In order to determine the pattern of expression of hb gene in anaerobic maize roots, two experimental systems were tested, a hydroponic and a flooded system. The hydroponic system could not discern between the treatments applied to the plants and the conditions of the system itself. On the other hand, the flooded system revealed that hb gene is expressed transiently during the first hours of anaerobiosis. Expression reaches its peak at 2 hrs and then declines. Under the same conditions, the pattern of expression for adh gene is also time dependent. The gene is strongly induced after 40 min of flooding and peaks at 12 hrs, then slowly decreases during the following 48 hrs. Oxygen transported via developing aerenchyma does not seem to have an effect on the pattern of expression of both hb and adh genes. Under anaerobic conditions, maize roots have an anaerobic stele and an aerobic cortex. The expression of reporter genes is directed by the Parasponia promoter in the tip and vascular cylinder of tobacco. Our results show that hb and adh genes are expressed referentially in the stele of flooded maize roots. mRNA production of hb and adh is 2 and 5 times greater respectively, in the flooded stele than in the cortex. In terms of the protein, we found Hb to be present in the roots of both flooded and unflooded maize roots. The protein level shows a slight increase after 12 hrs, even though it does not correlate to the levels of the message. (Abstract shortened by UMI.)