38
The Negative Action of Toxic Divalent Cations on
the Photosynthetic Apparatus
Robert Carpentier
Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada
763I. INTRODUCTION
Heavy metal and other toxic metal cations are widespread pollutants that are phytotoxic. Most of them
are absorbed by the plant roots, where they can accumulate. When the root tolerance is overloaded, the
metals are translocated toward the leaves and affect the photosynthetic apparatus. Lower plants are also
affected. In algae and cyanobacteria, the sensitivity to metal cations depends on their plasma membrane
permeability and their capacity to bioaccumulate and is thus variable between species. Similarly, higher
plants can be classified in three classes depending on their resistance to excess toxic divalent cations [1].
The first class of plants absorbs and translocates the ions as a function of metal concentration, the second
class includes plants that are tolerant and that can exclude the metals at the root level, and the third class
is represented by plants that can bioaccumulate heavy metals in their roots. The responses of the differ-
ent plant species at the level of the photosynthetic electron transport system will thus be widely variable
depending on species.
In this chapter, the inhibitory action of copper, mercury, cadmium, lead, zinc, and nickel will be re-
viewed. The sites and mechanisms of action will be discussed. The metals were studied in various plant
materials ranging from whole plants and plant seedlings to isolated chloroplasts or photosynthetic mem-
branes. The action during exposure of whole plants is often weaker than in isolated materials because of
the translocation process. However, the toxicity often increases and becomes more significant with pro-
longed exposure periods. Thus, the more precise studies of metal action are reported in isolated thylakoid
membranes and photosystem submembrane fractions, where the electron transfer components are more
readily accessible to metal cations and other reagents such as artificial electron acceptors and donors that
are used to localize the inhibitory active sites. Accurate knowledge of the inhibitory site and mode of ac-
tion of the toxic metal cations may lead to interesting applications of these inhibitors in studies of the
structure-function relationship of the photosynthetic apparatus [2].
II. ACTIVE SITES AND MODE OF ACTION OF TOXIC METAL CATIONS
A. Copper
Copper is an essential microelement in higher plants and algae as it occurs as part of the prosthetic groups
of several enzymes. It is involved in photosynthesis as part of plastocyanine, the nearest electron donor