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
A considerable decrease in the total yield, nitrogen content in the plant tissue and the protein content of the
seeds was noticed chickpea-rhizobium and green gram- bradyrhizobium symbiotic systems when treated with cadmium,
lead, copper, zinc, chromium and nickel added in combinations and separately Athar and Ahmad 2002. Adverse effects of
sludge application on N
2
fixation in faba bean Vicia faba have been reported Chaudri et al.1993. In white clover,
cadmium, lead and zinc caused reduction in growth and symbiosis when they were grown in soils highly contaminated
with these metals Rother et al.1983. Chickpea-rhizobium symbiotic system was more sensitive to the metals toxicity
than green gram-bradyrhizobium system Antipchuk et al.
2000.The adverse effects of mercury, cadmium, nickel 1mgml on nodulation and nitrogen fixation in Cicer
arietinum- rhizobium symbiotic system were reported as
decrease in dry weight of both total nodules and effective nodules indicating disturbances in the nodule function Pal
1996.
3.3.4.Nitrogenase activity and protein content
Soybean Glycine max L. nodules and roots in plants were subjected to two different concentrations 50 and 200
µM of CdCl
2
Karina et al. 2003. Nitrogenase activity decreased in nodules treated with 200 µM Cd
2+
. In 50 µM Cd
2+
-treated plants, NH
4 +
content increased by 55 in roots. Glutamate Glu and protein contents remained unaltered in
nodules treated with 50 µM Cd
2+
, while at the higher Cd
2+
concentration, both were decreased.Cadmium significantly depressed biological nitrogen fixation in 0-10mM treated one
month old pea Pisum sativum and also decreased nitrate reductase and glutamate synthase activities 6 days after
treatment Chugh et al. 1999. In Lablab purpureus, nodulation and nitrogenase activity were severely affected with 50-200
mgkg metal concentrations of Cd, Zn, Co, and Cu Younis 2007. Fig.3
3.4 Photosynthetic efficiency of some legumes
