Effects of manganese-excess on CO2 assimilation, ribulose-1,5-bisphosphate carboxylase/oxygenase, carbohydrates and photosynthetic electron transport of leaves, and antioxidant systems of leaves and roots in Citrus grandisseedlings

Abstract

BackgroundVery little is known about the effects of manganese (Mn)-excess on citrus photosynthesis and antioxidant systems. Seedlings of sour pummelo (Citrus grandis) were irrigated for 17 weeks with nutrient solution containing 2 μM (control) or 500 μM (excess) MnSO4. The objective of this study were to understand the mechanisms by which Mn-excess leads to a decrease in CO2 assimilation and to test the hypothesis that Mn-induced changes in antioxidant systems differ between roots and leaves.ResultsMn-excess decreased CO2 assimilation and stomatal conductance, increased intercellular CO2 concentration, but did not affect chlorophyll (Chl) level. Both initial and total ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity in Mn-excess leaves decreased to a lesser extent than CO2 assimilation. Contents of glucose, fructose, starch and total nonstructural carbohydrates did not differ between Mn-excess leaves and controls, while sucrose content was higher in the former. Chl a fluorescence (OJIP) transients from Mn-excess leaves showed increased O-step and decreased P-step, accompanied by positive L- and K-bands. Mn-excess decreased maximum quantum yield of primary photochemistry (Fv/Fm) and total performance index (PItot,abs), but increased relative variable fluorescence at I-steps (VI) and energy dissipation. On a protein basis, Mn-excess leaves displayed higher activities of monodehydroascorbate reductase (MDAR), glutathione reductase (GR), superoxide dismutase (SOD), catalase (CAT) and guaiacol peroxidase (GPX) and contents of antioxidants, similar ascorbate peroxidase (APX) activities and lower dehydroascorbate reductase (DHAR) activities; while Mn-excess roots had similar or lower activities of antioxidant enzymes and contents of antioxidants. Mn-excess did not affect malondialdehyde (MDA) content of roots and leaves.ConclusionsMn-excess impaired the whole photosynthetic electron transport chain from the donor side of photosystem II (PSII) up to the reduction of end acceptors of photosystem I (PSI), thus limiting the production of reducing equivalents, and hence the rate of CO2 assimilation. Both the energy dissipation and the antioxidant systems were enhanced in Mn-excess leaves, while the antioxidant systems in Mn-excess roots were not up-regulated, but still remained high activity. The antioxidant systems in Mn-excess roots and leaves provided sufficient protection to them against oxidative damage.

Chlorophyll a fluorescence in two varieties of sugar cane subjected to aluminum and water stress

“…The interference mechanisms of Al in the relations of energy absorption and electron transfer in PSII are not very clear. When in high concentrations, metals may cause a range of significantly different effects (Ralph and Burchett, 1998) like changes in both electron transfer and enzyme activities (Li et al 2010). Barazzouk et al (2005), based on electrochemical studies, attributed the effect of photoinducted electron transfer of excited chlorophyll to nanoparticles of gold.…”

Metal Uptake and Physiological Changes in Lemna gibba Exposed to Manganese and Nickel

“…This enzyme is one of the major scavengers of hydrogen peroxide produced by cells under heavy metal stress. Because of Mn and Ni stress, significant deductibility of POD activity was reported in V. vinifera (Mn), Pistia stratiotes (Ni), Citrus grandis (Mn), O. sativa (Mn), Zea mays (Ni) (Baccouch et al, 1998;Li et al, 2010;Mou et al, 2011;Singh & Pandey, 2011;Srivastava & Dubey, 2011). As seen from Figure 4, the increasing exposure time to both metals at different concentrations showed that the enzyme activity was more reduced at the dose of 16 mg/L than the other exposure doses in both heavy metals.…”

Antioxidant System in Citrus Under Nutrient Stress Conditions: Latest Developments

“…In roots, most of the antioxidant enzymes but MDAR and DHAR remained unaffected resulting in a decrease in reduced ascorbate pools (Li et al 2010 ) . Interestingly, no effect on MDA accumulation neither in leaves nor in roots was observed, probably because the induction of antioxidant enzymes is enough to cope with increased ROS.…”

“…In a recent publication, pummelo plants were grown under excessive Mn concentrations resulting in a severe impairment of the whole photosynthetic system from the donor side (PSII) to the reduction end of acceptor decrease, resulting in a reduction in the rate of CO 2 assimilation. On the contrary, the antioxidant system was enough to cope with overproduced ROS preventing further oxidative damage (Li et al 2010 ) . Changes in leaf and chloroplast ultrastructure linked to a decrease in electron transport rate and photochemical yield in response to different Mn levels have been reported in Citrus volkameriana (Papadakis et al 2007a, b ) .…”

“…In a recent paper, Li et al ( 2010 ) reported the effect of Mn excess on antioxidant mechanisms in citrus resulting in an increase in leaf total SOD activity as well as CAT and GPX respect to control conditions; however, DHAR activity was negatively affected by the treatment. In roots, most of the antioxidant enzymes but MDAR and DHAR remained unaffected resulting in a decrease in reduced ascorbate pools (Li et al 2010 ) .…”