Effects Of Manganese-toxicity On Photosynthesis And Antioxidant Systems In Sour Pummelo Seedlings

Citrus belongs to evergreen subtropical fruit trees and is cultivated in humid and sub-humid of tropical, sub-tropical, and temperate regions of the world mainly on acidic or highly acidic soils such as red soil, yellow soil and latosol and is liable to suffer from manganese (Mn)-toxicity.Sour pummelo [Citrus grandis (L.) Osbeck] seedling grown in pots containing sands were fertigated for 17 weeks with nutrient solution containing 2μM (control) or 500μM (Mn-toxicity) MnSO4. Thereafter, the effects of Mn-toxicity on growth, CO2 assimilation, Ribulose-1,5- bisphosphate carboxylase/oxygenase (Rubisco), non-structural carbohydrates and photosynthetic electron transport in leaves, and antioxidant systems in roots and leaves of sour pummelo seedlings. The objective of this study were to understand the mechanisms by which Mn-toxicity leads to a decrease in CO2 assimilation and to test the hypothesis that Mn-toxicity-induced changes in antioxidant systems differr between roots and leaves.Mn-toxicity decreased CO2 assimilation and stomatal conductance, increased intercellular CO2 concentration, but did not affect chlorophyll (Chl) content. Both initial and total Rubisco activity in Mn-toxicity leaves decreased to a lesser extent than CO2 assimilation. Contents of glucose, fructose, starch and total nonstructural carbohydrates did not differ between Mn-toxicity leaves and controls, while sucrose content was higher in the former. Chl a fluorescence (OJIP) transients from Mn-toxicity leaves showed increased O-step and decreased P-step, accompanied by postivieΔL- and K-bands. Mn-toxicity 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-toxicity 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-toxicity roots had similar or lower activities of antioxidant enzymes and contents of antioxidants. Mn-toxicity did not affect malondialdehyde (MDA) content of roots and leaves.In conclusion, Mn-toxicity impaired the whole photosynthetic electron transport chain from the donor side of photosystem II (PSII) up to the reduction of end acceptors of 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-toxicity leaves, while the antioxidant systems in Mn-toxicity roots were not enhanced, but still remained high activity. Antioxidant systems in Mn-toxicity roots and leaves provided sufficient protection to them against oxidative damage.