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 grandis seedlings

Abstract

Background: Very 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 microM (control) or 500 microM (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.

Results: Mn-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.

Conclusions: Mn-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.

Figure 1

Effects of Mn-excess on CO₂assimilation (A), stomatal conductance (B), and intercellular CO₂ concentration (C), initial Rubisco activity (D and E), total Rubisco activity (F and G) and Rubisco activation state (H) in sour pummelo leaves. Bars represent means ± standard errors (n = 4-8). Different letters above standard error bars indicate significant differences at P < 0.05.

Figure 2

Effects of Mn-excess on contents of glucose (Glu, A and G), fructose (Fru, B and H), sucrose (Suc, C and I), Glu + Fru + Suc (D and J), starch (E and K) and total nonstructural carbohydrates (TNC, F and L) of sour pummelo leaves expressed on a leaf area (A-F) or DW (G-L) basis. Bars represent means ± standard errors (n = 6). Different letters above standard error bars indicate significant differences at P < 0.05.

Figure 3

Effects of Mn-excess on OJIP transients (A and B), and the different expressions of relative variable fluorescence derived from the average OJIP transients: (C) between F_o and F_m: V_t= (F_t -- F_o)/(F_m -- F_o) and the differences of the two samples to the control sample (ΔV_t); (D) between F_o and F_{300 μs}: W_K = (F_t -- F_o)/(F_{300 μs}-- F_o) and the differences of the two samples to the control sample (ΔW_K) and (E) IP phase: (F_t -- F_o)/(F_I -- F_o) -- 1 = (F_t -- F_I)/(F_I -- F_o) in dark-adapted sour pummelo leaves.

Figure 4

Effects of Mn-excess on S_m(EC_o/RC), DI_o/RC, RE_o/RC, ET_o/RC, TR_o/RC, ABS/RC, δ_Ro (RE_o/ET_o), ψ_Eo (ET_o/TR_o), φ_Po (F_v/F_m or TR_o/ABS), φ_Ro (RE_o/ABS), PI_tot,abs and fraction of OEC in dark-adapted sour pummelo leaves. Each point is the mean of 9 or 10 replicates. All the values were expressed relative to the control (set as 1). All the parameters were significantly higher or lower in Mn-excess leaves than in controls.

Figure 5

Effects of Mn-excess on activities of APX (A and H), MDAR (B and I), DHAR (C and J), GR (D and K), SOD (E and L), CAT (F and M) and GPX (G and N) of sour pummelo leaves expressed on a leaf area (A-G) or protein (H-N) basis. Bars represent means ± standard errors (n = 5-6). Different letters above standard error bars indicate significant differences at P < 0.05.

Figure 6

Effects of Mn-excess on activities of APX (A and H), MDAR (B and I), DHAR (C and J), GR (D and K), SOD (E and L), CAT (F and M) and GPX (G and N) of sour pummelo roots expressed on a root FW (A-G) or protein (H-N) basis. Bars represent means ± standard errors (n = 5-7). Different letters above standard error bars indicate significant differences at P < 0.05.

Figure 7

Effects of Mn-excess on contents of ASC + DHA (A and F), ASC (B and G), GSH + GSSG (C and H), GSH (D and I) expressed on a leaf area (A-D) or protein (F-I) basis, and ratios of ASC to ASC + DHA (E) and GSH to GSH + GSSG (J) in sour pummelo leaves. Bars represent means ± standard errors (n = 5-8). Different letters above standard error bars indicate significant differences at P < 0.05.

Figure 8

Effects of Mn-excess on contents of ASC + DHA (A and F), ASC (B and G), GSH + GSSG (C and H), GSH (D and I) expressed on a root FW (A-D) or protein (F-I) basis, and ratios of ASC to ASC + DHA (E) and GSH to GSH + GSSG (J) in sour pummelo roots. Bars represent means ± standard errors (n = 5-7). Different letters above standard error bars indicate significant differences at P < 0.05.

Figure 9

Effects of Mn-excess on content of leaf MDA expressed on a leaf area (A) or FW (B) basis and of root MDA expressed on a FW basis (C) in sour pummelo seedlings. Bars represent means ± standard errors (n = 5-6). Different letters above standard error bars indicate significant differences at P < 0.05.