| As an essential trace nutrient element for plant growth and development,iron is involved in various physiological processes including photosynthesis,respiration and redox reactions in plants.Foliar application of iron fertilizer has been reported to increase crops yield,especially in iron-deficient condition.Nowadays,there are many researches on fruit trees chlorosis caused by iron deficiency.However,people easily ignore the demand for iron nutrition in vegetables because vegetables do not show any symptom of iron-deficiency chlorosis.Furthermore,soil,plant and microorganism can interact and communicate closely.It can be inferred that spraying iron fertilizers will promote growth and development of plants,which may change nutrient of soil due to different nutrient requirements of plants growth and alter the community structure of rhizosphere soil microbe.Therefore,in order to explore the connections,we use vegetables of Brassica chinensis and asparagus lettuce for experiments containing indoor pot experiment and outdoor field experiment,respectively.The whole experiments are conducted in alkaline soil developed from limestone to study the changes in growth indicators（above-ground and below-ground yield）and physiological indicators（chlorophyll content,antioxidant enzyme activity,and malondialdehyde content）of vegetables,soil physical and chemical properties and rhizosphere soil microbial community.Afterwards,we make a further research to analyse the differences of root activity,root morphology and root exudates metabolomics of Brassica chinensis in different treatments under hydroponic conditions.In a word,this study comprehensively explains the internal mechanism of plant differential growth by foliar iron fertilization,in order to provide effective solutions for the growth limitation of vegetables under iron deficiency condition.The main findings are as follows:（1）The indoor pot experiment showed that,there were significant differences in Brassica chinensis biomass after different iron fertilization treatments.Among them,the lowest biomass（20.24 g）was found in Y0 treatment,whereas the highest biomass（37.24 g）was appeared in Y4treatment.Compared with CK,different treatments all significantly increased the chlorophyll content of Brassica chinensis leaves.Different fertilization treatments have different effects on the antioxidant enzyme activity and malondialdehyde（MDA）content,soil nutrients,and microorganisms.The principal component analysis（PCA）results showed that the microbial community structure was significantly different under different fertilization treatments.The results of correlation analysis showed that there was a significant negative correlation between the yield of cabbage and soil available potassium,but no significant correlation with other indicators.（2）The outdoor field experiment showed that,there were significant differences in asparagus lettuce biomass after different iron fertilization treatments,in which Y0 had the lowest biomass of aboveground and belowground as 558.33g and 23.60g,respectively.Besides,different organic iron fertilization all promoted the increase of aboveground and belowground biomass of asparagus lettuce.Among them,the aboveground biomass of Y4 treatment was the highest（723.75g）,while Y3 had the highest belowground biomass,39.85g.Different organic iron fertilization increased the chlorophyll content of asparagus lettuce leaves,among which Y4 was the highest（29.17 SPAD）.Different iron fertilizer treatments have different effects on the antioxidant enzyme activity and malondialdehyde（MDA）content,soil nutrients and microorganisms of asparagus lettuce.Among them,the three antioxidant enzyme activities,and MDA content were the highest under Y0 treatment.The content of AP and AK of Y4 were the lowest,while total PLFA,various fatty acids,and various microorganisms were the highest.The PCA results showed that spraying different iron fertilizers had a significant effect on the microbial community structure of rhizosphere soil.The results of correlation analysis showed that the aboveground biomass of asparagus lettuce had a significant positive correlation and extremely significant positive correlation with chlorophyll conten and belowground biomass,respectively.However,the aboveground biomass had a significant negative correlation with MDA and available phosphorus.Similarly,the belowground biomass had a significant positive correlation with chlorophyll content,but a significant negative correlation with SOD.（3）The hydroponic experiment showed that,the biomass of Brassica chinensiss showed significant differences after spraying different iron fertilizers.Among them,the aboveground and belowground biomass of Brassica chinensiss in Y0 were the lowest as 7.26 g and 0.95 g,respectively.On the contrary,different concentrations of organic iron fertilizer all increased the biomass of aboveground and belowground,in which Y4 had the highest aboveground biomass（31.60 g）and Y3 had the highest belowground biomass（3.30 g）,respectively.Compared with CK,different treatments all significantly increased the chlorophyll content of Brassica chinensiss leaves,however,there was no significant difference between the treatments.Different treatments had different effects on the three antioxidant enzymes（SOD,POD and CAT）activities of Brassica chinensis leaves.（4）The result of root development showed that,different foliar iron fertilization had a significant effect on the root morphology and physiology.Compared to CK,Y0 significantly inhibited the root growth of Brassica chinensis.Therefore,the total root length,total root volume,and average root diameter were the lowest values.The root morphological parameters of Brassica chinensis treated with decreasing concentrations of organic iron fertilizers showed an increasing trend.Among them,Y4 significantly improved the root growth.The root activities of Brassica chinensis had significant differences.Compared to CK,Y0 significantly decreased the root activity,which was 438.86μg·g-1·h,however,Y4 significantly increased it(1163.60μg·g-1·h).The correlation analysis results showed that the aboveground biomass of cabbage had significant positive correlation with belowground biomass and total root surface area,and had extremely significant positive correlation with total root length,average root diameter,total root volume and root activity.Similarly,there was a significant positive correlation between belowground biomass and total root volume.（5）Further gas chromatography-mass spectrometer（GC-MS）analysis of root exduates of Brassica chinensis under different treatments showed that,the root exduates had significant differences after foliar application of iron fertilizers,which may be the most important reason for differences in root morphological indicators and pakchoi growth.A total of 290 metabolites were quantified via GC-MS.The shape and number of peaks varied,which could indicated the differences between the metabolite profiles.According to PCA and orthogonal projections to latent structures-discriminant analysis（OPLS-DA）analysis,all of the samples in the score plots were within the 95%Hotelling’s T-squared ellipse,thus indicated that there was no outlier among the analyzed samples,and the differences between the different treatments are far away each other.Therefore,the result also showed that there were significant differences in root exudates between different treatments.On the basis of the results,two obvious treatments of Y0 and Y4 were finally chosen to compare the differential metabolites between them and CK,respectively.A total of 12 metabolites were considered as significantly different metabolites（SDMs）between Y0 and CK.Similarly,there were27 SDMs between Y4 and CK.（6）Under the condition of iron deficiency,spraying different iron fertilizers has different effects on the yield of leafy vegetables.0.05%ferrous sulfate significantly reduces the yield.Organic iron fertilizers with different concentrations can increase the yield,of which 0.02%organic iron fertilizer has the best effect.Spraying iron fertilizer can increase the chlorophyll content to different degrees.In addition,the treatment with 0.05%ferrous sulfate increases the malondialdehyde content,causing greater damage to the plants.On the contrary,organic iron fertilizer can slow down the iron deficiency.Brought by the stress,thereby promoting the growth of vegetables and increasing yield.However,the effects of spraying iron fertilizer on the soil physicochemical properties and rhizosphere microbes of pakchoi and lettuce are different.The experimental soil available nutrients in pakchoi changed significantly,and the different available nutrients（alkaline nitrogen,available phosphorus and available potassium）decreased,but increased after treatment with ferrous sulfate.There was no obvious change in soil nutrients in the experiment of lettuce.Similarly,the content of microorganisms in the rhizosphere soil was inconsistent.The rhizosphere microorganisms in lettuce showed an increasing trend under different treatments,while the rhizosphere microorganisms in pakchoi decreased first and then increased. |
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