The Effect Mechanism Of Rare Earth Nanopriming On Seed Germination And Seedling Growth Of Alfalfa Under Salt Stress

Alfalfa is a perennial leguminous herb belonging to the genus Medicago,which is known as the"King of Forage".Soil salinization is serious in China,highly overlapping with the main cultivation areas of alfalfa.Salt stress reduces alfalfa yield by affecting seed germination and seedling growth.Therefore,exploring methods to improve the salt tolerance of alfalfa is of great practical significance,among which seed nanopriming is a convenient strategy.This study used La₂O₃NPs and Ce O₂NPs to prime alfalfa seeds.The effects of nanopriming on the germination characteristics,physiological characteristics and hormone levels of alfalfa seeds under control and salt stress conditions were explored,and physiological and biochemical processes such as growth,photosynthesis,osmotic regulation,and antioxidant system during the growth stage of alfalfa seedlings were analyzed.The following research results were obtained:1.Under different concentrations of salt stress treatment,the stress degree of alfalfa seed germination and early seedling increases with the increase of salt concentration,and Huanghou is more sensitive than WL656HQ.The identification concentrations of WL656HQ and Huanghou are 250 and 200 mmol·L^-1,respectively.2.The hydrodynamic dimensions of La₂O₃NPs and Ce O₂NPs are 1673±274nm and 1415±548 nm,respectively,with Zeta potentials of 16.6±0.1 m V and-9.0±0.2 m V,respectively.In vitro experiments have shown that La₂O₃NPs and Ce O₂NPs have nanocatalytic effects on amylase,with optimal catalytic concentrations of 10 and100 mg·L^-1,respectively.3.Different priming concentrations of La₂O₃NPs and Ce O₂NPs can significantly improve the germination and growth characteristics of Huanghou and WL656HQ under salt stress.Among them,the priming treatment of 10 mg·L^-1La₂O₃NPs and 50 mg·L^-1 Ce O₂NPs has the best effect on improving the salt tolerance of"Huanghou",which can increase the germination rate from 10.0%to 72.7%and56.7%,respectively;the priming treatment of La₂O₃NPs and Ce O₂NPs with a concentration of 00 mg·L^-1 has the best effect on improving the salt tolerance of"WL656HQ",which can increase the germination rate from 30.0%to 82.0%and66.0%,respectively.4.The reasons why La₂O₃NPs(10 mg·L^-1–“Huanghou”;100 mg·L^-1–“WL656HQ”)和Ce O₂NPs(50 mg·L^-1–“Huanghou”;100 mg·L^-1–“WL656HQ”)can promote seed germination and improve seed salt tolerance（Huanghou-200 m M;WL656HQ-250 m M）include the elimination of wax layer,modification of seed coat structure,significant increase in 24 h water absorption（P<0.05）,increaseα-amylase activity and produce higher soluble sugar content,promote seed germination metabolism,and improve salt tolerance of germinated seeds;Nano priming reduced the content of reactive oxygen species,malondialdehyde,and relative conductivity under salt stress,and increased catalase activity to protect seeds and alleviate salt stress damage;In addition,hydropriming and Ce O₂NPs priming decreased abscisic acid content,releasing seed dormancy.Metabolic analysis has shown that the main reason for promoting seed germination caused by Ce O₂NPs is the role of water factors,and under salt stress,Ce O₂NPs can better improve seed salt tolerance.5.La₂O₃NPs and Ce O₂NPs can promote seedling growth,increase root length,leaf area,and biomass of the seedlings,increase chlorophyll b content of the seedlings.Salt stress（200 m M）reduces the net photosynthetic rate,transpiration rate,and stomatal conductance of alfalfa seedlings.Nanopriming reduces the intercellular carbon dioxide concentration of the"Huanghou",while nanopriming causes the inhibition of alfalfa photo leaves,reducing Initial fluorescence and Maximum fluorescence.La₂O₃NPs priming reduced proline content,leading to increased osmotic stress,but increased catalase activity in seedlings and superoxide dismutase activity in roots.La₂O₃NPs priming eliminated reactive oxygen species by increasing the activity of antioxidant enzymes.Ce O₂NPs induce a decrease in malondialdehyde content,which has a positive effect on protecting cell membranes.This research indicated that nanopriming breaks the hard seeds phenomenon of alfalfa seeds,improves water absorption and activates amylase to initiate germination metabolism;Nanoenzyme properties enhance amylase activity;Reduce the content of abscisic acid to relieve seed dormancy,then ultimately promote seed germination.In addition,nanopriming can improve seed salt tolerance by reducing membrane damage and improving the metabolic level of stress resistant pathways;Increase seedling yield by increasing photosynthetic levels and other means.In summary,this study provides a theoretical basis for the effects of La₂O₃NPs and Ce O₂NPs on the germination and seedling growth of alfalfa seeds,as well as a basis for their cultivation and application in salt stress environments.