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.
Highlights
Very little is known about the effects of manganese (Mn)-excess on citrus photosynthesis and antioxidant systems
We investigated the effects of Mn-excess on CO2 assimilation, Rubisco, carbohydrates and photosynthetic electron transport in leaves, and antioxidant systems in roots and leaves of sour pummelo [Citrus grandis (L.) Osbeck]
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 Mninduced changes in antioxidant systems differ between roots and leaves
Summary
Very little is known about the effects of manganese (Mn)-excess on citrus photosynthesis and antioxidant systems. Previous studies have shown that Mn-excess can inhibit CO2 assimilation in many plants including tobacco (Nicotiana tabacum L.) [3,4], wheat (Triticum aestivum L.) [5,6], cucumber (Cucumis sativus L.) [7], ricebean (Vigna umbellata L.) [8], white birch (Betula platyphylla Suk.) [9], rice (Oryza sativa L.) [10], common bean (Phaseolus vulgaris L.) [11], mungbean (Vigna radiata L.) [12], Alnus hirsuta Turcz., Betula ermanii Charm., Ulmus davidiana Planch. Nable et al [4] showed that the inhibition of photosynthesis in tobacco leaves was not a consequence of decreased stomatal conductance, because both intercellular CO2 concentration and rate of transpiration were not affected. Similar results have been found in Alnus hirsuta Turcz., Betula ermanii Charm., Ulmus davidiana Planch. and Acer mono Maxim. [13]
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