Effects of aluminum on light energy utilization and photoprotective systems in citrus leaves

Abstract

Background and aims: Under high photon flux, excitation energy may be in excess in aluminum (Al)-treated leaves, which use a smaller fraction of the absorbed light in electron transport due to decreased CO2 assimilation compared with normal leaves. The objectives of this study were to test the hypothesis that the antioxidant systems are up-regulated in Al-treated citrus leaves and correlate with protection from photoxidative damage, and to test whether xanthophyll cycle-dependent thermal energy dissipation is involved in dissipating excess excitation energy. *

Methods: 'Cleopatra' tangerine seedlings were fertilized and irrigated daily for 8 weeks with quarter-strength Hoagland's nutrient solution containing Al at a concentration of 0 or 2 mM from Al2(SO4)3.18H2O. Thereafter, leaf absorptance, chlorophyll (Chl) fluorescence, Al, pigments, antioxidant enzymes and metabolites were measured on fully expanded leaves. *

Key results: Compared with control leaves, energy was in excess in Al-treated leaves, which had smaller thermal energy dissipation, indicated by non-photochemical quenching (NPQ). In contrast, conversion of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z) at midday increased in both treatments, but especially in Al-treated leaves, although A + Z accounted for less 40 % of the total xanthophyll cycle pool in them. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR) and catalase (CAT), and concentrations of ascorbate (AsA), dehydroascorbate (DASA), reduced glutathione (GSH) and oxidized glutathione (GSSG) were higher in Al-treated than in control leaves. *

Conclusions: These results corroborate the hypothesis that, compared with control leaves, antioxidant systems are up-regulated in Al-treated citrus leaves and protect from photoxidative damage, whereas thermal energy dissipation was decreased. Thus, antioxidant systems are more important than thermal energy dissipation in dissipating excess excitation energy in Al-treated citrus leaves.

Fig. 1.

(A) CO₂ assimilation; (B) non-photochemical quenching, NPQ; (C) photochemical quenching coefficient, qP; (D) PSII quantum efficiency; (E) electron transport rate; and (F) excess energy in Al-treated and control leaves. Bars represent mean ± s.e. (n = 8). Different letters above the bars indicate a significant difference at P < 0·05.

Fig. 2.

(A) Chlorophyll (Chl); (B) Chl a/b; (C, D) A + Z expressed on the basis of xanthophyll cycle pool or Chl; and (E, F) xanthophyll cycle pool expressed on the basis of area or Chl before dawn and midday in Al-treated and control leaves. Bars represent mean ± s.e. (n = 7). Significant differences were tested between pre-dawn and midday data for the same type of leaves, and Al-treated and control leaves taken at the same time. Different letters above the bars indicate a significant difference at P < 0·05.

Fig. 3.

Activities of (A) superoxide dismutase (SOD); (B) ascorbate peroxidase (APX); (C) monodehydroascorbate reductase (MDAR); (D) dehydroascorbate reductase (DHAR); (E) glutathione reductase (GR); (F) and catalase (CAT) in Al-treated and control leaves. Bars represent mean ± s.e. (n = 4). Different letters above the bars indicate a significant difference at P < 0·05.

Fig. 4.

(A) Concentrations of ascorbate (AsA); (B) dehydroascorbate (DAsA); (C) reduced glutathione (GSH); and (D) oxidized glutathione (GSSG); and ratios of (E) AsA/DAsA and (F) GSH/GSSG in Al-treated and control leaves. Bars represent mean ± s.e. (n = 4). Different letters above the bars indicate a significant difference at P < 0·05.