| The experiment was carried out with the seedlings of two apple rootstock cultivars(Malus hupehensis Rehd. and Malus xiaojinensis Cheng et Jiang) under zinc-deficiency stresswith hydroponics. Not only the physiological and root morphology responses of two applerootstocks under zinc-deficiency stress were investigated, but also the antioxidative capabilityand plant hormone level responses of two apple rootstocks with no zinc supply wereanalysised for exploring the difference of endurance and sensitivity to zinc-deficiency stressbetween cultivars of two apple rootstocks. Effect of IAA concentration, polar transportationand redistribution to root growth and development under zinc-deficiency condition wasresearched to explain the role of IAA in the root zinc-deficiency availability of the applerootstocks.(1) The physiological and root morphology responses of two apple rootstocks underzinc-deficiency stress were investigated for exploring the difference of endurance andsensitivity to zinc-deficiency stress between cultivars of two apple rootstocks. The experimentwas carried out with the seedlings of two apple rootstocks cultivars under zinc-deficiencystress with hydroponics. The changes in the biomass, the morphology and architecture of root,the activity of root, and the zinc accumulation in plant shoot and root were studied. Therewere significant differences between different apple rootstocks cultivars in response tozinc-deficiency stress. The obtained results showed that many obvious zinc-deficiencysymptoms were observed in the two apple rootstocks seedling receiving no Zn supply, such as,the plant dwarfed, the new leaves etiolated and rosetted, the length of stem node decreasedand the root tips enlarged. The results also indicated that zinc-deficiency treatment delayedthe occurrence of zinc-deficiency symptoms of the Malus xiaojinensis Cheng et Jiangseedling by7days than the Malus hupehensis Rehd.. The biomass of Malus hupehensis Rehd.root system was significantly inhibited under zinc-deficiency stress. Root is more sensitive to zinc-deficiency than that of shoot. Cultured for20days in a zinc-deficiency solution, the rootlength, root surface area, root volume and the number of root tips of the two apple rootstockscultivars are both decreased seriously, and the decreased ranges of Malus xiaojinensis Chenget Jiang are smaller than those of Malus hupehensis Rehd., Contrarily, the average diameter oftwo cultivars roots are increased under zinc-deficiency condition. Malus hupehensis Rehd.treatment plant had the maximum value of root activity of Malus hupehensis Rehd. treatmentplant appeared at the30th day, while that of Malus xiaojinensis Cheng et Jiang appeared at45th day. The content of Zn of two cultivars under zinc-deficiency stress was in the order asbelow: root>stem>leaf. Control plants> Malus xiaojinensis Cheng et Jiang>Malus hupehensisRehd. treatment plant. The sensitivity and endurance of the two apple rootstocks seeding tozinc-deficiency stress were different. The Malus hupehensis Rehd. was more sensitive to zincdeficiency and its root was vulnerable to zinc-deficiency stress, while Malus xiaojinensisCheng et Jiang presented less decrease in its zinc content and had better endurance andresistance to the zinc-deficiency stress.(2) Observe on the anatomical structure of root cross-sections in Malus hupehensis Rehd.and Malus xiaojinensis Cheng et Jiang seedling. Studies have been made shown that underzinc-deficiency stress, not only root cortical thickness and root cortical cell diameter havesignificantly changed, but also the number of root vessel and root vessel diameter havechanged correspondingly. The diameter of root parenchyma cell and root cortical thicknesshave been reduced significantly in the two apple rootstocks seedling without Zn supply,Cortical layers of the difference apple rootstocks seedling was not significant. The diameter ofstele was significantly decreased, and its roots presented a smaller number of vesselles withbigger diameters.(3) The antioxidative capability and plant hormone level responses of two applerootstocks under zinc-deficiency stress were investigated for exploring the difference ofendurance and sensitivity to zinc-deficiency stress between cultivars of two apple rootstocks.The experiment was carried out with the seedlings of two apple rootstocks cultivars underzinc-deficiency stress with hydroponics. The changes in the antioxidant enzyme activity ofsuperoxide dismutase(SOD), peroxidase(POD) and catalase(CAT), membrane lipidperoxidation level(MDA) and the endogenous hormones content of indole-3-acetic acid(IAA), gibberellin(GA3), zeatin nucleotide(ZR) and abscisic acid(ABA) in plant leaf and root werestudied. There were significant differences between different apple rootstocks cultivars inresponse to zinc-deficiency stress. The obtained results showed that many obviousphysiological changes were observed in the leaves and roots of the two apple rootstocksseedling receiving no Zn supply, such as, the SOD activity was significantly decreased, whilethe activities of POD and CAT and the content of MDA showed increase. Comparing with theMalus hupehensis Rehd.seedling, the decrement of activitity of SOD of the Malusxiaojinensis Cheng et Jiang was smaller, the activities of POD and CAT increase at a greaterextent and the accumulation content of MDA was less. Cultured for45days in azinc-deficiency solution, the content of IAA and GA3of the two apple rootstocks cultivars areboth increased seriously, and the increased ranges of Malus xiaojinensis Cheng et Jiang arebigger than those of Malus hupehensis Rehd., Contrarily, the content of ZR and ABA of twocultivars roots are decreased under zinc-deficiency condition. The sensitivity and enduranceof the two apple rootstocks seeding to zinc-deficiency stress were different. The Malushupehensis Rehd. was more sensitive to zinc-deficiency and its membrane lipid peroxidationlevel was high under zinc-deficiency stress, while Malus xiaojinensis Cheng et Jiangpresented more potent antioxidant effect, had more plant hormones that promotes plantgrowth and had better endurance and resistance to the zinc-deficiency stress.(4) It is concluded that zinc-deficiency stress perform its action on the endogenous IAAcontent of root through reducing of IAA synthesize, inhibiting IAA exportation from shootapex and IAA polar transportation, the inhibitory extent of root system architecture on Malusxiaojinensis Cheng et Jiang was lower than that on Malus hupehensis Rehd.. Two differenttemporal phases were observed in the response of RSA to zinc-deficiency stress. First, lateralroot development was promoted between days7and11after germination, but, after day11,all root growth parameters were negatively affected, leading to a general reduction of primaryroot and lateral root length. Zinc-deficiency availability had contrasting effects on variousstages of lateral root development, with a marked stimulation of lateral root primordialinitiation but a strong inhibition of activation of the initiated primordia. The involvement ofauxin signaling in these morphological changes was investigated in control plants treated withindole-3-acetic acid or2,3,5-triiodobenzoic acid. This shows that auxin plays a major role in the zinc starvation induced changes of root development. From these data, we hypothesizethat several aspects of the RSA response to Zinc-deficiency are triggered by localmodifications of auxin concentration. Measurements of local changes in auxin concentrationinduced by Zinc-deficiency is in line with these hypotheses. Furthermore, the observation thatZinc-deficiency availability mimicked the action of auxin in promoting LR development inthe Malus xiaojinensis Cheng et Jiang and Malus hupehensis Rehd. confirmed that zincstarvation stimulates primordial emergence through increased accumulation of auxin orchange in sensitivity to auxin in the primordia. Both the strong effect of2,3,5-triiodobenzoicacid and indole-3-acetic acid suggest that Zinc-deficiency availability modifies local auxinconcentrations within the root system through changes in auxin synthesis and auxin transport. |
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