Abstract: NH4+-grown plants are more sensitive to light stress than NO3−-grown plants, as indicated by reduced growth and intervenal chlorosis of French bean (Phaseolus vulgaris L.). Measuring the time course of Fv/Fm ratios under photoinhibitory light regimes did not reveal any difference in PS II damage between NO3−- and NH4+-grown plants, in spite of some indications of higher energy quenching in NO3−-grown plants. Also, a direct action of NH4+ as an uncoupler at the thylakoid membrane could be excluded. Instead, biochemical analysis revealed enhanced lipid peroxidation and higher activity of scavenging enzymes in NH4+-grown plants indicating that these plants make use of metabolic pathways with stronger radical formation. Evidence for higher rates of photorespiration in NH4+-grown plants came from experiments showing that electron flux and O2 evolution were decreased by SHAM in NH4+-grown plants, and by antimycin A in NO3−-grown plants. Further, the comparison of electron flux and of photoacoustic measurements of O2 evolution suggested that in NH4+-grown plants the Mehler reaction was also increased, at least in the induction phase. However, the major cause of N form-dependent stress sensitivity is assumed to be in the coupling between photosynthesis and respiration, i.e., NO3−-grown plants can utilize the TCA cycle for the generation of C skeletons for amino acid synthesis, thus improving the ATP: reductant balance, whereas NH4+-grown plants have enhanced rates of photorespiration.