Study On Cadmium (Cd) Stress Tolerance And Its Physiological Response Of Citrus

Cadmium (Cd) is one of nonessential and toxic heavy metals for both plants and animals, and its easily entrance into the food chain through the soil-plant system threatens human’s health. Citrus is the largest cultivated fruit-crop in China, but some citrus orchards have been polluted by Cd. Citrus are perennial plants with the possibility of easy accumulation. This character may affect the growth and development of tree and the fruit safety. While most of the commercial citrus trees are the combination of rootstocks and scions by grafting, so the characters of citrus tree’s tolerance to Cd in soil and its ability of Cd absorption, transportation and accumulation in different rootstocks determine the characters of the citrus tree. To study the citrus Cd tolerance and its physiological response is helpful to understand Cd affection on growth, cell membrane permeability and photosynthesis, to explicit the rule of Cd transportation and accumulation in different organs, and also helpful to screen citrus rootstocks and scions with the ability of high Cd endurance and low Cd accumulation, which can provide theory basis for production of the pollution-free and green citrus fruits.This thesis contains3parts. The first part is about the study on Cd endurance, transportation and accumulation in9citrus rootstock seedlings under Cd stress. The second part is about the study on physiological response of4citrus rootstock seedlings under Cd stress. The third part is about the study on Cd transportation and accumulation of2grafted young citrus trees under Cd stress. The results of each part were as follows.1. The study on Cd endurance, accumulation of9citrus rootstock seedlings under Cd stress9varieties of citrus rootstock seedlings were trifoliate orange (Poncirus trifoliate), red tangerine{Citrus reticulata Blanco cv. Tangerina), Ziyangxiangcheng (Citrus junos Sieb. ex Tanaka), Carriao citrange (C.sinensis Osb.×P.trifoliate Raf.), Ichang papeda (Ichangensis), Citrumelo (citrumelo), Goutou sour orange (C. aurantium L.), Brazil Sour orange (C. aurantium L.) and Cleopatra mandarin (C. reticulate Blanco.).4-month old rootstock seedlings grown in hydroponic culture under Cd stress [0mg·L-1,2.5 mg·L-1,10mg·L-1,20mg·L-1,50mg·L-1and100mg·L-1(Cd0、Cd2.5、Cd10、Cd20、 Cd50、Cd100for short)] were used as materials to study the growth, Cd content in aerial part and roots, Cd transport ratio (TR) and plant uptake factor (PUF). The results were as follows.(1) Under Cd stress in hydroponics culture for30days,9citrus rootstock seedlings under low Cd concentration (CdO, Cd2.5and Cd10) stress were alive. The higher concentration of Cd treatment was, the higher mortality the seedlings was. Under the highest Cd stress (Cd100), all varities of seedlings died.(2) Cd stress inhibited seedling growth, and this inhibitory effect increased with the increasing of Cd concentration. The order of Cd content resulted in20%growth decrease of citrus rootstock seedlings compared with the control was Cleopatra mandarin<Brazil Sour orange<Citrumelo<Ziyangxiangcheng<Carriao citrange<Goutou sour orange<Ichang papeda<Red tangerine<Trifoliate orange.(3) The roots of citrus rootstock seedlings were the main organs for the accumulation of cadmium. The Cd content Cd in the seedling organs increased with the increasing of the Cd stress concentration, except for Ziyangxiangcheng and Goutou sour orange (P＜0.05). The contents of Cd in roots were48-487times to the aerial parts.(4) The TR of9citrus rootstock seedlings was improving with the increasing of Cd content, while the rule of PUF was contrary. Under low Cd stress (Cd2.5), citrumelo had the lowest TR (0.20%), trifoliate orange and Ichang paped had the secondly low one (0.23%). Ziyangxiangcheng had the highest one (0.97%). However, under the high Cd stress (Cd50), citrumelo had the lowest TR (1.92%), red tangerine had the second low TR (2.87%), and Goutou sour orange had the highest TR (6.71%). Under low Cd stress, the PUF of Goutou sour orange was1.7-7.7times to the others. While the differences of PUF among the cultivars decreased with the increasing of the Cd stress concentrations.