Nitrogenase activity in response to restricted shoot growth in Alnus incana

Abstract

In the Alnus–Frankia symbiosis the nitrogen-fixing root nodules are one of the sinks for carbon compounds newly formed in photosynthesis and exported from the leaves (source). The competition for assimilates between shoot tips and root nodules was studied by reducing shoot growth. Cloned plants of Alnus incana (L.) Moench were grown without combined nitrogen in a growth chamber. Shoot growth was inhibited by excision of all buds or by induced dormancy. The experiments showed an increased root biomass in the treated plants, indicating a changed source–sink balance. The treatments never caused an increased nitrogenase activity (C2H2-dependent C2H4 production); rather it was decreased. The nitrogenase activity was always correlated with leaf areas. It was also correlated with growth, which is shown by the similar nitrogen percentage in intact and treated plants.