Re: Iron take up by plants

Dear Nancy -
 
Iron is available in the soil as Fe2+ and Fe3+ ions.  Fe2+ ions are the 
easiest for the plant to absorb.  The Fe3+ ions usually precipitate in the 
soil to form insoluble compounds:

Soluble                                               Insoluble
  2Fe3+     +     6OH-    ->     2Fe(OH)3        ->   Fe2O3.3H20

This insoluble form of iron can't diffuse to the plant roots to be 
absorbed.  Plants have two strategies to get at this insoluble Fe3+: 

1) In angiosperms:  
The plant secretes a phenol-like ligand (e.g. caffeic acid) from its roots. 
This ligand binds to Fe3+ ions and keep them in a soluble form (i.e. 
chelates the Fe3+ ions).  The plant also secretes reducing agents (such as 
NADPH), and once the Fe3+ chelate has diffused to the root surface the Fe3+ 
is reduced to Fe2+.  This Fe2+ is then immediately absorbed.  These plants 
often secrete H+ ions to try to make both types of Fe ions more soluble, 
although this often fails on basic (calcium rich) soil.

2) In grasses:
The plant secretes a compound which tightly chelates Fe3+ (called a 
phytosiderophore).  The complex of phytosiderophore-Fe3+ diffuses to the 
root surface where the whole complex is absorbed.  The iron is then reduced 
to the Fe2+ form inside the root, and the phytosiderophore is either 
degraded or released again to the soil.  This may also occur using 
siderophores produced by bacteria in the soil which are also iron stressed. 
It's probably likely that the plant steals siderophores from the bacteria, 
and bacteria steal phyosiderophores form the plant.

More on phytosiderophores.

It's important to remember that these methods only work in roots - Fe3+ 
given by foliar feeding won't be absorbed.

Hope this helps.  All the best, Carl.