Abstract: To investigate the effects of magnesium（Mg）stress on physiological responses and anatomical characteristics of‘Harumi’tangor [Citrus reticulate ×（C. reticulata × C. sinenesis）] plants grafted on Xiangcheng（C. junos）rootstock，Mg at five levels（0，2.4，12，24 and 240 mg · L-1）in nutrient solution was exogenously supplied to the plants grown at sand medium，in which 24 and 240 mg · L-1 were considered as control and excess treatments，respectively，and the other three were treated as low Mg. Our results showed that，Mg stress（either under low or excess conditions）significantly decreased plant height，root length，root viability，leaf chlorophyll content as well as dry mass in various parts，whereas it induced leaf relative conductivity and MDA content. The effects were more pronounced in plants supplied without Mg（0 mg · L-1 Mg 2+ treatment），while the activities of antioxidant enzymes（SOD and CAT）in leaves declined synchronously. Mg concentration in all plant parts increased significantly as external Mg concentration increased，and it was found that compared to the other tissues，Mg tended to be accumulated more in leaves，where upper younger leaves had higher Mg concentrations than those in basal old leaves（except under excess Mg conditions）. Zn concentration showed various degrees of decreases in various tissues of plants under Mg stress treatments，especially under the low Mg treatment 2.4 mg · L-1. Moreover，concentrations of boron（B）and calcium（Ca）were higher in basal old leaves than those in upper young leaves with the exception of 2.4 mg · L-1 Mg,and excess Mg decreased those concentrations in leaves of either upper or basal parts. According to the anatomical observation in leaf structure and chloroplast ultrastructure，Mg deficiency stress resulted not only in the disorganization of chloroplsts（in a darker appearance of chloroplast stroma and a reduced volume of thylakoid membranes)，but also of the other organelles like the mitochondria having blurred membranes with reduced volume and abnormal black spots）. Excess Mg treatment also caused the mitochondria having blurred membranes，however，it led to disappearance of chloroplast lamellar，increment of the number and enlargement of the volume of starch grains and plastoglobulus.

Key words: citrus, magnesium, stress, rootstock, growth, distribution of mineral element, ultrastructure