Ferredoxin NADP+ oxidoreductase (FNR) enzymes catalyse electron transfer between ferredoxin and NAD(P)H. A photosynthetic FNR (pFNR) in plants catalyses the final step of linear electron flow (LEF) in photosynthesis by transferring electrons from reduced ferredoxin to NADPH, providing reductant for carbon fixation. Various alternative electron flow pathways require reductant in the form of ferredoxin. pFNR may, therefore, be crucial to the regulation of reductant partition between carbon fixation and other metabolic pathways.