Ontogenetic interactions between photosynthesis and symbiotic nitrogen fixation in pigeonpea

SUMMARY Total nodule nitrogenase activity (TNA, μ mols ethylene plant ‐1 h ‐1 ) in pigeonpea (Cajanus cajari) increased with plant growth to reach maximum at flowering (75 days after sowing), decreasing thereafter until maturity (120 days after sowing). However, specific nodule nitrogenase activity (SNA, μ mols ethylene g ‐1 nodule fresh wt h ‐1 ) reached its maximum earlier (45 days after sowing). The rate of photosynthesis and shoot and nodule respiration followed a similar pattern to TNA. However, higest rates of root respiration were observed at flowering and again immediately before final harvest. 14 CO 2 feeding studies showed that assimilates produced in leaves before flowering were retained in the vegetative parts. Assimilates produced after flowering were exported to the reproductive structure at the expense of the nodules. It is suggested that the decreased availability of photosynthate to nodules decreased nitrogen fixation.