Abstract
The effect of nitrogen source (N(2) or nitrate) on carbon assimilation by photosynthesis and on carbon partitioning between shoots and roots was investigated in pea (Pisum sativum L. 'Baccara') plants at different growth stages using (13)C labelling. Plants were grown in the greenhouse on different occasions in 1999 and 2000. Atmospheric [CO(2)] and growth conditions were varied to alter the rate of photosynthesis. Carbon allocation to nodulated roots was unaffected by N source. At the beginning of the vegetative period, nodulated roots had priority for assimilates over shoots; this priority decreased during later stages and became identical to that of the shoot during seed filling. Carbon allocation to nodulated roots was always limited by competition with shoots, and could be predicted for each phenological stage: during vegetative and flowering stages a single, negative exponential relationship was established between sink intensity (percentage of C allocated to the nodulated root per unit biomass) and net photosynthesis. At seed filling, the amount of carbon allocated to the nodulated root was directly related to net photosynthesis. Respiration of nodulated roots accounted for more than 60 % of carbon allocated to them during growth. Only at flowering was respiration affected by N supply: it was significantly higher for strictly N(2)-fixing plants (83 %) than for plants fed with nitrate (71 %). At the vegetative stage, the increase in carbon in nodulated root biomass was probably limited by respiration losses.