. Uptake of phytotoxic amounts of metal by higher plants or algae can result in inhibition of several enzymes, and in increase in activity (= induction) of others. Two mechanisms of enzyme inhibition predominate: (1) binding of the metal to sulphydryl groups, involved in the catalytic actionor structural integrity of enzymes, and (2) deficiency of an essential metal in metalloproteins or metal-protein complexes, eventually combined with substitution of the toxic metal for the deficient element. Metal accumulation in the cellular compartment of the enzyme is a prerequisite for enzyme inhibition in vivo. The induction of some enzymes is considered to play a significant role in the stress metabolism, induced by metal phytotoxicity. Peroxidase induction is likely to be related to oxidative reactions at the biomembrane; several enzymes of the intermediary metabolism might be stimulated to compensate for metal-sensitive photosynthetic reactions. The induction of enzymes and metal-specific changes in isoperoxidase pattern can be used as diagnostic criteria to evaluate the phytotoxicity of soils, contaminated by several metals. Lines for future research on metal phytotoxicity are proposed, involving the study of inhibition and induction of enzymes at the different cell membranes (especially the plasmamembrane) in vivo.

Effects of metals on enzyme activity in plants

Effects of sewage sludge amendment on heavy metal accumulation and consequent responses of Beta vulgaris plants

Cadmium accumulation and its influence on lipid peroxidation and antioxidative system in an aquatic plant, Bacopa monnieri L.

Malate, oxalate and mustard oils were analysed in zinc-resistant and zinc-sensitive ecotypes of Silene cucubalus Wib., Rumex acetosa L., Thlaspi alpestre L. and Agrostis tenuis Sibth. The effect of zinc on the activities of carbonic anhydrase and peroxidase in Siiene cucubalus was tested. Carbonic anhydrase of the zinc-resistant ecotypes was stimulated by addition in vivo of high amounts of zinc. The high activity of peroxidase in the non-zinc-resistant ecotypes after the addition of zinc documented a poisoning of the sensitive plants by zinc. The amount of oxalate differed greatly between ecotypes. There was, however, no direct correlation between zinc-resistance and oxalate. When applying zinc to the nutrient medium, the synthesis of oxalate was inhibited in zinc-sensitive, but stimulated in zinc-resistant ecotypes of Silene cucubalus and Rumex acetosa. In Thlaspi alpestre high concentrations of mustard oil glucosides were found. Zinc-resistant plants produced twice as much glucosides as sensitive ones. A possible role of mustard oils in zinc-resistance is discussed. In the content of malate there were great quantitative differences between zinc-resistant and zinc-sensitive plants. All zinc-resistant ecotypes of all the species contained much higher concentrations in their green organs than the sensitive ones. It is assumed that malate is a major factor in the evolution of zinc-resistance. Malate may act as a complexing agent for zinc within the plasma, whereas oxalate and mustard oils may function as “terminal acceptors” of large amounts of zinc. The role of malate may be extended by a special transport mechanism, by which zinc is eliminated from the plasma into the vacuole.

The Role of Malate, Oxalate, and Mustard Oil Glucosides in the Evolution of Zinc‐Resistance in Herbage Plants

Accumulation and distribution of toxic metals in wheat (Triticum aestivum L.) and Indian mustard (Brassica campestris L.) irrigated with distillery and tannery effluents.

Well-lighted plants may contain considerable amounts of ascorbic acid (AA) particularly in their chloroplasts. AA is known to be a strong reductant which fulfils several functions in photosynthesis. AA may also influence detoxification of polluted plants, e.g. by reducing SO2. AA contents of forest tree species were distinctly decreased by shading, particularly in light demanding species. Continued SO2 fumigation depressed AA contents long before visible symptoms of injury appeared. AA thus deserves more attention in physiological air pollution research.

Air pollution and ascorbic acid

Peroxidase activity (p. a.) of several tree species was investigated in a forest influenced by F-containing exhalates of an aluminium smelter. P. a. increases with age of tissue, particularly under the influence of fluorides. It was used therefore to map the zone of plant reaction in apricot orchards in the vicinity of an aluminium smelter and it supplements foliar analysis.



P. a. in foliage of white ash was used to determine whether exhaust of automotive traffic in the city of Zurich does affect vegetation. A significant increase of p. a. was found in the city although the plants did not show any symptoms of injury.



P. a. thus is a very sensitive indicator of plant reaction to air pollutants and allows to detect “hidden” injury (no visible symptoms).

The use of peroxidase activity for monitoring and mapping air pollution areas1

The detection of hidden F-injury of forest trees by a simple colorimetric determination of peroxidase activity. The paper describes a simple routine method for determining colorimetrically peroxidase activity (p. a.) in foliage of forest tree species, including conifers. The effect of factors such as foliage age, F Content, external F dust, necrosis etc. on p. a. is investigated. Analyses of plants exposed at different distances to F exhalates of an aluminum plant show that with decreasing distance F content and p. a. increase. P. a. thus is an indicator of air pollution effects on tree physiology even in the range where no visible symptoms of injury occur.

Der Nachweis unsichtbarer („physiologischer”) Fluor-Immissionsschädigungen an Waldbäumen durch eine einfache kolorimetrische Bestimmung der Peroxidase-Aktivität

This study confirms that CO2 uptake of European conifers increases during needle maturation. Extensive investigations with grafts of several species show that this increase may even mask a deleterious effect of a weak SO2 stress. In plants which are not dependent on air-borne SO2 for S nutrition CO2 uptake was not favored by SO2. Peroxidase activity of tree foliage may be a valuable stress indication, particularly in areas with light air pollution. It is, however, not specific for air pollution and seems to be most suitable for “active” monitoring.

The influence of SO2 on CO2 uptake and peroxidase activity

Spruce cuttings (Picea abies) were subjected to low SO2 concentrations for almost half a year. Afterwards transpiration and stomatal conductance were measured with a porometer. Transpiration was decreased by SO2 in light but stomata reacted sluggishly to changed light conditions.

Th. Keller

Papers

Air pollution and ascorbic acid

The use of peroxidase activity for monitoring and mapping air pollution areas1

Der Nachweis unsichtbarer („physiologischer”) Fluor-Immissionsschädigungen an Waldbäumen durch eine einfache kolorimetrische Bestimmung der Peroxidase-Aktivität

The influence of SO2 on CO2 uptake and peroxidase activity

The influence of a prolonged SO2 fumigation on the stomatal reaction of spruce