Fine Root Morphology,Hormone Synthesis And Secretion Characteristics Of Ectomycorrhizal Pinus Massoniana Seedlings Under Acid Aluminum Stress

Aluminum toxicity is the main factor restricting plant growth in acid soil.In recent years,climate warming,industrial development,environmental pollution,acid deposition,and application of agricultural fertilizers have all exacerbated soil acidification,and also promoted a large amount of aluminum dissolution into activated aluminum,which in turn deepened aluminum poisoning and greatly reduced the productivity of forest land.Pinus massoniana Lamb.is the main afforestation tree species growing on the acid soil in South China.It plays an important role in the forestry production.It once suffered from a large area of decay due to aluminum toxicity.Some ectomycorrhizal fungi（ECMF）can significantly alleviate the toxicity of aluminum to P.massoniana,which is beneficial to the growth of P.massoniana seedlings under acidic aluminum stress and the absorption and utilization of nutrients.In addition,inoculation of ECMF may also improve the resistance of P.massoniana to aluminum toxicity by affecting the fine root morphology and structure,but there are few studies on this at present.Based on this,the Pt 715 and Ld 2strains of Pisolithus tinctorius and Lactarius deliciosus were used to infect the seeds of P.massoniana to synthesize ectomycorrhizal seedlings.After acidic aluminum stress treatment and culture for 30 weeks,the fine root morphology structure and hormone content of the P.massoniana seedlings and the physical and chemical properties of the rhizosphere soil were respectively analyzed.P.massoniana seedlings without ECMF inoculation（non-ectomycorrhizal seedlings）were used as control.In order to explore the influence and mechanism of mycorrhization on the fine root morphology and structure of P.massoniana under acid aluminum stress,and further reveal the mechanism of ECMF to improve the aluminum resistance of plants,so as to provide theoretical basis for applying ectomycorrhizal technology to the recovery and reconstruction of productivity of P.massoniana forest land.The main results are as follows:（1）Ectomycorrhizal significantly improve the fine root morphological parameters of P.massoniana seedlings under acid aluminum stress.The number of fine roots of 1st（0.0-0.5 mm diameter）and 2nd（0.5-1.0 mm diameter）,as well as the root surface area and root length of 1st fine roots of P.massoniana seedlings are significantly reduce under acid aluminum stress,for example,2.0 m M Al³⁺significantly reduce the number of 2nd fine roots by 8.9%compare with no Al³⁺treatment,and the surface area and root length of 1st fine roots decrease by 10.0%and 15.4%,respectively.Compare with non-ectomycorrhizal seedlings,inoculation with Ld 2 and Pt 715 significantly increase the length of 1st fine roots under 2.0 m M Al³⁺treatment by 14.1%and 16.4%,respectively.In addition,ectomycorrhizal significantly improved the configuration parameters of fine roots of 1st and 2nd,and did not affect the configuration parameters of roots of 3rd.Therefore,ectomycorrhizal promotes the growth and differentiation of fine roots of P.massoniana seedlings under acid aluminum stress,and expands the nutrient absorption area of the root system.（2）Ectomycorrhizal significantly improve the fine root anatomy of P.massoniana seedlings under acid aluminum stress.The fine root anatomical structure of P.massoniana seedlings is significantly damage by acid aluminum stress:the relative area of fine root deformed cells and cortical cell thickness both increase significantly,while the relative area of xylem decrease significantly,for example,the relative area of deformed cells and cortical thickness of 2.0 m M Al³⁺are 17.9 and 1.4 times higher than those without Al³⁺treatment,respectively.Compare with non-ectomycorrhizal seedlings,ECMF inoculation significantly improved the anatomical structure of fine roots of P.massoniana under acid aluminum stress:when treated with 2.0 m M Al³⁺,inoculation of Pt 715 and Ld 2 significantly reduce the relative area of deformed cells of fine roots by 48.9%and 53.6%,respectively,the relative area of xylem increase significantly by 102.7%and 129.7%,and the relative thickness of cortex decrease significantly by 17.1%and 19.8%.In addition,no mycelium appear in the root system of non-ectomycorrhizal seedlings,and mycelium was observed in both ectomycorrhizal seedlings,and the relative thickness of the mycelium increase with the increase of the Al³⁺concentration;the relative thickness of the mycelium of Ld 2 mycorrhizal seedlings was significantly higher than that of Pt 715 mycorrhizal seedlings,when treated with 2m M Al³⁺,the former is significantly80.5%higher than the latter.It can be seen that ectomycorrhizal significantly improves the anatomical structure of P.massoniana fine roots under acid aluminum stress,in which the ectomycorrhizal plays an important role.（3）Ectomycorrhizal significantly promote the synthesis and secretion of endogenous growth hormone of P.massoniana seedlings under acid aluminum stress.Under acidic aluminum stress,the synthesis of endogenous hormones such as indole-3-acetic acid（IAA）,zeatin（ZT）,betaine（BET）,gibberellins（GA）,and ethylene（ETH）are significantly reduce and the synthesis amount of abscisic acid（ABA）increase significantly.1.0 m M Al³⁺significantly reduce the IAA and BET content of fine roots by 71.8%and 18.4%compare with no Al³⁺treatment,and the ABA content increase significantly by 91.1%.Compare with the non-ectomycorrhizal seedlings,ECMF significantly promote the synthesis of IAA,ZT and other growth hormones and inhibite the synthesis of ABA under acid aluminum stress,for example,the GA content of the fine root of Ld 2 increased by 88.7%and the ABA content decrease by 41.1%when treated with 2mm Al³⁺.In addition,the content of hormones such as IAA and ZT in rhizosphere soil was also significantly higher in ectomycorrhizal seedlings than in non-ectomycorrhizal seedlings.When treated with 2.0 m M Al³⁺,the IAA content in ectomycorrhizal seedlings rhizosphere soil of Pt 715 and Ld 2 are significantly higher than that in non-ectomycorrhizal seedlings rhizosphere soil by 21.6%and 37.9%respectively.It can be seen that ECMF can significantly promote the synthesis and secretion of endogenous growth hormone of P.massoniana under acid aluminum stress.（4）Ectomycorrhizal significantly change the physical and chemical properties of rhizosphere soil of P.massoniana seedlings under acid aluminum stress.Compare with non-ectomycorrhizal seedlings,in the rhizosphere soil of ectomycorrhizal seedlings under acidic aluminum stress:p H decrease,soil aggregate stability increase,exchangeable Mg²⁺and K⁺decrease,and active aluminum content increase.Therefore,ectomycorrhizal promotes the secretion of H⁺and the absorption of exchangeable Mg²⁺and K⁺of P.massoniana seedlings under acid aluminum stress,inhibits the absorption of active Al³⁺,and improves the stability of soil aggregates.In conclusion,ECMF inoculation significantly improve the fine root morphology of P.massoniana seedlings under acid aluminum stress.which is beneficial to protect the fine roots from the toxicity of active aluminum,and promote the growth and development of fine roots through the synthesis and secretion of growth hormone,as well as the improvement of the physical and chemical properties of rhizosphere soil,so as to expand the area of contact and absorption of nutrients and promote the transport of nutrients from roots to the upper part of the ground,and thus enhancing the resistance of P.massoniana seedlings to aluminum toxicity.