Under high light conditions,ammonium nutrition has a negative effect on plant growth.This suggests that the ad- verse effects of ammonium nutrition on plant growth may be related to carbon gain,photosynthesis,and photorespiration. 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 CO_2.Nitrate reduc- tion 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 CO_2 refixation. Under high light conditions, ammonium nutrition has a negative effect on plant growth. This suggests that the ad- verse effects of ammonium nutrition on plant growth may be related to carbon gain, photosynthesis, and photorespiration. 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 CO_2.Nitrate reduc - tion 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 (ie, cell size, mesophyll thickness, and chloroplast volume) and enzymic (ie, ribulose-1,5-bisphosphate carboxylase / oxygenase (Rubisco) uvate 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 CO_2 refixation.