| Zinc(Zn)is one of the essential trace elements for humans and plants.Zn can bind to proteins and plays an important role in human health and plant growth.Wheat is one of the main food crops in China,but the content of Zn in wheat grains is low.Therefore,increasing the Zn content in wheat grains is essential for Zn supplementation of human.Nitrogen(N),as one of the essential elements of plants,has been proved to be beneficial to the absorption of Zn by plants.The combined application of N and Zn can significantly increase the Zn content in crops.At present,although the research on Zn absorption of plants improved by N combined with Zn application has been in-depth,the mechanism of how plant root exudates respond to N combined with zinc application to improve Zn absorption has not been studied in detail.Therefore,in this paper,the effects of N and Zn combination on root exudates and metabolic pathways of winter wheat were studied via hydroponic experiment,soil culture experiment,pot experiment and root box experiment.The effects of exogenous organic matter secreted by roots on soil p H,available Zn and Zn chemical forms,and the effects of exogenous cis-aconitic acid and root exudates on N and Zn content in winter wheat,available Zn and Zn chemical forms in soil were studied.The possible mechanism of N and Zn combination to improve Zn availability in soil through root exudates of winter wheat was revealed.The main results are as follows:1.The effects of N and Zn combination on the composition of root exudates and metabolic pathways in winter wheat were studied via metabolomics.The results showed that N,Zn and their combination had different effects on the composition of root exudates in winter wheat.N,Zn and N+Zn significantly up-regulated the secretion of3-methylthiopropylamine,3-isochromanone,cis-aconitic acid and cis-gondoic acid.N and N+Zn significantly up-regulated the secretion of nine metabolites such as 1-monopalmitin,palmitic acid,tetracosane and so on.Zn and N+Zn significantly increased the secretion of eleven metabolites such as 1-hydroxyanthraquinone,2-indanone,3-hydroxybutyric acid,4-hydroxymethyl-3-methoxyphenoxyacetic acid,and so on.N treatment significantly up-regulated six metabolites and down-regulated twenty metabolites.Zn application significantly up-regulated four metabolites and significantly down-regulated three metabolites.In addition,N+Zn treatment significantly up-regulated three metabolites and down-regulated seven metabolites.At the same time,N,Zn as well as N and Zn combination affected the metabolism of winter wheat,including the tricarboxylic acid cycle(TCA cycle),c5 branched dicarboxylic acid metabolism,glyoxylic acid and dicarboxylic acid metabolism,arginine and proline metabolism,2-oxycarboxylic acid metabolism and other metabolic pathways.2.The effects of exogenous organic matter to simulate root exudates of winter wheat on p H,available Zn and Zn chemical forms in soil were studied.The results showed that under the control treatment,exogenous organic matter had no significant effect on available Zn and transformation of Zn chemical forms in soil.Under the condition of N and Zn combination,exogenous cis-aconitic acid,3,5-dihydroxyphenylglycine and glycine could reduce soil p H.Exogenous cis-aconitic acid,3,5-dihydroxyphenylglycine,glycine and glucose-1-phosphate promoted the transformation of Zn from insoluble to carbonate-bound and organic-bound form,and improved the availability of Zn in soil.This result reveals that the organic matter secreted by roots can change the soil p H and promote the transformation of Zn from insoluble to a highly effective form,thereby improving the availability of Zn.3.The effects of exogenous cis-aconitic acid on available Zn in soil,N and Zn contents in different parts of winter wheat were studied.The results showed that the combination of N and Zn increased the contents of citric acid and cis-aconitic acid in the roots and shoots of winter wheat,which was related to the increased activities of citric acid synthase(CS)and cis-aconitic acid enzyme(ACO).The decrease of isocitric acid content in shoots indicated that the conversion of cis-aconitic acid to isocitric acid was inhibited.This inhibition may be the reason for the combination of N and Zn promoting the secretion of cis-aconitic acid.At the treatment of N and Zn combination,the addition of 10 μg/plant cis-aconitic acid significantly increased the available Zn content in soil and the Zn content in winter wheat.The results showed that exogenous addition of cis-aconitic acid could activate soil Zn and improve the absorption and accumulation of Zn in winter wheat.This result reveals the possible mechanism of regulating the metabolism and secretion of cis-aconitic acid under the condition of N and Zn combination to improve the Zn availability in winter wheat.4.The effects of exogenous root exudates on available Zn,chemical forms of Zn in rhizosphere and non-rhizosphere soil,as well as Zn content in different parts of winter wheat were studied.The results showed that under N and Zn combination,exogenous cis-aconitic acid could significantly reduce the content of residual and ironmanganese oxide-bound Zn in rhizosphere and non-rhizosphere soil,increase the content of exchangeable and carbonate-bound Zn,which was beneficial to the transformation of Zn to available forms,and significantly increase the available Zn content in rhizosphere and non-rhizosphere soil.Exogenous root exudates and cisaconitic acid could significantly increase the Zn content in roots and grains of winter wheat,which was conducive to the Zn accumulation in winter wheat,and the effect of cis-aconitic acid was stronger than root exudates.This result reveals the mechanism of cis-aconitic acid in improving the bioavailability of Zn in winter wheat under the condition of N and Zn combination. |
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