Interference in the photosynthetic function and machinery

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

Heavy metals are known to interfere with many vital processes of the plants including photosynthesis Clijsters and Assche 1985 and cause inhibition in this process. This inhibition has been attributed with an indirect action on plant water balance, stomatal conductance and CO 2 availability. The direct effects include effect on chloroplast organization, chlorophyll biosynthesis, electron transport and enzymes of photosynthetic carbon metabolism Fig. 4.

3.4.1 Interference in the photosynthetic function and machinery

Heavy metals lead to oxidative stress in the plants. As a result, there is decreased photosynthetic activity and growth of tissues, which is followed by reduction of plant productivity Ouzounidou 1995 Maksymiec 1997. The insufficiently utilized assimilatory force by Calvin cycle slowed down due to heavy Fig: 4 How do heavy metals effect the process of photosynthesis? Kaur Nayyar : Heavy metal toxicity to food legumes: effects, antioxidative defense and tolerance mechanisms 7 metal stress may, in consequence, enhance proton gradient formed in chloroplasts and increase non-photochemical dissipation of light energy andor decrease photochemical efficiency Maksymiec and Baszyn´ski 1996, Maksymiec 1997. Many heavy metals are known to interfere with the photosynthetic machinery Fig.4. For example cadmium interferes with the chloroplast function and electron transport system by damaging PSII of photosynthesis. Copper shows negative effect on the components of both the light reactions e.g., PSII, thylakoid membrane structure andchlorophyll content Ralph and Burchett 1998, Szalontai et al.1999, Pätsikkä et al. 2002 and CO 2 -fixation reactions Angelov et al.1993. However, in studies that examined both light and CO 2 -fixation components, the relative sensitivity of each to Cu varies among studies Moustakas et al. 1994. Krupa and Baszynski 1995 investigated the heavy metals treated legumes and reported that they severely affect the rate of photosynthesis by inhibiting the light and dark photosynthetic reactions, inhibiting the enzymes of the carbon reduction pathways and disturbing the photosynthetic apparatus. 3.4.2.Interference in chlorophyll synthesis Studies indicate that heavy metals have deleterious effects on the total chlorophyll content in plants Fig.4. The effect of Cr on chloroplast pigment content in mungbean showed that irrespective of concentration, chlorophyll a, chlorophyll b and total chlorophyll decreased in 6-day-old mungbean seedlings Bera et al.1999. Effects of some heavy metals on content of chlorophyll in bean Phaseolus vulgaris seedlings was investigated by Zengin and Munzuroglu 2005 grown in Hoagland solution spiked with various concentrations of Pb, Cu, Cd and Hg. It was reported that the total chlorophyll content declined progressively with increasing concentrations of heavy metals. The total chlorophyll content, chlorophyll b content and carotenoids content was severely affected with in blackgram varieties treated with lead and copper Bibi and Hussain 2005. Also, it was concluded that application of lead and copper to both the black gram cultivars caused significant reduction in the photosynthetic gas exchange, inactivation of enzymes such as Rubisco, Rubisco activase and carbonic anhydrase. Total chlorophylls and carotenoids were calculated from the seedlings of Cyamopis tetraganoloba treated with heavy metals Cd, Pd, Ni, Zn and Cu. It was concluded that Cd and Pb in comparison to Zn, Cu, and Ni reduced the total chlorophyll content at 1000 ppm. Shi and Cai 2008 reported the effects of cadmium treatments on Arachis hypogea plants and concluded that these treatments caused a decrease in the net photosynthetic rate and reduced the content of the photosynthetic pigments as well. Phaseolus vulgaris L. plants grown in soil 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 showed decrease in the content of photosynthetic pigments, total soluble sugars, starch content as well as soluble protein. However, total free amino acid content and lipid peroxidation were increased with increasing concentration of heavy metals Bhardwaj et al. 2009. Kamel 2008 treated Vicia faba plants with different concentrations of lead nitrate ranging from 0-48 mM in hydroponic solution. It was observed that low doses of Pb 0.49 mM increased the chlorophyll content while the chl-a content decreased at high concentrations of Pb 48 mM. It was also observed that the 14 C-fixation decreased at all the applied Pb concentrations.

3.4.3. Reduction in the activities of photosynthetic enzymes and inhibition of photosynthetic rate