Heavy metal (HM) toxicity is one of the major abiotic stresses leading to hazardous effects in plants. A common consequence of HM toxicity is the excessive accumulation of reactive oxygen species (ROS) and methylglyoxal (MG), both of which can cause peroxidation of lipids, oxidation of protein, inactivation of enzymes, DNA damage and/or interact with other vital constituents of plant cells. Higher plants have evolved a sophisticated antioxidant defense system and a glyoxalase system to scavenge ROS and MG. In addition, HMs that enter the cell may be sequestered by amino acids, organic acids, glutathione (GSH), or by specific metal-binding ligands. Being a central molecule of both the antioxidant defense system and the glyoxalase system, GSH is involved in both direct and indirect control of ROS and MG and their reaction products in plant cells, thus protecting the plant from HM-induced oxidative damage. Recent plant molecular studies have shown that GSH by itself and its metabolizing enzymes—notably glutathioneS-transferase, glutathione peroxidase, dehydroascorbate reductase, glutathione reductase, glyoxalase I and glyoxalase II—act additively and coordinately for efficient protection against ROS- and MG-induced damage in addition to detoxification, complexation, chelation and compartmentation of HMs. The aim of this review is to integrate a recent understanding of physiological and biochemical mechanisms of HM-induced plant stress response and tolerance based on the findings of current plant molecular biology research.

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The inhibitory effects of aqueous methanol extract of Cymbopogon nardus (L.) Rendle were determined on seedling growth of eight test plant species: alfalfa (Medicago sativa L.), cress (Lepidum sativum L.), lettuce (Lactuca sativa L.), rapeseed (Brassica napus L.), barnyard grass (Echinochloa crus-galli L.), Italian ryegrass (Lolium moltiflorum Lam.), jungle rice (Echinochloa colonum (L.) P. Beauv.) and timothy (Phleum pratense L.). The bioassay was conducted with four extract concentrations (0.01, 0.03, 0.1 and 0.3 g dry weight equivalent extract/mL). The extracts inhibited significantly shoot and root growth of four test plants such as cress, lettuce, rapeseed and Italian ryegrass at the concentration ≥ 0.03 g dry weight equivalent extract/mL. The inhibitions of shoots and roots increased with increasing extract concentrations. The concentrations required for 50% growth inhibition of all test plants ranged 0.007-0.090 g dry weight equivalent extract/mL. Roots of all test plants were more sensitive to the extract than their shoots. Lettuce was most sensitive, follows by cress and timothy. The results suggest that C. nardus may have allelopathic compounds and may be a candidate for isolation and identification of allelopathic compounds to develop an alternative weed management option.

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