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