Effects Of Boron On Organic Acids And Cell Wall Components Of Trichosanthes Kirilowii Maxim.Under Aluminum Stress

Trichosanthes kirilowii Maxim.is a kind of crop with economic value,due to the serious pollution of Al by the acidification of the soil,the promotion and cultivation of T.kirilowii is hindered.Therefore,this study uses hydroponic method to study the mitigation effect of boron on Al stress.To explore the specific mechanism of action of boron in mitigating aluminum stress,two Trichosanthes kirilowii cultivars with different Al resistance were used as experimental materials.Boron and Al were treated under hydroponic conditions,the growth index and photosynthesis parameters of T.kirilowii were measured.Then Al and boron content of root tips,antioxidant enzyme activities,cell wall components,and enzyme activity related to organic acid metabolism were measured.The findings are as follows:1.All kinds of growth index of T.kirilowii decreased after Al treatment.Among them,the root length was the most sensitive to Al toxicity.The toxic effect of Al on plants was positively correlated with the duration of stress.At the same time,there were differences in the aluminum tolerance between the two kinds of T.kirilowii.under stress treatment,the Al-sensitive Pujiang T.kirilowii was more strongly stressed.At the same time,aluminum toxicity significantly reduced the contents of Chla and Chlb in the leaves of the T.kirilowii,but the ratio of chlorophyll content did not change significantly.By observing chlorophyll fluorescence,we found F0increased,and F_m,F_v/F_m,and ETR significantly decreased.Therefore,Al stress inhibited chlorophyll synthesis,reduce photosynthesis efficiency to reduce the photosynthesis capacity of the T.kirilowii.After the application of boron,the growth index of each index was significantly improved,the chlorophyll content and fluorescence parameters were restored to normal levels.2.After Al treatment,Al³⁺accumulated in the apex,the root tip was thickened,while boron treatment effectively reduced the Al content in the apex and maintained the normal morphology of the root tip.The content of boron and Al in the root tip showed that boron inhibit the accumulation of Al in the root tip,but the content of boron was not regulated by the Al concentration.The reduction of POD and CAT enzyme activities under Al stress hindered the process of removing reactive oxygen species in the cell,leading to the occurrence of oxidative stress and the increasing of MDA content.Evans blue staining results also confirmed this result.After adding boron,various enzyme activities were up-regulated compared with the Al treatment group,and the MDA content was reduced.Therefore,boron treatment effectively relieved the oxidative stress caused by Al toxicity and maintained the integrity of the cell membrane structure.The activities of SOD enzymes in antioxidant enzymes showed different trends under stress treatment,so this enzyme may be one of the key enzymes in anti-Al toxicity of T.kirilowii.3.After the Al treatment,the content of cellulose and hemi-cellulose in the cell wall of the root of T.kirilowii increased,the rigidity of the cell wall increased,thus cell elongation was inhibited.This appearance was restored after the addition of boron.Al stress changes the pectin component of the cell wall through three ways to enhance the Al accumulation process of the cell wall:First,Al stress specifically induces the synthesis of pectin II,which has a stronger ability to bind Al³⁺,and enhances the cation exchange capacity of the cell wall.Secondly,the degree of esterification of two types of pectin is reduced by regulating PME activity,more free carboxyl groups are generated,which is beneficial to the binding of Al³⁺.Finally,Al toxicity reduced the proportion of RG-II in total pectin,weakened the pectin complexation,and enhanced the accumulation of Al in the cell wall by inducing the synthesis of other pectin components other than its RG-II.However,the Al phenotype was inhibited after boron application.Therefore,boron prevents Al³⁺from entering the cell by regulating changes in the polysaccharide composition of the cell wall under Al stress.4.After Al treatment,activities of PEPC and MDH increased significantly,but the effect of boron treatment on the enzyme activity was not significant.The results show that boron does not participate in the Al resistance process of the T.kirilowii by regulating malate metabolism.The CS enzyme activity increased significantly under aluminum stress.With the enhancement of citric acid synthesis,the ability of root exudates to chelate Al³⁺increased,and the CS enzyme activity decreased after boron treatment.Therefore,the mechanism of boron's mitigation of Al toxicity was not by strengthening citric acid compositing.The application of boron inhibited ACO enzyme activity,so it can be speculated that boron reducing the conversion efficiency of citric acid.Leading to an imbalance in the synthesis and decomposition of citric acid,which causes a high level of citric acid to accumulate in the root tip and eventually secrete more citric acid relieves Al toxicity.In summary,this study confirmed that boron enhanced the activity of the antioxidative enzyme system in T.kirilowii under Al stress,and inhibited the occurrence of Al-induced oxidative stress.Boron can also hinder the accumulation of aluminum at the apex of the plant by enhancing the cell wall polysaccharide component,organic acid secretion and other external aluminum tolerance mechanisms,alleviating the inhibition of aluminum growth and photosynthesis,thus reducing the aluminum poisoning phenotype of the plant.