Studies On Mechanisms Of Root Exudation Ameliorating Aluminum Toxicity In Soybean (Glycine Max L.)

In these experiments,19 high quality varieties and lines of soybean(Glycine max L.)from East and nabe were tested for their tolerance to aluminum toxicity. With solution culture,sand culture and pot culture,two varieties of soybean (Zhechun No.2 and Zhechun No.3)were employed as the main material for the research.The important contents were comparatively study of nutritional components and exudating characteristics of soybean root exudations under aluminum;Synthetically analysis of the effect of soybean root exudates on mobilization of soil sparingly soluble phosphorus and potassium,chelation of Al³⁺,scavenging activity against DPPH free radical,and allelopathy of soybean seedings,as well as microorganism population;biochemical function and soil enzyme;Primarily probe into chelation with almuimun and transportation of important organic acids in soybean and lucubrating the inducing system of aluminum on root exudation of soybean.Further more,the dynamic changes of root morphology,accumulation of Al and respiratory metabolism in root tips were studied.The main research were as follows:1.19 cultivars were analysed and found to differ considerably in aluminum (Al)resistance,it shows that relative root length of Zhechun No.2 was 127%which was universally higher than others for strongest resistance of Al. By contraries,2DD06097 and Zhechun No.3 were most sensitive to Al.So Zhechun No.2(Al-resistant)and Zhechun No.3(Al-sensitive)were selected for further analysis,considering the facts.2.Citric acid was the idiocratic substance in soybean exudation under Al,but amino acid,isoflavone,phenolic acid,carbohydrate and so on were also detected with dose and time matter.The effects of Al stress on soybean exudation had beguin after 2-h induration period,yet Al had a continuous effect on all these components when roots was removed from Al treatment solution within 6-h.But the effect palled on after 6-h.3.The result shows that soybean root exudates has remarkable mobilization ability of soil sparingly soluble phosphorus and potassium;The mobilization effect enhanced slowly with the concentration of root exudate and culture time of seedings,and the activation ability is stronger of root exudates under Al treatment than that of the CK.Furthermore,it has the efficacity of chelating Al and scavenging activity against DPPH free radical by soybean root exudates.The result of allelopathy reveals that it restrains the seed germination and radicle length of soybean as be considered the notable autotoxicity for 3 days of breeding;But it showed promotion effect for 7days of breeding.4.Results indicate that in treatments low in soil Al(0.2 and 0.4 g kg^-1),irritated secretion from the soybean roots increased the population of soil microbes, strengthened their capability of material transformation and stimulated the activity of soil enzymes,whereas in treatments high in soil Al(0.8g kg^-1), root exudation was restrained comparatively,resulting in inhibition of the activity of microorganisms and soil enzymes.The results also demonstrated that exogenous root exudation could also influence the microbial ecosystem.5.Result indicates that exchangeable aluminum[Al³⁺]and[(AlOH)²⁺, Al(OH)₂⁺]of Zhechun NO.2 decreased,while[HA-Al]increased compare to non-rhizosphere soils.It reveals that the root exudates had something to do with the acidification of soil,and the form of aluminium had a direct relationship with pH,so the root secretion also had a certain impact on the form of aluminum indirectly.6.The NMR results show that Al in soybean roots was mainly chelated to citrate and malate in Al-citrate(1:3)and Al-malate(1:1)stoichiometry with the corresponding chemical shifts at 11.16ppm and-0.38ppm respectively, but no Al-organic acid complexes were found in soybean stems or leaves. Dissolubility Al in soybean root,stems or leaves exists in [AlO₄Al₁₃(OH)₂₄(H₂O)₁₂]form with the chemical shift at63.2ppm.It make clear that it is mainly in roots that some important organic acids chelate with almuimun and form complexes with little toxicity which can be transport to shoots.7.The experiment also showed that the activity of H⁺-ATPase had relationship with citrate acids efflux of soybean root,as well as the activity of CS and AP. But MDH,NADP-ICDH and PEPCase associate little with citrate acids efflux.Moreover,citrate carrier inhibitor A9Cand anion channel inhibitor Mersal restrain availably the exudation of citrate acids.8.Results showed it was significant that aluminum accumulated in the cell wall of soybean root tip cells,and it became more and more remarkable with dose and time increased;in the present of exogenous citric acid,the content of aluminum accumulated in the cell wall decreased,especially in Zhechun No.3.The change of comparative Al content in root tips of Zhechun No.3 and Zhechun No.2 did not play the same pattern in dose-and time-dependent manner.From the analysis of aluminum speciation,it could be seen that the change pattern of the content of Al_a and Al_b are parallel;the content of the two form decreased but Al_c increased with exogenous organic acids.9.Results of water-soluble protein's expression showed that lower aluminum concentration can promote water-soluble protein' expression,but higher aluminum concentration and longer treatment time inhibited its expression; the degree of water-soluble protein's expressions in Zhechun NO.3 were obviously lower than that in Zhechun NO.2 after 12 h and 24 h treatments in the same aluminum concentration.With increasing of aluminum concentration and treatment time,the respiratory rate of root tips fell to different extent in both varieties;with the treatment of extraneous source citric acid.The change of Mitochondria' H⁺-PPase activity was similar to the respiration's,with increasing of aluminum concentration and treatment time,activity in both varieties fell; with the treatment of extraneous source citric acid,H⁺-PPase activity in Zhechun NO.3 is higher than that in Zhechun NO.2.