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Photosynthesis: The process of photosynthesis is adversely affected by Pb toxicity. Plants exposed to Pb ions show a decline in photosynthetic rate which results from distorted chloroplast ultrastructure, restrained synthesis of chlorophyll, plastoquinone and carotenoids, obstructed electron transport, inhibited activities of Calvin cycle enzymes, as well as deficiency of CO2 as a result of stomatal closure. Ceratophyllum demersum plants when grown in aquatic medium containing Pb(NO3)2 showed distinct changes in chloroplast fine structure (Rebechini and Hanzely, 1974). Leaf cells of such plants exhibited a reduction in grana stacks together with a reduction in the amount of stroma in relation to the lamellar system as well as absence of starch grains. Pb treatment also changes the lipid composition of thylakoid membranes (Stefanov et al., 1995). Effects of Pb on various components of photosynthesis, mitotic irregularities, respiration, water regime and nutrient uptake are shown in figure 3.
Pb inhibits chlorophyll synthesis by causing impaired uptake of essential elements such as Mg and Fe by plants (Burzynski, 1987). It damages the photosynthetic apparatus due to its affinity for protein N- and S- ligands (Ahmed and Tajmir-Riahi, 1993). An enhancement of chlorophyll degradation occurs in Pb-treated plants due to increased chlorophyllase activity (Drazkiewicz, 1994). Chlorophyll b is reported to be more affected than chlorophyll a by Pb treatment (Vodnik et al., 1999). Pb also inhibits electron transport (Rashid et al., 1994). Pb effects have been reported for both donor and acceptor sites of PS II, the cytochrome b/f complex and PS I. It is largely accepted that PS I electron transport is less sensitive to inhibition by Pb than PS II (Mohanty et al., 1989; Sersen et al., 1998).
Pb also causes strong dissociation of the oxygen evolving extrinsic polypeptide of PS II and displacement of Ca, Cl-, Mn from the oxygen-evolving complex (Rashid et al., 1991). Ahmed and Tajmir-Riahi (1993) found conformational changes in light-harvesting chlorophyll (LHC II) subunits, following binding with Pb in vitro. It is proposed that conformational changes induced by Pb treatment might lead to incomplete assembly followed by degradation (Ahmed and Tajmir-Riahi, 1993).
A strong relationship exists between Pb application and a decrease in photosynthesis of the whole plant and is believed to result from stomatal closure rather than a direct effect of Pb on the process of photosynthesis (Bazzaz et al., 1975). According to Kosobrukhov and coworkers (2004), the photosynthetic activity of plant is governed by many factors including stomatal cell size, number of stomata, stomatal conductance, leaf area etc. While studying the effects of Pb on the development of thylakoid of cucumber and poplar
plants Savari and coworkers (2002) observed increased chlorophyll content either in the PS II core or LHC II at low concentrations of Pb treatment, whereas a strong decrease in chlorophyll level of seedlings was seen at the 50 mM Pb. At 50 mM Pb treatment level the concentration of Pb inside the leaf might have been high enough to directly inhibit chlorophyll synthesis (Sengar and Pandey, 1996).
Respiration and ATP content: Pb exerts a significant effect on respiration and ATP content of photosynthetic organisms. In vitro application of Pb to mitochondrial preparations from plant cells revealed a decrease in respiration rate with increasing Pb concentrations (Reese and Roberts, 1985). Using isolated chloroplasts and mitochondria in different plant species it has been shown that Pb affects the flow of electrons via the electron transport system (Miles et al., 1972; Bazzaz and Govindjee, 1974). The inhibitory effect of Pb at higher concentrations appears to be due to uncoupling of oxidative phosphorylation (Miller et al., 1973). At lower concentrations, however, a stimulation of respiration is observed in whole plants (Lee et al., 1976), detached leaves (Lemoreaux and Chaney, 1978), isolated protoplasts (Parys et al., 1998) and mitochondria (Koeppe and Miller, 1970). The exposure of detached leaves of C3 plants (pea, barley) and C4 plants (maize) to 5 mM Pb(NO3)2 for 24 h caused stimulation of the respiratory rate by 20-50 % (Romanowska et al., 2002). Mitochondria isolated from Pb-treated pea leaves oxidized substrates (glycine, succinate, malate) at higher rates than mitochondria from control leaves (Romanowska et al., 2002). The respiratory control and the ADP/O were not affected by Pb treatment. Pb caused an increase in ATP content as well as an increase in the ATP/ADP ratio in pea and maize leaves (Romanowska et al., 2002). Rapid fractionation of barley protoplasts incubated at low and high CO2 conditions, indicated that the increased ATP/ADP ratio in Pb-treated leaves resulted mainly from the production of mitochondrial ATP. The activity of NAD+-malate dehydrogenase in protoplasts of barley leaves treated with Pb was 3-fold higher than the protoplasts from control leaves (Romanowska et al., 2002). The activities of photorespiratory enzymes NADH-hydroxypyruvate reductase and glycolate oxidase as well as of NAD-malic enzyme were however, not affected by Pb treatment (Romanowska et al., 2002). The mechanism underlying the stimulation of respiration by Pb is not clear. According to Ernst (1980) the higher respiration rate observed under Pb toxicity could be due to an increased demand for ATP production through oxidative phosphorylation.
