However, there is no consistent evidence of a specific mechanism that could explain the plant growth reduction under
ammonium supply. It is generally accepted that during the light reaction, a surplus of nicotinamide adenine dinucleotide
hydrogen phosphate (NADPH) is produced, which is not completely used during the assimilation of CO2. Nitrate reduction in the leaf represents an additional sink for NADPH that is not available to ammonium-grown plants. Nitrate and
ammonium nutrition may use different pathways for NADPH consumption, which leads to differences in photosynthesis
and photorespiration. The morphological (i.e., cell size, mesophyll thickness, and chloroplast volume) and enzymic (i.e.,
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase), and glutamine
synthetase/glutamate synthetase (GS/GOGAT)) differences that develop when plants are treated with either nitrate
or ammonium nitrogen forms are related to photosynthesis and photorespiration. The differences in photorespiration
rate for plants treated with nitrate or ammonium are related to the conversion of citrate to 2-oxoglutarate (2-OG) and
photorespiratory CO2refixation.