Effects Of Aluminum Toxicity On The Growth And IAA Distribution In Alfalfa

Alfalfa (Medicago sativa L.) is a kind of good perennial leguminousgrasses, known as “the queen of the forage grass”. As one of the mostimportant forage grass, However, alfalfa can not grow well in red soil ofsouthern China due to aluminum toxicity in red soil, and seriously restrictedanimal husbandry development in southern China. Therefore, it is necessaryto study the mechanism of aluminum toxicity on alfalfa, and to do the workon breeding new variety of aluminum tolerance alfalfa.The paper focused on the research of IAA（Indole-3-acetic acid）distribution, and influence of mineral nutrients on IAA accumulation inAl-stressed alfalfa. The main results were below:1. Aluminum stress significantly inhibited the growth of the alfalfaseedling. The root length, aboveground biomass and underground biomass inAl-stressed alfalfa were obviously lower than that of control without Al(p<0.05), root activity and chlorophyll content reduced significantly, andMDA content increased significantly in Al-stressed alfalfa (p<0.05), whichmeans that the stability of cellulose membrane structure was damaged.Exogenous application of IAA can significantly increase the growth ofAl-stressed alfalfa seedling, allevillated physiological injury of Al toxicity onalfalfa. Among the diffident IAA contents,6mg/L IAA was the mosteffective content on promoting the growth of Al-stress alfalfa.2. Al stress significantly reduced the IAA concentration in root tip andshoot top bud of Al-stressed alfalfa seedlings compared to control treatment,and the IAA concentration in root tip of alfalfa without shoot top bud weresignificantly lower than that with shoot top bud under Al-stressed andnone-Al-stressed conditions. However, the IAA concentration in root tip ofalfalfa without shoot top bud were equal to or higher than that with shoot top bud after spring IAA to Al-stressed alfalfa seedling, suggested that IAAsynthesis was mainly occurred in shoot top bud, and little amount of IAA wassynthesized in root tip. Al toxicity significantly inhabited the IAA synthesisin alfalfa.3. Al3+content in the roots of Al-stressed alfalfa was significantly higherthan control, and the Al3+cation located mainly in cell wall and vascularbundle of root, which caused callose content in root tip of Al-stressed alfalfaobviously higher than control. The increased callose content in root tip ofAl-stressed alfalfa directly caused sieve mesh block, and decreased IAAtransport from top bud to root tip, consequently decreased IAA concentrationin root tip.4. The mineral nutrition contents of Ca2+、Mg2+、Zn2+、Mn2+、Na+in rootsof Al-stressed alfalfa were significantly decreased compared to control,which directly influenced the growth of Al-stressed alfalfa. The absence ofCa2+, Mg2+, Zn2+and B in culture solution significantly decreased IAAconcentration in roots of Al-stressed alfalfa, however, exogenous appliedCa2+, Mg2+, Zn2+and B to culture solution can obviously increase the IAAconcentration in roots of Al-stressed alfalfa. There was a closed relationshipbetween the deficiency of mineral nutrient and decrease of IAA synthesis andtransport.These results indicated that Al toxicity led to inhibition of IAA synthesisand transport in Al-stressed alfalfa, which was the main reason of growthreduction in Al-stressed alfalfa.