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.

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

Heavy metals interfere with chlorophyll synthesis either through direct inhibition of an enzymatic step or by inducing deficiency of an essential nutrient Van Assche and Clíjsters 1990. Sheoran et al.1990 studied the effect of Cd 2+ and Ni 2+ on the rate of photosynthesis and activities of key enzymes of the photosynthetic carbon reduction cycle in leaves from pigeonpea Cajanus cajan L., cv. UPAS-120 grown in nitrogen free sand culture. It was concluded that the application of Cd 2+ and Ni 2+ 0.5 and 1.0 mM at an early vegetative stage 30 days after sowing resulted in about 50 and 32 reduction in net photosynthesis, respectively. The activities of the photosynthetic enzymes were decreased to different levels 2–61 depending upon the enzyme and the concentration of the metal ion. It was found that Cd toxicity caused notable reduction in photosynthetic rate in different plant species Baszynski et al.1980. In case of pigeon pea, Cd concentrations of 56 and 112 mgL inhibited net photosynthesis to about 50 at early development stages 30-day-old plants and did not exert any significant effect on that process at later stages 70-day-old plants. Sheoran et al. 1990 also reported that at the early stage of pigeonpea, CO 2 exchange rate was quite susceptible to Cd stress. Application of copper and lead at concentrations 25 or 50 mg L -1 in two Mungbean cultivars Vigna radiata L. Wilczek] Mung-1 Mung-6 caused significant reduction in the CO 2 exchange and photosynthetic pigments. Also, there was significant inhibition of photosynthetic and transpiration rates and stomatal conductance compared to the same doses of copper at higher concentration of lead 50 mg L 1 compared to the same doses of copper Ahmad et al. 2008. In the excised leaf segments of pea, it was reported by Sengar and Pandey 1996 that Pb lowered photosynthesis specifically by the inhibition of –amino levulinic acid synthesis and the decrease in the 2-oxoglutamate and glutamate pool, which may be caused by the competition between the essential ions required for chlorophyll synthesis and lead. Applying Cd upto 1 mM concentration to pea Pisum sativum seedlings caused a sharp decline in the chlorophyll content, photosynthetic rates, activity of photosystems and photosynthetic enzymes RUBISCO etc. in 6 days and these effects became more stressed during extended treatment. 8 Journal of Food Legumes 263 4, 2013

3.5 Reproductive biology: Pollen function