Effects Of Al³⁺ Stree On Performances Of Medicago Sativa L. And Mechanisms Of Relieving Al³⁺ Stress Using Exogenous Organic Acids

Medicago sativa L.,“the queen of forage grasses”, is one of the best foragegrasses. However, M. sativa, as one of the most important forage grasses that havebeen cultivated, can not grow well in in northern China where extensive soils are acidin general, due to Al³⁺stress. The present paper investigated tolerances of27differentM. sativa cultivators and mechanisms of how M. sativa plants can tolerate Al³⁺stress.The results show that,1. The27different cultivators of M. sativa described above differed significantlyin their tolerances to Al³⁺stress. Based on their tolerances to Al³⁺stress, these27M.sativa cultivators were classified into four groups （I、II、III、IV）. The four species ofthe group IV （i.e. Juren802, Fengbao, WL525and Saranduo） had the widesttolerances to Al³⁺stress.2. The acid treatment （pH4.5） did not influence the growth of M. sativa （P>0.05）. Above-ground biomass, below-ground biomass, root length, root vigor, leafmalondialdehyde content and peroxidase content under the acid treatment did notdiffer from those under the control treatment （pH6.0）. However, the acid treatmentdid affect the metabolism of organic acids in M. sativa roots. Under the acid treatment,the contents of oxalic acid, malic acid, citric acid and succinic acid in M. sativa rootsdecreased significantly （P <0.05）.3. The acid and Al³⁺stress treatment (pH4.5, Al³⁺100μmol L-1) significantly inhibited the growth of M. sativa （P>0.05）. Above-ground biomass, below-groundbiomass, root length, leaf Fv/Fm value and root vigor under the acid and Al³⁺stresstreatment were significantly lower than those under the control treatment (pH6.0, noAl³⁺added)（P <0.05）. Contrarily, leaf malondialdehyde content and peroxidasecontent under the acid and Al³⁺stress treatment were significantly elevated （P <0.05）.The acid and Al³⁺stress treatment also altered the processes of nitrogen metabolism.Leaf alanine content, ammonia content and praline content were significantly elevated（P <0.05） under the acid and Al³⁺stress treatment. Moreover, antagonism amongmetallic ions absorbed by M. sativa roots was found under the acid and Al³⁺stresstreatment. Al³⁺content in M. sativa roots under the acid and Al³⁺stress treatment wastwice as high as that under the control treatment, while K+、Zn2+、Na+、Mn2+and Mg2+contents under the acid and Al³⁺stress treatment were decreased by a large extent.4. Spraying four acids （i.e. oxalic acid, malic acid, citric acid and succinic acid）significantly relieved Al³⁺stress for M. sativa. Above-ground biomass, below-groundbiomass and root length of M. sativa plants under the acid spraying treatment weresignificantly higher than those under the control treatment, and root vigor, leafmalondialdehyde content and peroxidase content were improved. The effects ofantagonism among metallic ions absorbed by M. sativa roots were also relived. Of thefour acids, the effects of spraying oxalic acid were the shortest in duration and lowerin magnitude, and the effects of spraying citric acid and succinic acid were the longestin duration and most significant in magnitude.These results suggest that different cultivators of M. sativa differed significantlyin their tolerances to Al³⁺stress and that spraying organic acids could relieve Al³⁺stress for M. sativa. These results may benefit to the breeding of M. sativa cultivatorsthat are tolerant to Al³⁺stress.