Agriculture Reference
In-Depth Information
the ability to transform into The colonised cells eventually form aggregations
called cortex nodules, usually a few millimetres in diameter. The formation of shoot
nodules in
Sesbania rostrata
follows a similar pattern although the initial phase differs
(Dommergues
et al
., 1985.).
Frankia
organisms penetrate the root via the root hairs or by inter-cellular penetration
and form infectious hyphae which extend to the cortex. Once in the cortex, these organisms
form subspherical vesicles within which N-fixation occurs (Huss-Danell, 1997).
The biochemistry of N-fixation
Biological nitrogen fixation (N-fixation) is the reduction of atmospheric nitrogen
to ammonia in a reaction controlled by the nitrogenase enzyme. Nitrogenase consists
of two components. Component I is dinitrogenase, the site of reduction, and is a
molybdo-ferro-protein of about 230 kDa. Component II is dinitrogenase reductase, an
iron protein (molecular weight about 60 kDa) which provides electrons to Component
I for reduction. Dinitrogenase is associated with the iron-molybdenum cofactor
which is believed to contribute a surface for reduction (Sprent, 1984; Giller and
Wilson, 1991). The nitrogenase enzyme is similar among all N-fixing organisms
although alternative systems have been identified for
Azotobacter
spp. that do not
contain Mo in the cofactor (Pau, 1989).
Nitrogenase is extremely sensitive to oxygen, with both enzyme components becoming
irreversibly deactivated upon exposure to atmospheric concentrations. Consequently, free-
living, nitrogen-fixing bacteria only fix nitrogen in anaerobic or low oxygen environments
(Eady, 1992). Some cyanobacteria develop thick walled heterocysts, within which
nitrogenase occurs and where it remains isolated from aerobic processes (Elkin, 1992).
Nitrogenase is also restricted to the heterocysts in the cyanobacteria-plant symbioses
(Bergman
et al.,
1992b). Oxygen is excluded from nitrogenase in the legume-rhizobia
symbiosis by two mechanisms. The nodule cortex acts as a physical barrier to oxygen
diffusion (Witty and Minchen, 1990) and leghaemoglobin binds oxygen and transports
it to respiratory sites while excluding it from nitrogenase (Gallon and Chaplin, 1987).
Leghaemoglobin is the red pigment which colours the interiors of legume root nodules.
Nitrogen fixation is a highly energy-demanding process, whether carried out
biochemically within a prokaryote, or within a pressure vessel in fertiliser factory.
Sprent (1984) estimated that about 15 tonnes of photosynthate are required to assimilate
1 tonne of nitrogen by biological fixation. The energy requirements for N-fixation are
met by the host plant and are estimated to be 2.5 mg C mg (Warembourg and
Roumet, 1989), about 16 % less efficient than nitrate assimilation (see Neves, 1982).
Saari and Ludden (1986) estimated that 28 moles of ATP are required for each mole
of N fixed.
