The Influence of Rhizosphere Nitrogen Form and Rising Atmospheric CO2 on the Balance between Root and Shoot Processes.

Ammonium and nitrate that roots absorb from the rhizosphere are the major sources of nitrogen for most plants. Ammonium can be toxic if it accumulates in plant tissues, and so plants generally assimilate ammonium into amino acids in their roots near the site of absorption. By contrast, plants can accumulate nitrate without toxic effects, and so plants assimilate the absorbed nitrate in the roots or translocate it to the shoots where it is stored or assimilated. The balance between root and shoot nitrate assimilation varies within and among species. When plants are exposed to ambient CO2 and receive sufficient rhizosphere nitrate to support full growth, shoots usually account for the majority of plant nitrate assimilation. Under such conditions, the energy costs of nitrate assimilation are borne by photorespiration rather than by mitochondrial respiration, and thereby do not divert energy from other plant activities such as growth. CO2 inhibition of both photorespiration and protein content under nitrate nutrition in a wide variety of C3 plants indicates that photorespiratory nitrate assimilation is a major contributor to the performance of a whole plant. Mechanisms other than photorespiration empower nitrate assimilation because mature C3 plants under nitrate nutrition continue to grow at elevated CO2, albeit often at a slower pace than under ammonium nutrition. One such mechanism is the coupling of nitrate assimilation to mitochondrial respiration. Root nitrate assimilation is entirely dependent on mitochondrial respiration and may even accelerate at elevated CO2 because carbohydrate translocation to roots increases. Enhanced root nitrate assimilation at elevated CO2 partially compensates for the CO2 inhibition of shoot nitrate assimilation.