Effects Of Magnesium Deficiency On Citrus Sinensis Elements And Root And Leaf Anatomical Structures

Seedlings of’Xuegan’[Citrus sinensis (L.) Osbeck] grown in pots containing fine river sand were irrigated for 16 weeks with nutrient solution including 0 mM (Mg-deficiency) or 1 mM (control) MgSO4 every other day. Thereafter, we investigated the effects of magnesium (Mg)-deficiency on seedling growth, photosynthesis, chlorophyll (Chl) content, Fv/Fm and root, stem and leaf content of nutrient elements as well as the anatomical structures of roots and leaves. The main results are as follows:1. Mg-deficiency first led to yellowing on both sides of main (midrib) veins and between the lateral veins in the older leaves, leaving an inverted V-shaped green area at the leaf base. Mg-deficient midribs and main veins became corky, swollen and spitting. Later with increase in leaf age, these yellow leaves shed premature. Mg-deficiency inhibited seedling growth.2. Mg-deficiency significantly decreased Mg, P, B, Cu, Fe and Zn content, increased K and Ca content, but had no significant influence on S, N and Mn content in roots, stems and leaves.3. Mg-deficiency significantly decreased Chl content, CO2 assimilation, transpiration rate, stomatal conductance (Gs) and Fv/Fm, but significantly increased the intercellular CO2 concentration (Ci) in C. sinensis leaves.4. Paraffin section showed that Mg-deficiency mainly affected the zone of differentiation in C. sinensis roots. Mg-deficiency led to alteration in the endodermis cell wall structure under the epidermal cells, decreased the volume of the primary xylem cells, and impaired the development of cell wall.. Further observation demonstrated that mitochondria, Golgi apparatus and endoplasmic reticulum were closely related to the lignification of the secondary cell wall during the development of vessel element. Mg-deficiency caused the degradation of organelle within the vessel element as autophagosome, and accelerated the programmed cell death, thus imparing the structures of xylem, hence inhibiting the long distance transport of nutrient elements in the root system, eventually affecting shoot development.5. Mg-deficiency abnormally increased the volume of mesophyll cells, including sponge and palisade cells and the thickness of leaves, and led to the irregular lignification of sponge cell walls and the abnormal hyperplasia of the main and lateral vein vascular cambium. Transmission electron microscopy observation showed that Mg-deficiency did not significantly affect the number of starch grains and plastoglobulus in chloroplasts, but significantly increased their volume. Under Mg-deficiency, the stacked structures of thylakoids were relatively complete, but many grana existed between the membranes of thylakoids. Mg-deficiency impaired the development of secondary phloem, led to abnormal proliferation of vascular tissues, thus causing deformation of primary phloem due to serious extrusion, eventually resulting in the cracking of leaf veins. Dysfunctions of leaf phloem tissues might hinder the export of photoassimilates, thus affecting the transport of assimilates from leaves to roots, hence inhibiting root growth, development and functions.