Various physiological effects of ammonium, nitrate and mixed ammonium-nitrate nutrition of plants have been studied in this laboratory during the last years. Some of the characteristic distinctions observed between plants growing on these nitrogen sources are described and discussed. Biomass production of ammonium-grown plants increased with K+ concentration in the nutrient medium between 0.1 to 3 mM, while nitrate-fed plants reached maximal growth around 0.25 mM K+. The water use efficiency (WUE) of ammonium-fed plants was lower and also more dependent on K+ than that of plants receiving nitrate. At low K+ levels (0.1 mM) in the medium, plants growing on ammonium-N spent nearly twice as much water through transpiration per unit mass than nitrate-fed plants. WUE of plants receiving NH4NO3 was poor at low K+ (similar to ammonium-fed plants) but at high K+ this plants grew even better than nitrate-grown plants. Overall transpiration per plant was little affected by K+ levels in the medium. Consequently, the main effect of K+ was on the overall photosynthetic capacity of the shoot. Similar studies were carried out on the efficiency of the plants to convert inorganic nitrogen taken up from the nutrient media into organic nitrogen. Ammonium-fed plants assimilated into organic compounds only between 20% and 30 % of the nitrogen taken up from the solution. NUE of nitrate-fed plants was much higher (about 100%) regardless of the K+ levels in the medium. In additional studies to characterize the preferential growth rate of plants fed mixed ammonium and nitrate sources, we examined root respiration rates, oxidation of C-14-labeled assimilates by the root, balance between root respiration and leaf net photosynthesis, patterns of assimilate allocation and effect of CO2 level.
