Coupling Between Photosynthesis and Nitrogen Fixation

The first experimental demonstration of a coupling between photosynthesis (PS) and nitrogen fixation (NF) appeared in the literature 45 yr ago and involved a demonstration of increased N content of clover and alfalfa plants grown in N-free media with C_O2 enrichment (24). Early CO₂ enrichment studies with soybeans (2) were done with soil-grown plants and nodulation and nitrogen fixation were not accurately quantified. However, several studies published within the past 5 yr demonstrate clearly that these two vital processes are coupled in soybean plants. We now find ourselves in a position where this coupling can and should be more thoroughly explored at the cellular and chemical levels.