| Aluminum (Al) is an important limited factor of rape production and quality in acid soil, rape was considered as a middle Al-resistant plant with a lower resistance to Al toxicity when compared with other crops like soybean and buckwheat, therefore it’s more susceptible to Al toxicity. In recent years, the mechanism of plant alleviate Al toxicity under Al stress are mainly focused on root exudation (exudate site, ingredient of exudation, exudate time, and exudation content), cell wall, border cells, and so on. Among them the alleviate mechanism of organic acid exudation from plant root has become a research hotspot, but study about effect of exogenous organic acid on plant growth, nutritional element uptaking and transportation, Al forms in plant under Al stress has rarely reported. In our study, Al-resistant variety of Heza No.7and Al-sensitive variety of Zhongnongyou No.6which were chosen from11different varieties of rape as experiment material. We have studied the effect of exogenous organic acid (citrate and malate) on rape root growth, biomass, antioxidant system, changes of elements uptaking and transportation, Al content, and Al forms under Al stress by hydroponic experiments. We were supposed to reveal the role and mechanism of exogenous organic acid on rape under Al stress, and provide a theoretical basis for further research. The results of this study were summarized as followings:1. The selection result of Al resistant and sensitive rape varieties:11different varieties of rape were used in pre-experiment, and root elongation was measured. The result showed that the root relative elongation in Heza No.7and Zhongnongyou No.13 were much higher than that in other varieties of rape when under Al stress (100μmol·L-1) after3days treatment. The root relative elongation of them was99.84%and98.43%, respectively. The root relative elongation of Xiwang No.98and Zhongnongyou No.6were82.59%and76.40%, respectively. When we extended the processing time to6days, we found that Heza No.7still had a higher root relative elongation of99.66%, but the root relative elongation in Zhongnongyou No.6has decreased to69.03%, which suggested that Heza No.7had a higher tolerance to Al toxicity, and Zhongnongyou No.6was much sensitive to Al stress when compared with other varieties of rape in the same condition. So the most Al-resistant variety of Heza No.7and the most Al-sensitive variety of Zhongnongyou No.6were chosen as our experiment materials.2. The experiment on the characteristics of root morphology showed that under Al stress, the root length, surface area, and volume were significantly decreased when compared with the control in Zhongnongyou No.6, when exogenous organic acid added, the root length, surface area, and volume were significantly increased. Exogenous organic acid has a mitigation effect on both varieties of rape, but it has more obvious effect on Zhongnongyou No.6than on Heza No.7. Effect of exogenous organic acid on biomass under Al stress was also studied, we found that the biomass (including fresh weight, dry weight, water content, and root-shoot ratio) in root of Heza No.7were increased in single Al and Al organic acids treatments when compared with the control, and in treatment of Al+citrate has a significantly increased. The biomass in shoot of Heza No.7decreased in single Al treatment when compared with the control and organic acid treatments. However, the biomass in root and shoot in Zhongnongyou No.6were decreased in single Al when compared with the control, among them the root dry weight, and root-shoot ratio was significantly decreased. When exogenous organic acid added the biomass was increased compared with single Al treatment.3. Effect of exogenous organic acid on rape root antioxidant system:the SOD, POD, and APX activity in both varieties of rape were obviously increased when exogenous organic acid added compared with single Al treatment (except Al+citrate+malate in Zhongnongyou No.6). The activity in Zhongnongyou No.6(Al-sensitive) was much higher than that in Heza No.7(Al-tolerant). In addition, the content of MDA, soluble protein, and cell wall polysaccharide in root were significantly increased in single Al treatment in both varieties of rape, but all of these contents were significantly decreased when exogenous organic acid added. Comprehensive analyzing evaluation by the method of subjection function showed that malate has the best mitigation effects for root morphology, antioxidant system, and cell wall polysaccharide of rape.4. Exogenous organic acid application enhanced the accumulation of Al in rape root, but only a small part of Al was transported to stem and leaf, and this was very advantageous for plant to resistant to Al toxicity. Different organic acid have different effects in different varieties of rape. Citrate can significantly improve the absorption of Al in Heza No.7, Al content in Al+citrate treatment increased by50%compared with single Al treatment. While, the highest Al content appeared in mixed treatment of citrate and malate, and Al content was nearly three times larger than that in single Al treatment. The results of organic acid involved in the regulation of nutrient elements uptake and transportation in rape under Al stress as follow:the Ca、K、Fe content in stem and leaf in Zhongnongyou No.6and Heza No.7were significantly increased when organic acid added, and organic acid also promote the absorbtion of Mg, Ca, K, Fe, and P in root of Zhongnongyou No.6. Although organic acid has less effect on element absorption in Heza No.7, it enhanced the Mg, Ca, K, and Fe transported from root to leaf. We also found that, except Fe and P element, content of Mg, Ca, and K in stem and leaf were much higher than that in root. Comprehensive analyzing evaluation by the method of subjection function showed that malate has the best mitigation effects for Heza No.7and citrate for Zhongnongyou No.6.5. The NMR results showed that:Aluminum in rape root mainly exists in [AlO4Al12(OH)24(H2O)12]7+form with the chemical shift at63.2ppm in single Al treatment, and this is a most toxicity form for plant. When exogenous organic acid added, Al-citrate acid (1:1) was been detected, and with the corresponding chemical shift at11.13ppm and 11.21ppm. Al-malate (1:3) complexes was also found and the main chemical shift appeared at11.4ppm and11.36ppm, but when the concentration of-OH increased, parts of Al could formed as Al(OH)24+and chemical shift appeared at2.39ppm and2.42ppm. In addition, some other non-toxic or less toxic Al forms like Al-F and Al2(OH)42+also have been found in our study. It makes clear that Al mainly chelated with organic acids in rape root and formed complexes with little toxicity to alleviate Al toxicity.6. By determination the content of the root exudation, we found that, the added exogenous organic acid was significantly increased the content of citrate and malate on two varieties of rape when compared with single Al. But exogenous organic acids have a little effect on the acivity of APA and content of soluble sugar. Otherewise, the content of proline in two varieties were significantly increased in single Al treatment when compared with control, and it decreased obviously when exogenous organic acid added. The proline content in Zhongnongyou No.6decreased more then that in Heza No.7. We can conclude that organic acids can regulate the secretion of rape root exudation, and also have a mitigation effect on aluminum toxicity. |
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