Continuous use of land for cultivation of crops along with the  endless  use of agro- chemicals are  leading the soil  to become  problematic by making them highly saline and accumulation of heavy metals.  At  times,  abiotic factors like flood and  drought  also add to amplify the problems.   Research have shown,  growing of actinorhizal tree species may  become a solution for such soil to replenish them. 

          We generally know about leguminous plants which are known for their symbiotic association with bacteria like Rhizobium to fix the nitrogen in soil. Crop rotation with the pulses  is of generally recommended practice to restore the soil fertility.  But there are some tree species which plays same role of nitrogen fixation in soil in association with bacteria like Frankia. These are called as Actinorhizal plants or trees.

Actinorhizal Plants

        Frankia,  a gram positive bacteria,  is of  filamentous shape,   known for its  host specificity. It has the ability to convert atmospheric nitrogen into ammonia, a plant utilizable form.  This process occurs  in the presence of enzyme nitrogenase  in association with roots of actinorhizal plants.  Firstly,  they form nodules on host roots and fixes nitrogen in them   and  in turn bacteria get reduced carbon for their utilization. Frankia even fall in the category of free living nitrogen fixing bacteria and perform  the same role in absence of host plants.  The bacteria   are the diazotropic kind,  which  can survive without external source of nitrogen.

Importance of Actinorhizal plants:

        There are 24 genera belonging to 8 angiosperm families fall in category of actinorhizal plants. Plant families like  Betulaceae, Casuarinaceae, Coriariaceae, Datiscaceae, Eleagnaceae, Myricaceae, Rhamnaceae, and Rosaceae are some of them. The genera Casuarina in tropical and subtropical region and Alnus in temperate are well known  and beneficial kind  of actinorhgizal plants. These species contribute for  production of fire woods  and industrial purpose, even in degraded soils. Alongside,  they help to replenish the soil with nitrogen fixing. Their leaves and trashes become the important source of biomass.

           Plants like Hippophae rhamnoides (Sea buck thorn) is an actinorhizal tree famous  for  production of  fruits rich in  Vitamin C and carotinoides. In addition, they are the  source of fire wood and fodder. Rubus elipticus (Yellow Himalayan Raspberry) is  another such plant found in  temperate climate. They  bear fruits rich in ascorbic acids, beta carotenes and antioxidants. Roots of this tree  are known to have some medicinal properties.

Frankia, A Nitrogen Fixing Bacteria

Casuarina as an actinorhizal tree:

         Casuarina sps. (Casurina equisetifolia, Casuarina cunninghamiana) are evergreen angiosperms, belong to family casuarinaceae. Trees are  known for its ability to tolerate high soil salinity, frequent drought and flood and heavy metal pollution. They are also reported give notable contributions in restoring  soil fertility in a way of  symbiotic nitrogen fixing  in association with  Frankia.  The trees   may also  symbiotically associate with Ectomycorrhizal fungi (EMF) and Arbuscular Mycorrhizal Fungi (AMF) for better phospahte absorption. Casurina trees are important component of social forestry, especially along the sea coast to mitigate soil erosion and  as wind breakers.