Physiological impacts of magnesium-deficiency in Citrus seedlings: photosynthesis, antioxidant system and carbohydrates

Magnesium (Mg)-deficiency affects productivity and quality in agriculture, yet at a physiological level it is not well understood. Citrus grandis and Citrus sinensis seedlings were irrigated for 12 weeks with 0, 50, 500 or 2,000 mu M MgSO4. Thereafter, Mg-deficiency-induced changes in photosynthesis, antioxidant system and carbohydrates were investigated. Mg-deficiency affected CO2 assimilation more in C. grandis leaves than in C. sinensis ones, but Mg-deficiency-induced accumulation of sugars was not higher in the former except for sucrose. Mg-deficiency-induced photoinhibitory impairment occurring on the whole photosynthetic electron transport chain was more severe in C. grandis leaves than in C. sinensis ones. Mg-deficient leaves had higher or similar activities of antioxidant enzymes and contents of antioxidant metabolites except for catalase (CAT) activity and reduced glutathione (GSH) content. However, Mg-deficiency increased leaf malondialdehyde (MDA) content. In conclusion, the greater decrease in CO2 assimilation in Mg-deficient C. grandis leaves may be caused by the greater decrease in the photosynthetic electron transport capacity. Mg-deficiency-induced up-regulation in leaf antioxidant system does not provide enough protection to Mg-deficient leaves against the oxidative damage.