(5) Trifoliate orange showed tolerance to Cd with low absorption and accumulation capacity. Goutou sour orange showed middle tolerance to Cd with low absorption ability but high translocation ability. Ziyangxiangcheng and Cleopatra mandarin showed susceptible to Cd with high absorbing-translocation ability. The other cultivars showed undifferentiated tolerant characters.2. The study on physiological responses of4citrus rootstock seedlings under Cd stress4varieties of citrus rootstock seedlings are trifoliate orange (Poncirus trifoliate), red tangerine (Citrus reticulata Blanco cv. Tangerina), Carriao citrange (C.sinensis Osb. ×P.trifoliate Raf.) and Ziyangxiangcheng(Citrus junos Sieb. ex Tanaka).4-month old seedlings grown in hydroponic culture under different Cd stresses (0,2.5mg-L-"ZHI-CK, HONG-CK,KA-CK and ZI-CK" and "ZHI-Cd2.5, HONG-Cd2.5, KA-Cd2.5and ZI-Cd2.5" for short) were used as materials to study. The change of cell membrane permeability, photosynthesis antioxidant enzyme activities and osmoregulation substance were detected under different cadmium treatment. The results were as follow.(1) Cd stress leded to membrane lipid peroxidation of4varieties of citrus rootstock seedlings. The longer the processing time was, the higher content of malondialdehyde (MDA) in trifoliate orange was. While the MDA contents in other3rootstock seedlings rose first and then fell. The relative conductivity of the4citrus rootstock seedlings under Cd stress was significantly increased by106.4%(trifoliate orange),8.1%(red tangerine),25.7%(Carriao citrange) and10.1%(Ziyangxiangcheng) than the control, respectively.(2) Leaf chlorophyll content (Chl a, Chl b) and chlorophyll fluorescence of the4citrus rootstock seedlings under Cd stress were significantly lower than those of control (P＜0.05). Rate of chlorophyll fluorescence decreased slowly at first and rapidly in the end.(3) Cd stress induced the change of antioxidant enzyme activity. Superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities rose at beginning, and then performed different activities with the extension of the stress time. Overall, the SOD and POD activities of trifoliate orange were higher, at beginning, but the two enzyme activities of red tangerine were higher in the end.(4) Cd stress led to the change of the contents of membrane permeability related materials (proline and soluble protein). The proline contents in seedling leaves of trifoliate orange, red tangerine and Carriao citrange rose under Cd stress, while the proline content in seedlings leaves of Ziyangxiangcheng rose first and then fell. There were different changes of the protein contents in the4rootstock seedlings, trifoliate orange rose, red tangerine and Ziyangxiangcheng rose first and then fell. There was no significant change of soluble protein in leaves of Carriao citrange.3. The study on Cd transportation and accumulation of2grafted young citrus trees under Cd stress.2grafted young citrus trees were Fengwan navel orange (FWO)(Citrus sinensis Osbeck) and Satsuma mandarin(Citrus unshiu cv.’Okitsu wase’) on Carrizo citrange (C.sinensis Osb.×P.trifoliate Raf.) rootstock. The3-year old grafted young trees grown in soil under different Cd stresses (0g/2m2,15g/2m230g/2m260g/2m2(CK, Cdl, Cd2and Cd3for short) were used as materials to study Cd transportation and accumulation. The results were as follow.(1) The leaf Cd content of in the two varieties of citrus increased as the stress time extended. At the end of the experiment, the leaf Cd contents of FWO stressed by Cdl, Cd2and Cd3treatments were2.9,2.7and2.1times to those of Satsuma mandarin, respectively.(2) The accumulation of Cd in organs of the two citrus varieties showed roots> leaves>xylem>phloem>peel>pulp. The Cd contents in all citrus fruits reached the pollution-free food standards of China (＜0.05mg·kg-1FW).(3)(3) According to the pollution-free food standards of China and above study results, the threshold Cd contents of soil for producing pollution-free citrus fruit were7.6mg·kg-1(total Cd content) and5.7mg·kg-1(available Cd content) for FWO, and22.2mg·kg-1(total Cd content) and17.9mg·kg-1(available Cd content) for Satsuma mandarin.