Different Tolerance to Light Stress in NO₃⁻‐ and NH₄⁺‐Grown Phaseolus vulgaris L.

NH 4 + ‐grown plants are more sensitive to light stress than NO 3 − ‐grown plants, as indicated by reduced growth and intervenal chlorosis of French bean ( Phaseolus vulgaris L.). Measuring the time course of F v /F m ratios under photoinhibitory light regimes did not reveal any difference in PS II damage between NO 3 − ‐ and NH 4 + ‐grown plants, in spite of some indications of higher energy quenching in NO 3 − ‐grown plants. Also, a direct action of NH 4 + as an uncoupler at the thylakoid membrane could be excluded. Instead, biochemical analysis revealed enhanced lipid peroxidation and higher activity of scavenging enzymes in NH 4 + ‐grown plants indicating that these plants make use of metabolic pathways with stronger radical formation. Evidence for higher rates of photorespiration in NH 4 + ‐grown plants came from experiments showing that electron flux and O 2 evolution were decreased by SHAM in NH 4 + ‐grown plants, and by antimycin A in NO 3 − ‐grown plants. Further, the comparison of electron flux and of photoacoustic measurements of O 2 evolution suggested that in NH 4 + ‐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., NO 3 − ‐grown plants can utilize the TCA cycle for the generation of C skeletons for amino acid synthesis, thus improving the ATP: reductant balance, whereas NH 4 + ‐grown plants have enhanced rates of photorespiration.