Page 58 - Plant-Based Remediation Processes

46 M. Mleczek et al.

Almost all enzymes involved in the removal of ROS are dependent on the presence
of certain metal ions in their active centres, e.g. Cu, Zn, Mn or Fe for SOD and Fe
for CAT or APX. Excessive levels of free metal ions with similar properties can
lead to displacement and ion substitution and consequently inhibition of protein
activity. The stunted growth and decrease of biomass observed in plants grown in a
polluted environment is the result of numerous changes in cell functioning.
A prominent one is disorder in chloroplast structure and functioning. Although
most plants accumulate heavy metals in roots and only 1–5 % of absorbed ions are
transported to above-ground parts (Piechalak et al. 2002, 2003) even such a small
amount has a significant impact on leaf structure and functioning. Chloroplasts in
these plants are smaller and the number of both grana and thylakoids is reduced.
However, the negative metal effect is not expressed to the same degree in all
chloroplasts; particularly exposed are chloroplasts located near the vascular system,
where the concentration of metals is the highest (Krupa 1988). There are numerous
reports indicating a decline in chlorophyll content during exposure to heavy metals
in almond, bean, corn, sunflower, Norway spruce or oak (Nada et al. 2007). This
decrease is caused by several factors, for example imbalance in nutrient level or
inhibition of enzymes involved in chlorophyll biosynthesis. Due to competition of
transporters, disruption of water management and membrane permeability, heavy
metals cause disturbances in uptake of elements; it was reported that the most
affected is the absorption of N, K, Mg and Mn. The effects on absorption of P, S,
Ca, Zn and Fe are more complex; their uptake is related to plant species, environ-
mental stress, pH and soil. For chlorophyll biosynthesis, especially important is
decrease in Mg, Fe, Ca and Zn level observed in plants exposed to Cd, Pb, Cu or Mn
(Van Assche and Clijsters 1990;Ku ¨pper et al. 1996). Presence of cadmium or lead
entailed decrease or inhibition of activity of δ-aminolevulinic acid dehydratase and
protochlorophyllide reductase, which are involved in chlorophyll biosynthesis
(Padmaja et al. 1990; Van Assche and Clijsters 1990). It was shown that heavy
metals affected the function of both PSI and PSII, although PSII remains the main
target of metal toxicity. The authors reported that the proteins which took protons
for photosynthesis in PS II were decomposed and decreased under Cd stress.
Heavy metal stress has been shown to both induce and inhibit expression of
various protein genes which results in changes in protein content (Shah and Dubey
1997). Palma et al. (2002) observed a decrease in general protein content during
exposure of Brassica juncea to high concentrations of Cd and Pb. The author
postulated that this may be caused by enhanced protein degradation as a result of
increased protease activity, which is found to increase under stress conditions. It is
also likely that these heavy metals may have induced lipid peroxidation and
fragmentation of proteins due to toxic effects of reactive oxygen species which
led to reduced protein content. Decrease in the protein content has also been found
in aquatic plants when treated with metalliferous wastewater (Aravind and Prasad
2005). In plants exposed to heavy metal there was reported an increase of various
molecules that can bind with metal ions and form stable complexes, which greatly
decreases metal toxicity. It was shown that organic and amino acids (such as citric
acid, oxalic acid, succinic acid, aspartic acid, glutamic acid, histidine, and cysteine)