Photosynthesis, Antioxidant System And Organic Acid Metabolism Of Citrus In Response To Magnesium Deficiency

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 magnesium (Mg) deficiency. So far, limited data are available on the effects of Mg deficiency on photosynthesis, antioxidant system and organic acid (OA) in citrus seedlings.'Sour pummelo'(Citrus grandis) and'Xuegan'(C. sinensis) seedlings grown in pots containing sands were irrigated for 12 weeks with 0, 50, 500 or 2000μM Mg. Thereafter, the effects of Mg deficiency on seedling growth, CO₂ assimilation, photosynthetic electron transport and antioxidant system in leaves, and carbohydrates and organic acid metabolism in roots and leaves were investigated. This objective of this study were to understand the mechanisms by which Mg deficiency leads to a decrease in CO₂ assimilation and to test the hypothesis that Mg-deficiency-induced changes in the contents of carbohydrates and OA metabolism differ between roots and leaves.Mg content in roots and leaves decreased with increasing Mg supply. Mg deficiency decreased the ratio of root DW to shoot DW, because root DW decreased to a larger extent than shoot DW in response to Mg deficiency. Mg-deficient plants firstly developed Mg-deficient symptoms in the older leaves. The first symptoms were small, interveinal, yellow areas on both sides of midrib and between the main veins, leaving an inverted"V"of green at the leaf base under severe Mg deficiency. The midribs and main veins on 0μM Mg-treated leaves showed enlargement and corkiness, as in B deficiency. The Mg-deficient symptoms were more pronounced in C. grandis leaves than in C. sinensis ones.Mg deficiency decreased CO₂ assimilation and stomatal conductance, but increased or unaffected intercellular CO₂ concentration. Mg deficiency affected CO₂ assimilation and stomatal conductance more in C. grandis than in C. sinensis. Chl, Chl a and Chl b contents in C. sinensis and C. grandis leaves increased with increasing Mg supply. There were no significant differences in Chl, Chl a and Chl b contents between both species. OJIP transients from Mg-deficient leaves showed increased O-step, accompanied by postivieΔL-,ΔK-,ΔJ- andΔI-bands, but Mg deficiency had less effect on P-step. Compared with controls, Mg-deficient leaves had decreasedφPo (Fv/Fm or TRo/ABS) and PItot,abs, but increased DIo/RC and deactivstion of OEC. Generally speaking, Mg-deficiency-induced changes in OJIP transients and related parameters were higher in C. grandis leaves than in C. sinensis ones.Compared with controls, Mg-deficient roots had similar or lower contents of carbohydrates and OA (malate and citrate), while Mg deficiency increased or did not affect their contents except for leaf citrate. The contents of glucose, fructose, sucrose and OA were not higher in C. grandis leaves than in C. sinensis ones except that the contents of sucrose soluble sugars under 500μM Mg and the contents of starch, TNC and citrate under 500 and 2000μM Mg were significantly higher in the former. In addition, the Mg-deficiency-induced changes in the activities of OA-metabolizing enzymes differed between roots and leaves.Mg-deficient leaves had higher or similar activities of antioxidant enzymes (SOD, APX, MDAR, DHAR, GR and GPX) and centents of antioxidant metabolites (ASC, ASC + DHA and GSH + GSSG) except for CAT and reduced gulathione concentration. However, Mg deficiency increased the content of MDA.In conclusion, Mg deficiency affects CO₂ assimilation more in C. grandis leaves than in C. sinensis ones. Although lower proportions of photosynthates are partitioned to the roots of Mg-deficient seedlings, the Mg-deficiency-induced increase in sugars cannot explain why CO₂ assimilation decreased to a larger degree in Mg-deficient C. grandis leaves than in Mg-deficient C. sinensis ones. 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, because the Mg-deficiency-induced photoinhibitory impairment occurring on the whole photosynthetic electron transport chain from the donor side of PSII (i.e. OEC) up to the reduction of end acceptors of PSI is more severe in C. grandis leaves than in C. sinensis ones. The antioxidant system is up-regulated in response to Mg-deficiency, but this up-regulation does not provide enough protection against the photoxidative damage. The Mg-deficiency-induced changes in the contents of carbohydrates and the metabolism of OA differ between roots and leaves.