Coronatine is a non-specific, chlorosis-inducing phytotoxin produced by several pathovars of Pseudomonas syringae, including pv. atropurpurea, the causal agent of bacterial blight of ryegrass. The physiological development of chlorosis caused by coronatine was investigated in Nicotiana tabacum, a plant which was found to be particularly sensitive to this toxin. The disappearance of chlorophylls from the affected tissue was independent of light intensity, indicating that chlorosis was not the consequence of photochemical bleaching. The loss of chlorophylls was associated with a proportional decline in the rate of photosynthesis, expressed as nmol O2.cm-2 min-1, and protein synthesis, expressed as pmol leucine incorporated cm-2 min-1. However, expressed on a chlorophyll basis, neither photosynthesis nor protein synthesis were reduced in the coronatineaffected tissue, compared to controls, revealing the physiological competence of the chlorotic tissue. Quantitative changes were observed in the tissue content of free amino acids. During the development of chlorosis there was an increase in the tissue content of tryptophan and asparagine, while the content of alanine and aspartate declined. Coronatine-affected leaves were visibly thickened, and this was associated with an increase in the number of cells in the palisade mesophyll layer. This latter observation indicates that coronatine may interfere with the growth regulation of tobacco leaves.