Antioxidative Enzymatic systems Antioxidative Defense mechanisms

8 Journal of Food Legumes 263 4, 2013

3.5 Reproductive biology: Pollen function

It has been found, that inhibitory effects of sulfur dioxide on pollen germination and tube elongation have occurred at concentrations lower than those at which foliar effects have been recorded Varshney and Varshney 1980. This suggests that heavy metals, even if present at low concentrations in plant tissue, may affect pollen germination and pollen tube growth. Pollen tubes are excellent standard systems with which the effects of drugs and pollutants can be investigated Kristen et al.1993. Studies report that heavy metals at toxic levels inhibit pollen germination, pollen tube growth Tuna et al. 2002 by causing ultra-structural changes. Xiong and Peng2001tested 5 herb species Vicia augustifolia, V.tetrasperma, Pisum sativum, Plantago depressa, and Medicago hispida for their responses against Cd exposure for pollen germination and tube growth to Cd exposure in vitro. Results revealed that pollen germination of all the species was inhibited at Cd concentrations of 2.51µgml and higher, and tube growth was inhibited at concentrations of 1.58µgml and higher. The pollen response to Cd stress exhibited interspecies differences. Vicia angustifolia and V. tetrasperma were sensitive to Cd and were inhibited in either pollen germination or tube growth by Cd at 0.01 µgml. At 1 µgml, pollen tube growth of V. angustifolia, V. tetrasperma, and P. sativum was inhibited. Results suggested that cadmium at such a low concentration as 0.01 lgmL is able to exert adverse effects on pollen germination in some sensitive species, while it fails to do so for less-sensitive species.

3.6 Phyto-hormones

Quantitative determination of endogenous chemical compounds including hormones in plants growing under controlled environmental conditions in presence and absence of heavy metals is used to study the effect of heavy metals Varga et al.1999. Atici et al. 2005 investigated the changes in abscissic acid, gibberellic acid, zeatin and zeatin riboside hormones of chickpea seeds germinating under Pb or Zn heavy metals exposure. Pb increased abcisic acid and zeatin contents while decreased gibbrellic acid content in the germinating seeds. High concentrations of Zn 1.0 and 10 mM decreased contents of zeatin, zeatin riboside and giberellic acid while 0.1 mM Zn increased the content of the same hormones. ABA content was enhanced by Zn in all concentrations used.Cakmak et al. 1989 estimated the concentrations of phytohormones particularly IAA and concluded that its concentration in zinc-deficient bean Phaseolus vulgaris L. plants are clearly lower compared to those of Zn-sufficient plantsZn 2+ , changes in concentrations of ABA are less distinct. Re-supply of Zn to deficient plants restores the IAA level to that of the Zn-rich plants within 96 h, whereas the ABAconcentrations are only slightly increased after Zn resupply.In contrast, the effect of Zn nutritional status oncytokinin levels is less clear. Information on effect of phytohormones is sparse and is required to be worked out.

3.7 Adverse effects on nutritional value of legumes

Limited data suggests that some heavy metals induce substantial reduction in the nutritional quality of the seeds in terms of accumulation of starch, proteins, amino acids, and minerals. It has been reported that As interferes in the uptake and accumulation of minerals in seeds and shoots Paivoke and Simola 2002 and may alter nutritional composition Tu and Ma 2005. Paivoke and Simola 2002 reported that As 12.5 to 73.3 mg of sodium arsenatekg dry weight of soil caused interference in mineral nutrient balance of Zn, Mg, and Mn in peas. Similarly, in pea, growing in 2.5mM cadmium a decrease in starch content of its seeds was observed, however, the protein content remained unaffected Dewan and Dhingra 2004. In case of Phaseolus vulgaris L. plants supplemented with different Pb and Cd concentrations 2, 4, 6, 8 g Kg -1 for lead and 1.5, 2.0, 2.5, 3.0 g Kg -1 for cadmium, total soluble sugars, starch content as well as soluble proteins content decreased as concentration of metals was increased in comparison of control plants. However, the total free amino acid content was increased with increasing concentration of heavy metals Bhardwaj et al. 2009. In case of chickpea grown in As 5mgkg of dry soil, there was a significant inhibition in the accumulation of seed reserves such as starch, proteins, sugars, and minerals as compared to the controls, which indicated that As application markedly reduced the quality of the chickpea seeds Malik et al. 2011.

