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