Kaur Nayyar : Heavy metal toxicity to food legumes: effects, antioxidative defense and tolerance mechanisms 5
3.2 Germination of legume seeds
Germination of the seed is one of the most highly sensitive processes and toxicity with the essential and non
essential heavy metals invariably affects the germination rate and subsequent seedling growth. It represents a limiting stage
of plant life cycle under heavy metal stress situation Rahoui et al.
2008, Smiri et al. 2009. High levels 500 ppm of hexavalent Cr in soil reduced germination up to 48 in the
bush bean Phaseolus vulgaris Parr and Taylor 1982. The growth of embryonic axis of germinating pea seeds Pisum
sativum cv. Bonneville was significantly inhibited by as low
as 0.25 mM cadmium and the elongation of the radicle was affected more severely than that of the plumule Chugh and
Sawhney 1996. In another experiment, Al-Yemini and Nasser 2001 reported a significant decrease in the percentage of
seed germination, seedling growth and an increase in radicle length of Vigna ambacensis L.after treatment with cadmium,
mercury and lead at concentrations 0.05-50mM. Seed germination of the alfalfa plant cultivar Malone was seriously
affected by a concentration of 20 ppm of CdII, CrVI, and by 40 ppm of CuII and NiII Peralta et al. 2001. However, the
root and shoot growth of the alfalfa plant was stimulated by a concentration of 5 ppm of CrVI, CuII, NiII, and ZnII. In
a study, Al-Rumaih et al. 2001 revealed that cadmium chloride adversely influenced the germination process of Vigna
unguiculata
seeds. The germination percentage and the germination rate index GRl expressed as percentage
germination per day showed a significant p~0.05 decrease at 40 ppm, and the decline in these measures became highly
significant p~0.01 at 80 and 160 ppm cadmium chloride concentrations, as compared with the untreated control ones.
Cr VI at 40 ppm reduced seed germination by 23 of seeds of lucerne Medicago sativa cv. Malone and growth in the
contaminated medium Peralta et al.2001.The effect of several doses of As V, Cd II, Pb II, Hg II, Cu II, Zn II on the
seed germination of four common pulses Vigna mungo L.Hepper, Vigna radiata L.Wilzek, Pisum sativum L. and
Lens culinaris
L. were observed by Mandal and Bhattacharyya 2007. Certain levels of some heavy metals
were favourable for seed germination while others were the inhibitory ones. Exposure of 20 ppm Hg II showed higher
toxicity than other heavymetals and reduced the germination potential to 50 as compared to control. It was also shown
that the order of toxicity of the metal elements on the four pulses decreased as follows: Hg As Cd Pb Cu Zn. In
alfalfa, it was reported that the seed germination is seriously affected by 20 ppm of Cd
+2
, Cr
+6
, and by 40 ppm of Cu
+2
, Ni
+2
while the root and shoot growth are stimulated by 5 ppm of Cr
+6
, Cu
+2
, Ni
+2
, and Zn
+2
Aydinalp and Marinova 2009. In another experiment Al-Qurainy 2009 studied the toxicity of
Al and Ni individually on P. vulgaris which affected root and shoot length and their effects on length was appeared after 3
days of germination but germination was not inhibited in both metal treatments. A reduction in seed germination and
seedling growth in chickpea treated with 50, 100, 200 and 400 ppm of nickel and cobalt was reported by Khan and Khan
et al. 2010. The germination of
Vigna unguiculata seeds after treatment in solution containing varying concentration of
cadmium chloride CdCl
2
.H
2
O was observed Egharevba and Omoregie 2010.The concentrations of cadmium Cd in the
solution used for the treatment were 0.00 ppm, 0.80 ppm, 8.00 ppm, 40.00 ppm, 100.00 ppm and 180 ppm. Results showed
that the percentage germination and rate of increment in shoot height decreased as cadmium level in the treatment solution
increased. However, no growth was observed at 100 and
120 ppm. There can be a number of reasons behind the decreased
rate of seed germination in plants. The decrease in seed germination of heavy metal treated plants can be attributed to
the accelerated breakdown of stored food materials in seed Shafiq et al. 2008. Several authors reported that the inhibition
of root elongation caused by heavy metals may be due to metal interference with cell division, including inducement of
chromosomal aberrations and abnormal mitosis Radha et al.