The key enzyme of CO2 assimilation in C3 plants ribulose-bisphosphate carboxylase, is sensitive to Pb whereas the oxygenase activity remains relatively unaffected (van Assche and Clijsters, 1990). Therefore, it is quite possible that after Pb treatment when photosynthesis is significantly reduced, photorespiration could continue at a similar rate. This would increase the relative rate of photorespiration to photosynthesis. The inhibition of photosynthesis observed after Pb treatment leads to decreased utilization of ATP for CO2 fixation. In leaf extracts of Pb-treated plants higher in ATP/ADP ratios have been observed compared to untreated plants (van Assche and Clijsters, 1990). Leaves of Pb-treated plants show increased respiration, which appears to be a result of oxidation of excess photosynthetic reducing equivalents, which are produced under conditions of limited CO2 fixation (Poskuta et al., 1996).
At higher concentrations of Pb, inhibition of respiration is observed. Respiration of corn root tips decreased by 10-17 % after 1 h treatment with 20 mM Pb and by 28-40 % after 3 h treatment (Koeppe, 1977). Pb is regarded as one of the most potent metal ions for the inhibition of chloroplastic ATP synthetase/ATPase activity and for the destruction of the membranes (Tu Shu and Brouillette, 1987). Although the sensitivity of photophosphorylation to heavy metal ions is well documented, there is no general agreement regarding their site of action nor on the underlying mechanism. Some experiments suggest that different mechanisms exist for the action of heavy metal ions on chloroplastic ATPase activity when these metal ions are applied under in vivo and in vitro conditions.
Nutrient uptake: High concentrations of Pb the in soil environment causes imbalance of mineral nutrients in growing plants. Many of the observed actions of Pb appear to be indirect as a result of mineral imbalance within the tissues. Significant changes in nutrient contents as well as in internal ratios of nutrients occur in plants under Pb toxicity (Kabata-Pendias and Pendias, 1992). In most cases Pb blocks the entry of cations (K+, Ca , Mg, Mn, Zn, Cu, Fe 3+) and anions (NO3-) in the root system (Figure 3). Two mechanisms for decreased uptake of micro and macronutrients under Pb toxicity have been suggested. The first mechanism, termed physical, relies on the size of metal ion radii, whereas the second mechanism, which is a chemical one, relies on the metal-induced disorder in the the cell metabolism leading to changes in membrane enzyme activities and membrane structure. The efflux of K+ from roots, apparently due to the extreme sensitivity of K+-ATPase and –SH groups of cell membrane proteins to Pb, is an example of the second type of mechanism.
Pb physically blocks the access of many ions from absorption sites of the roots (Godbold and Kettner, 1991). Although Pb levels in root tips and the basal root may appear to be similar, Pb alters the levels of mineral elements in the roots. In root tips the levels of Ca, Fe, Zn decrease after exposure to Pb. In root tips of Norway spruce 40 % of the Ca taken up was used in root tips growth. The inhibition of root growth after exposure to Pb may be due to a decrease in Ca in the root tips, leading to a decrease in cell division or cell elongation (Haussling et al., 1988). In Norway spruce needles, the level of Ca and Mn decrease with Pb treatment, which could be a result of a decrease in number of root tips and sites for apoplastic solute flux through the endodermis. In Picea abies Pb treatment lowered the Mn level of the needles (Sieghardt, 1988). In Cucumis sativus seedlings Pb decreased the uptake of K, Ca, Mg, Fe and NO3- and in Zea mays the uptake of Ca, Mg, K and P (Walker et al., 1997). Pb influences the overall distribution of nutritional elements within the different organs of the plant. The overall distribution ratio of Mn and S changed in favour of root over shoot under Pb toxicity, which may represent retention of these ions in root. Phosphorus content was found to be negatively correlated with soil Pb (Paivoke, 2002). Root nitrogen content is significantly reduced under Pb toxicity. Nitrate uptake declines in plants under exposure to Pb with a concomitant lowering of nitrate reductase activity and disturbed nitrogen metabolism (Burzynski and Gabrowski, 1984). The decline in nitrate uptake due to Pb may be as a result of moisture stress created by Pb (Burzynski and Gabrowski, 1984). In certain plant species like Pisum sativum, elevated nitrogen content is observed in roots at a Pb treatment level of 2 mM kg-1 soil which probably occurs due to inhibitory effects of Pb on NR activity (Paivoke, 2002).
Water status: A decline in transpiration rate and water content in tissues occurs in plants growing under Pb exposure. Various mechanisms have been suggested for the Pb-induced decline in transpiration rate and water content. Pb treatment causes growth retardation, which results in a reduced leaf area, the major transpiring organ (Iqbal and Moshtaq, 1987). Guard cells are generally smaller in size in plants treated with Pb. Pb lowers the level of compounds that are associated with maintaining cell turgor and cell wall plasticity and thus lowers the water potential within the cell. Metal ions including Pb increase the content of ABA and induce stomatal closure (Figure 3). Disordered respiration and oxidative phosphorylation observed under Pb toxicity may also cause disarray in the plant water regime. Experiments using epidermal peels floating on Pb solutions have shown that Pb induces stomatal closure (Bazzaz et al., 1974). Experiments using excised leaves have indicated that metals increase stomatal resistance not only when directly applied to guard or epidermal cells but also when they reach the cells via xylem (Bazzaz et al., 1974). A unified hypothesis regarding Pb-induced stomatal closure indicates that such effect is due to the inhibition of an energy system or due to alterations of K+ fluxes through membranes (Bazzaz et al., 1974).
this was taken from brazillan journal of plant physiology.