruction of the macrocyclic structure. These degradative reactions are compared with the reactions implicated in the breakdown of other porphyrins including haems in plants and animals.

A brief description is given of the occurrence of breakdown products of chlorophyll in past biomass, including those of geological significance and those in a more recent archaeological context.

Finally, the economic significance of chlorophyll breakdown is considered in the context of agriculture and horticulture, veterinary and medical sciences, food colouring and cosmetic industries, and the multi-million-dollar attraction of autumn leaf fall to tourism.

The degradation of chlorophyll — a biological enigma

Publisher Summary Important physiological processes, like photosynthesis, respiration, the stomatal mechanism, phloem loading, and carbon allocation, are all known to be affected by air pollution. A wide range in sensitivity in physiological responses to SO2, NOx, O3, acid precipitation, and HF is evident from the literature. Some of this variation is clearly because of genetic factors, though much of it is in response to differences in environmental conditions both prior to, during, and after pollutant exposure. Recognizing the importance of the environment, this chapter provides an overview of different fumigation methods and discusses their relevance. It deals on direct effects of SO2, NOx, O3, and acid precipitation on photosynthesis, respiration, and stomata, under the influence of climate and exposure strategies: short-term, long-term, single gas, and mixed gases. As the physiological and biochemical responses are key factors to the health of plants, they may be used as diagnostic tools for predicting injury by air pollution and general stress to individual species and the decline of whole populations. The chapter concludes with an evaluation of such methods of bioindication, because they could be of great relevance to the future and ongoing battle to diminish the deterioration of the physical and biological environment.

Photosynthesis and Stomatal Responses to Polluted Air, and the Use of Physiological and Biochemical Responses for Early Detection and Diagnostic Tools

Photosynthesis is simultaneously regulated by many factors. Environmental factors, consisting primarily of light, water, heat, carbon dioxide, and other gases, can have direct effects on photosynthesis when they alter rates of chemical processes in the photosynthetic pathway. Environmental factors, along with nutrients and sink demands for carbohydrates, may also have indirect effects on photosynthesis. Indirect effects are the result of changes in nonphotosynthetic processes that, in turn, alter the rate of photosynthesis. A final category of regulators, photosynthetic framework, consists of substances that compose the physiologically active compounds and structures that form the basis of the light and dark reactions responsible for photosynthesis. The framework determines the photosynthetic capacity of a plant or organ, i.e., the rate of photosynthesis achievable when direct and indirect effects are nonlimiting. In this chapter we have divided the discussion of factors that regulate photosynthesis in conifers into these three categories, framework, direct effects, and indirect effects, because of an expanding appreciation in the field of environmental physiology that external and internal factors can simultaneously regulate photosynthesis by both direct and indirect means. We offer this outline as a logical way of presenting and discussing these issues.

External and internal factors regulating photosynthesis

Urban polluted forest soils induce elevated root peroxidase activity in Scots pine (Pinus sylvestris L.) seedlings.

Effect of removal of snow cover in winter was investigated in an 80-year-old sugar maple (Acer saccharum Marsh.) stand in southern Quebec. We hypothesized that winter soil frost would induce some of the decline symptoms observed in sugar maple stands in southern Quebec in the early 1980's. Snow was continuously removed from around trees for a one week (partial removal) or for a four-month period (complete removal) during the 1990–1991 winter. Foliage and soils were sampled periodically during the summer of 1991. The complete snow removal treated trees showed decreased leaf water potential and increased peroxidase activity over most of the growing season. Foliar Ca was reduced in both snow removal treatments early in the growing season while foliar N was reduced in the complete snow removal trees late in the growing season. Soil NO
 3
 −
 and K+ were elevated in both snow removal treatments at various times throughout the growing season. Prolonged soil frost in a sugar maple stand can induce lower leaf water potential, higher leaf peroxidase activity and early leaf senescence during the following growing season. Soil frost may have reduced nutrient uptake without affecting significantly the leaf nutrient status.

Effect of snow removal on leaf water potential, soil moisture, leaf and soil nutrient status and leaf peroxidase activity of sugar maple

Effects of SO2 on the photosynthetic and respiration rates in scots pine seedlings.

Short-term exposure to high concentrations of SO2 caused a temporary decrease in the photosynthetic rate, while both short-term and long-term exposure to low concentrations of SO2 increased photo-synthesis after the exposure. Low SO2 concentrations did not cause any change in peroxidase activity. Thus peroxidase activity does not seem a suitable method in monitoring latent injury caused by low SO2 concentrations.

E. Mäkinen

Papers

Effects of SO2 on the photosynthetic and respiration rates in scots pine seedlings.

Effects of SO2 on photosynthesis and peroxidase activity in Scots pine needles