2010, Liu et al. 2003, which can affect the seedling growth. These observations indicated variable effects of
metals, though inhibitory in each of the legume species tested.
3.3. Nitrogen fixation and symbiosis
It is reported that excessive metal concentrations in the polluted soils cause damage to Rhizobia, legumes and their
symbiosis Ahmad et al. 2012. The establishment of symbiosis i.e. root nodulation is an orderly process, which is influenced
by various edaphic factors and the presence of pollutants in the soil. However, very little is known about how legume–
Rhizobium
symbiosis is affected by varying metal concentration Huang et al.1974, Mcilveen and Cole 1974,
Decarvalho et al. 1982, Paivoke 1983, Yakoleva 1984. Various effects of heavy metals on nitrogen fixation are shown in Fig.3.
3.3.1 Heavy metals inhibit the activity of the symbiotic nitrogen fixers
Heavy metals affect the nitrogen fixation process by interfering with the performance of the symbiotic bacteria.
Fig:3 Effect of heavy metals on the nitrogen fixation efficacy of legumes
6 Journal of Food Legumes 263 4, 2013
Various metals e.g. Cu, Ni. Zn, Cd, As are known to inhibit the growth, morphology and activities of various symbiotic
N
2
fixers Stan et al. 2011 like R. leguminosarum, Mesorhizobium ciceri
, Rhizobium sp. and Bradyrhizobium sp. and Sinorhizobium Arora et al. 2010, Bianucci et al. 2011.
The population of R. leguminosarum bv. trifolii was radically altered by long-term exposure to heavy metals and it lost the
ability to form functional symbiosis with white and red clover Hirsch et al. 1993. Chaudri et al. 2000 reported a decrease
in two agriculturally important species of Rhizobia, R. leguminosarum bv. viciae
and R. leguminosarum bv. trifolii, in soils, which were irrigated with sewage sludge containing
Zn or Cu or mixture of Zn and Cu. Similarly, there are numerousreports where elevated amounts of heavy metals
have been found to limit the rhizobial growth and their host legumes Heckman et al. 1987, Broos et al.2005 and
therebyreducing the total crop yields Moftah 2000.
3.3.2 Inhibition and delay in nodulation in the legume roots
Nodulation in the soybean roots was greatly inhibited by the addition of Cd 10-20mgkg to the soil Chen et al.
2003 but the nitrogen fixation of the root nodule was stimulated with low concentrations of Cd, which decreased
sharply with the further additions. The impact of heavy metals such as cadmium 23mgkg and lead 390mgkg on nitrogen
uptake in chickpea was studied by Wani et al. 2008. It was reported that cadmium and lead reduced the number of nodules
by considerable percentages. The total nitrogen content of the shoots, nodule weight, nodule number and N
2
C
2
H
2
- fixation were reduced significantly in dry beans treated with
10µM Cd L Vigue et al.1981. The responses of Lablab purpureus-rhizobium
symbiosis to the effect of different levels of heavy metals Cd, Zn Co and Cu at concentrations control,
50, 100, 150 and 200 mgkg soil was reported Younis 2007. It was reported that there was enhancement in the nodule
number and their mass in the soil treated with 100 mgkg soil of Co and Cu, respectively while there was inhibition at other
levels. There was severe inhibition in the nitrogenase activity. Delay in the nodulation process in some legume crops has
also been observed. For example, with increasing concentration of arsenic As in the nutrient solution, there
was greater time required for Bradyrhizobium japonicum strain CB1809 to inoculate soybean. Riechman 2007.
3.3.3 Decrease in the rate of symbiosis
