THE EFFECT OF THE ACCUMULATION OF CARBOHYDRATE AND ORGANIC NITROGEN ON NITROGEN-FIXATION (ACETYLENE-REDUCTION) OF FABA BEAN CV FJORD

Plants of faba bean cv. Fiord were grown under controlled conditions, without mineral N, in coarse river sand. Twenty-five days after sowing when plants had at least eight fully opened leaves and were nodulated and actively fixing N2, half were topped and kept debudded for 21 d. Changes in dry weight, N2 fixation (acetylene reduction activity), soluble carbohydrate, starch, soluble N and total N in plants were monitored over the period. Both debudded and control plants grew and accumulated dry matter. Debudding resulted in a significant increase in the concentration of soluble carbohydrate, starch and soluble N. but had only a small effect on the total N concentration. A strong positive linear relation between total plant weight and N content of both control and debudded plants showed that even under conditions of excess supply of carbohydrate, faba beans have little capacity to store N. Soluble N accumulated in debudded plants presumably because less N was needed for the formation of new tissues than in control plants. AR continued to increase throughout the experiment in control plants but declined in debudded plants from 6 to 13 d after debudding and remained low until the end of the experiment. The decline was associated with an increase in available carbohydrate and in soluble N, The results of this experiment are consistent with a feed back control of N2 fixation by the soluble pool of N. © 1994 Annals of Botany Company.