Studies On Responses And Their Physiological Mechanism Of The Three Citrus Cultivars Subjected To Magnesium Deficiency

In major regions of citrus production in China, distinct symptom of magnesium (Mg) deficiency was very common. In this study,"shiranuhi"(Citrus reticulata×(C. reticulata×C.sinensis)) ,"ponkan"(Citrus reticulate) seedlings grafted on trifoliate orange(Poncirus trifoliata), and"harumi"tangor (Citrus reticulate×(C. reticulata×C. sinenesis) seedlings grafted on xiangcheng (C. junos) rootstock were taken as experimental materials, by using sand culture, we studied the effects of Mg stress on physiological index, absorption and transportation of Mg, and leaf cell ultrastructural of the three citrus cultivars, as well as the effects of Mg and/or zinc(Zn) stress on growth, and distribution of mineral element in different parts of"shiranuhi"(Citrus reticulata×(C. reticulata×C.sinensis)) and"ponkan"(Citrus reticulata) seedlings, with aim of illustrating the response differences and their physiological mechanism of different citrus cultivars subjected to Mg stress.The main results were as follows:(1)Changes of physiological and anatomical features and characteristics of Mg distribution of citrus cultivars"shiranuhi"and"ponkan"seedlings to Mg deficiency were studied. The results showed that: 1) The deficiency of Mg decreased significantly the plant biomass, root viability, leaf chlorophyll content and superoxidase dismutase(SOD) activity of both cultivars, especially on"shiranuhi". Magnesium deficiency increased significantly peroxidase(POD) activities, malondialdehyde(MDA) contents and leaf relative conductivity of both plants leaves. Furthermore, MDA contents and relative conductivity in"shiranuhi"were significantly higher than them in"ponkan". In addition, leaf Catalase(CAT) activity of"shiranuhi"in Mg deficiency condition increased significantly, but it wasn't observed on"ponkan". 2) Mg contents of both cultivars'leaves at different positions increased with increasing Mg concentration in solution, and they decreased obviously with the leaf aged. Furthermore, Mg content of"shiranuhi"were significantly higher than that of"ponkan"under Mg deficient condition. 3) Ultrastructural observation of leaf structure and chloroplast showed that the organelle of"shiranuhi"was damaged seriously under Mg deficient condition, which was revealed by the vague thylakoid lamella with less number, drastic increase of starch grain volume, and the blurred membranes of chloroplast and mitochondria. As to"ponkan", distortion, plasmolysis and less grana lamella of chloroplast were observed, but the starch grain and platoglobuli numbers were increased. Thus, it's suggested that the influences of Mg defeciency on physiological and anatomical features were more obvious in"shiranuhi"than in"ponkan".(2) To investigate the effects of Mg, Zn and their interactions on plant growth, mineral content of different parts of"shiranuhi"and"ponkan"grafted on trifoliate orange rootstock, The results showed that: 1)Mg stress remarkably inhibited the growth of"shiranuhi", whereas had little influcence on growth of"ponkan", the total dry mass of"ponkan"was lower than that of"shiranuhi"under Zn deficiency treatment, and total dry mass of both citrus cultivars were notably decreased under Mg and Zn deficiency. 2) Mg absorption of the two citrus cultivars were increased under Zn deficiency treatment, but Zn absorption of both were decreased under Mg and Zn deficiency, especially on"shiranuhi". 3) There existed obvious differences on Mg, Zn, boron(B), iron(Fe) uptake and transportation in the two citrus cultivars, under Zn deficiency, contents of Zn, B, Fe were significantly different between the two citrus cultivars, however, under Mg deficiency, obvious differences were observed on contents of Mg, B, Manganese(Mn) in both cultivars.(3) To investigate the effects of magnesium (Mg) stress on physiological responses and anatomical characteristics of"harumi"tangor plants grafted on xiangcheng 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 weight 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^-1Mg²⁺ 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^-1Mg, 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.