OXYGEN RELATIONS OF THE ROOT-NODULES OF ALNUS-RUBRA BONG

The acetylene reducing activity of Alnus rubra nodules is maximal in 15 to 30% oxygen. Within the nodule, the large number and size of many of the intercellular spaces between the uninfected cells in the outer cortex facilitates the diffusion of air through the lenticeb to the infected cells in the mid‐cortex. Access of air to the infected cells, which are arranged in longitudinal bundles separated by one or more lines of starch‐containing uninfected cells and which run at an angle away from the stele towards the nodule apex, is restricted by the reduced number and size of air spaces in this region. The cytoplasm of the infected cells shows high activity of peroxidase and o‐diphenol oxidase, but the lack of correlation between the activity of these enzymes and nitrogenase activity suggests that they are not involved in the control of access of oxygen to the endophyte. It is suggested that one role of nodule peroxidase is the ignification of the infected cells associated with cellular senescence. Copyright © 1979, Wiley Blackwell. All rights reserved