Modelling the relative contribution of seed nitrogen reserves and external nitrogen uptake during heterotrophic growth in Medicago truncatula

Abstract

Heterotrophic growth relies on remobilisation of seed reserves and mineral absorption. We used a compartmental model to investigate the fluxes of N absorption and remobilisation of N reserves in a legume seed with high protein content. Seedling growth was studied during the heterotrophic stage in two genotypes of Medicago truncatula as a function of N supply. N absorption and seed remobilisation fluxes were distinguished in a 15 N labelling experiment. Remobilisation of seed N reserves was high during germination, but N uptake started as soon as the radicle protruded. Both sources contributed to high elongation rates of the radicle and hypocotyl. When organ lengths stabilised, there was an efflux of N from the cotyledons and roots indicating that seedling growth was limited by carbohydrate production. No significant differences between genotypes were observed except for early N uptake, which was lower in the genotype with the highest initial seed N content. N fluxes were similar to those of other non-legume dicotyledonous species but differed from monocotyledonous species. These results improve our understanding of the effects of mineral fertilisation on crop establishment. The compartmental model is a useful tool to analyse N fluxes patterns within and between diverse species, in relation to seed characteristics and soil N availability.