Oxygen concentration affects nodule anatomy and nitrogenase activity of <i>Alnus maritima</i>

Abstract

ABSTRACT Alnus maritima is a shrub that associates with N 2 ‐fixing Frankia in the wetlands in which it is native. Despite low concentrations in waterlogged soils, O 2 is critical to the maintenance of this symbiosis, and Frankia ‐infected nodules exist on roots of plants in native stands. The objective of the present study was to determine how root‐zone O 2 concentration influences N 2 fixation and the anatomy of nodules on A. maritima . Root zones of plants inoculated with soil from native stands were exposed to eight O 2 concentrations. Nitrogenase activity increased with increasing O 2 concentration. Photosynthetic rate, plant dry mass, leaf N content, and nodule fresh mass were maximal in plants maintained with 15–25% O 2 in the root zone. Nodule counts were maximal on roots maintained at 10% and above 25% O 2 , and nodules that developed at ≤ 2% O 2 were &lt; 2 mm in diameter and single‐lobed. Mean total area of air spaces within nodules decreased, and mean area per space increased, with increasing O 2 concentration. Seasonal and O 2 ‐dependent nodule pigmentation was observed. Our data illustrate that O 2 is critical to the development of functional symbioses, and that nodules of this species, which are submersed in nature, possess mechanisms for responding to their low‐O 2 environment.