PHOTOSYNTHATE COSTS ASSOCIATED WITH THE UTILIZATION OF DIFFERENT NITROGEN-FORMS - INFLUENCE ON THE CARBON BALANCE OF PLANTS AND SHOOT-ROOT BIOMASS PARTITIONING

The photosynthate costs of processes (amino acid and protein synthesis and turnover, and pH regulation) associated with the utilization of n itrate (NO3-), ammonium (NH4+) or glutamine (Gln) for plant growth wer e estimated. Based on these estimates, the effects of these forms of n itrogen (N) on the carbon balance of plants and on shoot-root biomass allocation were evaluated. The results indicated that NO3- as an N sou rce for plant growth is not substantially more expensive to utilize th an either NH4+ or Gln, particularly in the long term when costs due to protein turnover dominate the total costs of N utilization. It is als o suggested that the photosynthate use in processes associated with N assimilation has little impact on the carbon balance of plants, and he nce on shoot-root biomass allocation.