Dark CO2 fixation in leaves of tomato plants grown with ammonium and nitrate as nitrogen sources

Abstract

The dark (non-photosynthetic) CO2 fixation was studied in the leaves of ammonium-fed and nitratefed tomato plants. The ability to fix14CO2 in the dark of ammonium-fed plants was remarkably lower as compared with nitrate-fed plants, supporting the previous finding that the synthesis of C4-compounds from C3-compounds was reduced in the leaves of ammonium-fed plants. There was no difference in the activity of PEP carboxylase in extracts prepared from the leaves between both the plants during an early period of the treatment. However, the enzyme activity began to decrease rapidly in ammonium-fed plants 4 days after the treatment. By long-term treatments, the enzyme activity in ammonium-fed plants became half as high as that of nitrate-fed plants. The decreased PEP carboxylase activity in ammonium-fed plants was not associated with the presence of NH4-N and the absence of NO3-N in the leaf extract, and was not restored by the addition of the leaf extract from nitrate-fed plants. It is concluded that the decreased rate of synthesis of C4-compounds from C3-compounds in ammonium-fed plants is closely associated with a decrease in the dark fixation involving PEP carboxylase.