Study On The Physiological Mechanism Of Boron Alleviating Aluminum Toxicity Of Pinus Massoniana

The phenomenon of soil acidification is becoming increasingly serious.When the p H of the soil lower than 5.5,insoluble aluminum are converted into active Al[Al³⁺,Al（OH）³⁺,Al（OH）²⁺,etc.],which is released into the soil solution.Active Al can inhibit the growth of plant roots,and seriously disrupt the oxidation and metabolic system stability of plants,leading to cell lipid peroxidation and programmed death;Aluminum toxicity also affects photosynthesis and respiration of plants,affecting their normal growth and development.Pinus massoniana is an important timber and pioneer tree species for afforestation in barren mountains in China.It plays a very important role in forestry development and forest resource cultivation in China in whicih mainly distributed acidic soil areas.Previous studies have confirmed that Al toxicity in acidic soils can lead to imbalances in the antioxidant system of Pinus massoniana,hindering its normal growth and development.Aluminum toxicity has seriously restricted the sustainable development of masson pine forestry industry,and how to alleviate aluminum toxicity has become a key issue that urgently needs to be resolved for the sustainable and healthy development of masson pine industry in acid aluminum environment areas.Boron（B）,as a necessary micronutrient for promoting plant growth,plays a crucial role in the construction of plant cell walls and the physiology of growth and development.Recent studies have found that B can promote the growth of plant roots under aluminum toxicity,alleviate aluminum induced lipid peroxidation and cell wall structural disorders.As a fertilizer that can be directly applied to plant production,B is different from artificial compounds such as plant hormones and organic acids that alleviate aluminum toxicity,It has practical application value,which is of great significance for improving the plant growth environment in acid aluminum regions.Our previous experiment found that applying an appropriate amount of boron can promote the growth of Pinus massoniana under aluminum toxicity,but the specific mitigation mechanism is not yet clear.Therefore,this study used Hunan No.2 aluminum sensitive Pinus massoniana as the experimental material,using water culture treatment,through studying the impact of B on the growth,antioxidant properties,cell wall properties,and other characteristics of Pinus massoniana under aluminum toxicity,reveals the physiological mechanism of B participating in alleviating aluminum toxicity of Pinus massoniana,and constructs a pathway model.We hope to provide theoretical support for the production of Pinus massoniana trees in acid aluminum regions by exploring the specific mitigation mechanism of boron on aluminum toxicity of Pinus massoniana.After screening the optimal concentration of B to alleviate aluminum toxicity,we set up boron deficiency and boron addition treatments to study the effects of boron on the growth,physiological properties,and cell wall components of Pinus massoniana under aluminum toxicity.The main research results are as follows:1.This experiment measured the relative root elongation of 16 genotypes of Pinus massoniana,and used this as a classification standard to classify them into three categories using cluster analysis:aluminum sensitive category represented by Hunan 2,medium aluminum tolerant category represented by Fujian 2,and high aluminum tolerant category represented by Guangxi 2.2.In the screening test for the optimal concentration of B to alleviate aluminum toxicity in Pinus massoniana,different concentrations of B have different effects on the growth of Pinus massoniana under the acid aluminum environment.Among them,boron deficiency also has a certain inhibitory effect on the growth of Pinus massoniana,but it is not as obvious as that of herbaceous plants.The appropriate concentration of boron in the acid aluminum environment can promote the growth recovery of Pinus massoniana,but excessive B concentration may cause boron and aluminum co stress with aluminum,thereby inhibiting the growth of Pinus massoniana.Finally,we obtained the optimal concentration of exogenous boron to alleviate aluminum toxicity in Pinus massoniana seedlings under acidic conditions（p H=4.0±0.5）through case type cluster analysis,which was 40μM.3.Under aluminum toxicity（boron deficiency）,the root elongation of Pinus massoniana seedlings is inhibited,the number of lateral roots decreases,the plant becomes shorter,the content of photosynthetic pigments decreases,and aluminum is highly aggregated at the root tips.After adding 40μAfter MB,the growth inhibition effect caused by aluminum toxicity can be alleviated;At the same time,the aluminum content in the roots of Pinus massoniana seedlings continuously increases with the biological upper end direction.Combined with the coloring situation of the root hematoxylin staining test,the transition from deep immersion to shallow surface coloring of aluminum indicates that boron can effectively reduce the accumulation of aluminum in the root tip cells,shielding Al³⁺from entering the cells.In addition,the analysis of aluminum distribution coefficients in various parts found that boron had a tendency to promote the upward transport of Al³⁺,promoting the transfer from stem to leaf.4.Aluminum toxicity（boron deficiency）leads to disorder of the oxidative system of Pinus massoniana,a large accumulation of MDA and reactive oxygen species,resulting in lipid peroxidation and cell damage.This study found that exogenous addition of boric acid can regulate the antioxidant system of roots and leaves of Pinus massoniana under aluminum toxicity to resist oxidative stress:the activities of major antioxidant enzymes such as SOD,CAT,and APX are enhanced,and the increase in proline content indicates that B induces the enhancement of the antioxidant defense system of Pinus massoniana,and it is clear that excess oxygen free radicals alleviate the oxidative damage caused by aluminum toxicity.The results of weight analysis also showed that B had a strong regulatory effect on proline content in Pinus massoniana under aluminum toxicity.5.Aluminum toxicity（boron deficiency）resulted in a significant increase in polysaccharide content in the root cell wall of Pinus massoniana,an increase in PME activity,and a significant decrease in DM and KDO values.When Pinus massoniana lacks boron,aluminum induces the activity of PME enzymes and reduces the content of KDO in the cell wall,thereby reducing the proportion of RG-Ⅱin total pectin.This result may disrupt the stable cell wall structure of Pinus massoniana,increase the network of cell walls,accelerate the entry of active aluminum into the cells,causing physiological damage.After applying boron,the KDO content of Pinus massoniana increased,while the total pectin content decreased,indicating that the structural stability of the cell wall was restored,the pore size of the cell wall was reduced,the formation of pectin dimers was promoted,the aluminum binding sites in the cell wall were shielded,and the accumulation of aluminum in the cell was reduced.Boron may play two roles in alleviating aluminum toxicity of Pinus massoniana:under aluminum stress,boron is added to enhance the content of antioxidants and related enzyme activities to strengthen the antioxidant defense system and reduce peroxidation damage;regulating the accumulation of cellulose in the cell wall and the degree of methyl esterification of pectin improved the root cell wall hardening caused by aluminum toxicity,thereby improving root ductility and promoting root growth.The results of this study reveal some physiological mechanisms of boron alleviating aluminum toxicity of Pinus massoniana.According to the results of this study,we suggest that the use of boron fertilizer in Pinus massoniana afforestation in acid aluminum area can effectively avoid and resist aluminum toxicity.