Hemoglobins are heme containing proteins, capable of supporting reversible binding of oxygen. They represent a widespread family of proteins found in animals, plants, fungi and bacteria.
   The tertiary structure of these proteins appears to be highly conserved suggesting commonality of functions.
   The basic role of
hemoglobins is closely related to the delivery of oxygen to respiring tissues.  Until recently, the knowledge on plant hemoglobins was limited to the symbiotic systems in which fixation of free nitrogen occurs (Appleby, 1984).
Leghemoglobins facilitate the diffusion of oxygen to the bacteroids in the root nodule and at the same time, by sequestering the oxygen, they prevent the bacterial nitrogenase from being inactivated.
   The discovery of
hemoglobins in cereals (Taylor et al., 1994) and of a nonsymbiotic type hemoglobin in soybean tissues (Andersson et al., 1996) suggests that in plants the role of hemoglobins is not restricted to the nodulation process.