4. Antioxidative Defense mechanisms

Heavy metals cause oxidative stress by damaging the cells and disrupting the cellular homeostasis by the enhanced generation of toxic reactive oxygen species ROS. The ROS produced during stress are harmful for the plants and can pose a threat to cells by damaging membranes, nucleic acids and chloroplast pigments Chen and Goldsbrough 1994, Dra,_zkiewicz et al. 2004. The plants possess anti-oxidative systems to protect themselves against the damage produced by ROS. This system is composed of antioxidant enzymes: ascorbate peroxidase APOX, glutathione reductase GR, superoxide dismutase SOD, catalase CAT and non- enzymatic compounds ascorbic acid, glutathione, carotenoids, -tocopherol Gill and Tuteja 2010.

4.1 Antioxidative Enzymatic systems

There is evidence that in pea plants exposed to Cd 2+ , the antioxidant system might play a role in detoxification mechanisms Dixit et al. 2001. Cupric stress 50 uM and 75 uM induced changes in antioxidant enzymes GPX and CAT activities of Phaseolus vulgaris. L plants. GPX guaiacol peroxidase, EC 1.11.1.7 activity was decreased in 50 µM Cu- stressed leaves whereas 75 µM of CuSO 4 resulted in an increase of enzyme activity while CAT catalase, EC 1.11.1.6 activity was stimulated at 50 µM CuSO 4 but remained the same at 75 µM CuSO 4 Bouazizi et al. 2010. It was reported that in pea plants that there was a decrease in catalase, SOD, Kaur Nayyar : Heavy metal toxicity to food legumes: effects, antioxidative defense and tolerance mechanisms 9 and guaiacol peroxidase activities when treated with CdCl 2 0-50 mM. No significant changes in the glutathione reductase activity were shown by the treated plants Sandalio et al. 2001. Dixit et al. 2001 also reported the effects of 4 and 40uM cadmium on the antioxidants and antioxidant enzymes in the pea roots and leaves, separately. The results indicated that the levels of lipid peroxidation and H 2 O 2 increased in both the roots and leaves. Activities of SOD, APX, GST and GR were more at 40 uM concentration while GPOX decreased in the roots. Aluminium phytotoxicity causes oxidative stress in developing green gram seedlings and a significant increase in lipid peroxidation, peroxide content accompanied by a decrease in catalase activity Panda et al. 2003. However, superoxide dismutase, peroxidase and glutathione reductase activities increased with increasing aluminium concentrations. Both the contents of glutathione and ascorbate decreased with the elevated metal concentrations. In another experiment the effect of aluminium on lipid peroxidation, superoxide dismutase, catalase, and peroxidase activities in root tips of soybean Glycine max were investigated Cakmak and Horst 1991. Soybean seedlings treated with Al AlCI 3 concentrations ranging from 10 to 75 µM showed the enhancement of lipid peroxidation in the crude extracts of the root tips, the activities of SOD and peroxidase increased while catalase decreased. The effects of cadmium 5 uM and zinc 100 uM on the antioxidant enzyme activities in bean Phaseolus vulgaris were reported Chaoni et al.1997. Lipid peroxidation was enhanced in all plant organs of the plant and the catalase activity was decreased in both roots and leaves but not in stems. Mercury toxicity in alfaalfa Medicago sativa by treating the plants with 0–40 µM HgCl 2 for 7 d resulted in oxidative stress Zhou 2007. It was observed that treatment with Hg 2+ increased the activities of NADH oxidase and lipoxygenase LOX and damaged the biomembrane lipids. There was enhancement in the total activities of APX, POD and CAT. Several antioxidative metabolites such as ascorbate and glutathione GSH differentially accumulated in leaves. Table:1 Location and function of nonenzymatic antioxidants in plant cell Antioxidant Location in the plant Function α –tocopherol Chloroplast envelope, thylakoid membranes and plastoglobuli. Deactivating the photosynthesis-derived ROS and scavenging lipid peroxyl radicals in thylakoid membranes Ascorbic Acid Usually higher in photosynthetic cells and meristems and some fruits and highest in mature leaves with fully developed chloroplast Powerful water soluble antioxidant, prevent or minimize the damage caused by ROS. Glutathione Cell compartments like cytosol, ER, vacuole, mitochondria, chloroplasts, peroxisomes and in apoplast Role in antioxidative defense, regulation of sulfate transport, signal transduction, detoxification of xenobiotics and the expression of stress-responsive genes Carotenoids Leaves, fruits and floral parts Photoprotective role in addition to scavenging of ROS. Proline Cytosol Osmoregulation, seed germination, membrane integrity, inhibition of water loss and an antioxidant Salicylic Acid Cytosol Seed germination, stomatal closure, an antioxidant Flavonoids Leaves, floral parts, and pollens. Role as an antioxidant, flowers, fruits, and seed pigmentation, protection against UV light, defence against phytopathogens role in plant fertility and germination of pollen.

4.2 Non enzymatic defense systems