| About 50% of the world’s arable land and potential available arable land are acidic soils.In China,acidic soils are mainly located in the south,and aluminum(Al)toxicity has emerged as a significant barrier to crop development and yield enhancement in acidic soils.Al toxicity inhibits the growth of plant roots,reduces plant height and leaf area,and inhibits the absorption of mineral nutrients and water,which leading to a reduction in crop yield.So,the toxicity of Al has become a major issue in agriculture,due to human activities,the proportion of acidic soils is increasing in worldwide.Boron(B)is one of the essential nutrients for plant growth and development,mainly playing a vital role in the cell wall.Previous studies have shown that B can enhance the capability of tolerance when plants are subject to abiotic stresses such as heavy metals,drought,and salt.In China,rapeseed(Brassica napus L.)is one of the most important oilseed crops,accounting for 1/3 of the world’s planting area and total yield.As a medium Al tolerant plant,Al toxicity seriously affects the growth and yield improvement of rapeseed.Currently,research about alleviating Al toxicity in plants is mostly focused on plant hormones.However,mitigation substances of Al toxicity cannot be widely used as the high cost.Therefore,this study was performed on rapeseed to explore the regulatory effects of B on the growth,antioxidant system,organic acid synthesis,and exudates of root in rapeseed under Al toxicity.The objective of the study is to reveal the physiological mechanism of alleviating Al toxicity on rapeseed induced by B application.The main findings of the study are as follows:(1)Effects of B on growth and nutrient uptake of rapeseed under Al toxicityAl toxicity significantly inhibited the growth of rapeseed,reduced plant height and root length,and affected root morphology.Meanwhile,Al toxicity also lead to the reduced absorption of elements such as calcium(Ca),magnesium(Mg),iron(Fe),manganese(Mn),copper(Cu),and zinc(Zn).Furthermore,the content of chlorophyll in leaves were all reduced with Al existences,which reduced the photosynthetic rate and thus decreased the biomass of rapeseed.The growth potential of rapeseed was significantly improved under Al toxicity after B application.The inhibitory effect of Al toxicity on plant height and root length was alleviated.Meanwhile,the absorption of Ca,Mg,Fe,Mn,Cu,and Zn were increased.More importantly,the content of Mg in leaves was also increased as B application.The increment promoted chlorophyll synthesis,enhanced photosynthetic rate,and thus improved rapeseed biomass.These consequences proved that B application could alleviate limitation of rapeseed growth caused by Al toxicity,and the most significant relief effect was shown in the treatment with 100 μmol/L B treatment.(2)Effects of B on membrane osmoregulation substances and antioxidant system in rapeseed under Al toxicityThe study showed that rapeseed root was directly exposed to Al toxicity,which damaged the rapeseed root,increased plasma membrane permeability,and enhanced the content of malondialdehyde(MDA),a membrane lipid peroxidation product.Meanwhile,exposing to the circumstance of Al toxicity could also result in leakage of intracellular electrolytes.However,the content of membrane osmoregulation substances(soluble protein and proline)increased with Al addition.With B application,the plasma membrane permeability of roots significantly decreased,while the content of soluble protein,proline and glutathione(GSH)further increased.In the treatment with high Al concentration,B could also enhance the Al tolerance of rapeseed by reducing ascorbic acid(As A)in the root and increasing glutathione.The soluble sugar content in leaves increased under Al toxicity,and B further regulated the soluble sugar content to better respond to Al stress.The above results showed that B might alleviate oxidative stress caused by Al toxicity through promoting the increment of the content of membrane osmoregulation substances,regulating the antioxidant system,and reducing the production of membrane lipid peroxidation products.(3)Regulation of B on pectin KDO and B and Al content in the cell wall of rapeseed roots under Al toxicityIn the study,we found that Al mainly accumulated in the root because the highest Al content in the root was observed under Al toxicity treatments.The Al content in the root was greatly decreased under Al toxicity after B addition,while the B content further increased,which significantly reduced the absorption of Al by the root.The alteration of B and Al content in the cell wall was similar to that in the root.B supply significantly limited the entrance of Al into the cell wall,which reduced the side effect of Al toxicity on rapeseed growth.Under 100 μmol/L B treatment,the content of alkali soluble pectin and 3-deoxy-D-manno-2-octanoic acid(KDO)significantly increased.Consequently,in the cell wall,B could reduce the Al binding site,decreasing its accumulation and ultimately slowing down the toxic effect of Al on plants.(4)Regulation of B on root exudates and organic acid synthesis in rapeseed under Al toxicityThe results showed that Al toxicity significantly increased the content of total organic acids,soluble sugars,citric acid,and malic acid in the root exudates of rapeseed,while decreasing the content of total amino acids.In the treatment with suitable B concentration,the content of total organic acid and soluble sugars further increased.The content of citric acid and malic acid in rapeseed leaves and roots significantly increased under Al toxicity.Except that,compared to the leaves,the root contained more malic acid,and the malic acid display far higher content than citric acid.After B was supplied,the amount of malic acid synthesis significantly increased,while there was no significant change in citric acid in root.By measuring the enzymes involved in the synthesis of citric acid and malic acid,it was discovered that under Al toxicity,citric acid synthase activity increased,while the activities of malic acid dehydrogenase and cis aconitase decreased.After applying B,the activities of citric acid synthase,malic acid dehydrogenase,and cis aconitase decreased.A reduction in the conversion of citric acid and the decomposition of malic acid resulted in an increase in citric acid and malic acid synthesis.Our results indicated that B could increase the secretion and synthesis of citric acid and malic acid by regulating the secretion of rapeseed roots and by regulating the activity of related synthetases.Moreover,altering the content of citric acid and malic acid could transform Al to a stable form and then reducing its toxic effects.In summary,notably growth inhibition of rapeseed under Al toxicity was observed.The malondialdehyde content and plasma membrane permeability also raised.Limitation impact could be alleviated after B supplying.Firstly,B could improve the absorption of nutrients by root.Secondly,B utilization could regulate the antioxidant system and increase the content of osmoregulation substances,thereby reducing oxidative stress caused by Al toxicity.Finally,to reduce the negative impact of Al on rapeseed growth,increase the secretion of organic acids by regulating the activity of key enzymes involved in organic acid synthesis. |
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