Physiological responses of higher plants to soil contamination with metals

Abstract

Net photosynthesis of bean (Phaseolus vulgaris L.) seedlings decreased after assimilation of toxic amounts of zinc by the roots. This effect was related to the substitution of essential bivalent cations by zinc in at least two important metalloproteins of the chloroplast: 1) replacement of Mn²⁺ by Zn²⁺ in the water splitting enzyme of the thylakoid membrane inhibited photosynthetic electron transport at high electron flow rates; and 2) partial substitution of Mg²⁺ by Zn²⁺ in the ternary ribulose 1,5 bisphosphate carboxylase-CO₂-metal²⁺ complex in vivo decreased the carboxylase/oxygenase capacity ratio. These effects, obtained after application of toxic metal doses to intact plants, were compared with data on isolated chloroplasts to which zinc was applied in vitro. The relevance of both experimental approaches for assessing the physiological effects of heavy metals in vivo is discussed. The importance of metal concentration measurements at the subcellular level is emphasized. Enhancement of malondialdehyde content and ethane production in plants, treated with toxic amounts of Cu and Zn indicates that these metals affected membrane integrity by lipid peroxidation; the capacity of the enzyme lipoxygenase, involved in this process, also increased. Cellular decompartmentalization is a consequence of this effect. Similar changes were also observed after several kinds of environmental stress, and even during plant senescence.