Alleviation And Regulation Of Exogenous Organic Acids On Aluminum Toxicity Of Pinus Massoniana

In recent years,the problem of soil acidification has become increasingly prominent.Acidified soil contains a large number of active aluminum ions,which seriously affects the normal growth of plants.Active aluminum can significantly inhibit the growth of plant roots,make the leaves chlorosis and yellowing,leaf margin curl;affect the enzyme activity in plants,resulting in internal system disorder;at the same time,inhibit the normal progress of plant photosynthesis,reduce the net photosynthetic rate of plants,causing serious impact on plants.Pinus massoniana,as a Typical Subtropical Woody Plant in China,has the characteristics of drought resistance,barren,excellent fast-growing,and is often used in forest products processing and paper industry.However,due to the dissolution of active aluminum in acidic soil,its toxic effect leads to the decline and even death of Pinus massoniana forest in a large area,which greatly restricts the sustainable development of Pinus massoniana industry.Therefore,how to effectively alleviate the toxicity of aluminum to Pinus massoniana in acid soil,improve the aluminum tolerance of Pinus massoniana,and ensure the normal growth of Pinus massoniana forest in acid soil is an urgent problem to be solved.Root exudation of organic acids is one of the important mechanisms of aluminum tolerance in plants.It can reduce aluminum toxicity by chelating and precipitating active aluminum in soil,regulating the physical and chemical properties of Rhizosphere Soil and microorganisms.This mechanism is closely related to the organic acid metabolism system in plants.At present,a lot of progress has been made in the study of aluminum toxicity of herbaceous plants,but there are few studies on woody plants.The aluminum tolerance and aluminum toxicity mitigation mechanism of Masson pine,a typical woody plant in acid soil,are still unclear.Therefore,according to the previous screening of Pinus massoniana seedlings as experimental materials,the effects of exogenous organic acids on the growth index,root tip aluminum content,root tip TCA cycle related enzyme activities and photosynthetic characteristics of needles of Pinus massoniana under aluminum stress were studied by hydroponics method,and the physiological mechanism of exogenous organic acids in alleviating aluminum toxicity of Pinus massoniana was revealed.The purpose is to provide a theoretical basis for the sustainable development of Pinus massoniana cultivation and industry in acid soil.The main results are as follows:1.Under aluminum stress,the root elongation of Pinus massoniana seedlings was inhibited,and the plant was short and the stem became thin.The addition of organic acids could alleviate the aluminum toxicity.Under aluminum stress,the distribution of aluminum in roots of Pinus massoniana seedlings was not uniform,and the aluminum content in root tips was the lowest and increased upward.Root hematoxylin staining showed that the root tip color of organic acid treatment group was lighter than that of Al treatment group.Exogenous organic acid could effectively reduce the accumulation of aluminum in the root tip,and oxalic acid was more conducive to alleviate the absorption of Al³⁺by Masson pine roots.Combined with the results of root aluminum content,Al³⁺had the characteristics of upward transportation.The activity of pectin methylesterase in root tips increased significantly under aluminum stress,which indicated that the mechanism of cell wall efflux might be one of the effective mechanisms of alleviating aluminum toxicity in Pinus massoniana.The addition of exogenous organic acids significantly reduced the activity of pectin methylesterase in root tips,which indicated that organic acids could chelate with active aluminum directly in vitro,but not through the mechanism of cell wall efflux.2.The TCA cycle of Pinus massoniana seedling roots was affected by aluminum stress,resulting in the increase of PEPCase and MDH activities,and the decrease of ICDH and MDH activities.The addition of organic acids could alleviate the effect.To a certain extent,the activities of PEPCase and MDH were decreased,and the activities of ICDH and MDH were increased.The performance of each enzyme in roots and needles was different,indicating that organic acids could regulate the TCA cycle of roots In addition,the regulation mechanism of different growth parts was different.Through the study of acid phosphatase activity,it was found that aluminum stress led to the increase of ACP activity in root system.The addition of organic acid alleviated the pressure of the pathway of organic phosphorus inorganic phosphorus combined with active aluminum,resulting in the decrease of ACP activity.Therefore,it is speculated that the chelation of organic acid and active aluminum in vitro is the main way to alleviate aluminum toxicity in Pinus massoniana seedlings.3.The photosynthesis of Pinus massoniana seedlings was significantly inhibited under aluminum stress,which directly led to the inhibition of the synthesis and accumulation of photosynthetic pigments and the decrease of their content.The addition of citric acid and oxalic acid could effectively alleviate this situation,while malic acid was not obvious.At the same time,aluminum stress also reduced net photosynthetic rate,stomatal conductance and transpiration rate,and increased intercellular CO₂ concentration,but had no significant effect on water use efficiency and stomatal limit.Exogenous organic acids could alleviate the blocked photosynthesis.