Development of flood-induced lenticels in red alder nodules prior to the restoration of nitrogenase activity

Red alder (Alnus rubra Bong.) is a nitrogen-fixing woody plant that is comm on on wetland sites and tolerates flooding through a variety of induced mor phological and physiological changes. Among these changes are the formation of hypertrophied nodule lenticels and the subsequent full restoration of n itrogenase activity after 50 days of flooding. The objective of this study was to examine fine structural changes within red alder nodules during lent icel development that correspond to changes in nitrogenase activity during a 50-day experimental flood. Nodulated seedlings of red alder were grown un der greenhouse conditions and then exposed to root flooding for 1, 20, 35, or 50 days. At each harvest, estimates of nitrogenase activity were made vi a acetylene reduction, and nodule samples were taken for light-microscope e xamination. Only after 50 days of flooding did red alder show restoration o f nitrogenase to pretreatment levels. At this time, Frankia vesicles were f ound to be directly adjacent to developing lenticel tissue and large interc ellular spaces. Intercellular space within the nodule increased from 0.6% i n nonflooded tissue to 5.7% after 50 days of flooding. Our results demonstr ate the sensitivity of the nitrogenase enzyme to low oxygen soil conditions and indicate that substantial morphological change within the nodule must occur before red alder can regain the capacity to fix nitrogen under floode d conditions.