The photodynamic damage of the sensitive plants wheat and mustard, treated with chlorophyll (Chl) precursors 5-aminolevulinic acid (ALA) and glutamic acid (Glu) and with 1,10-phenanthroline (Phen), was caused by tetrapyrroles, which accumulated after 17 h in the dark period, followed by 12 h of irradiation with "white light". The effect of accumulated Chl in mustard plants was accompanied by changes in the amounts of the Chls and carotenoids and by dehydration of the tissues, partial chlorosis, and necrosis. The molecular nature of the specific photodynamic sensitivity of the mustard and wheat plants under the influence of Phen and Chl precursors was important: accumulation of tetrapyrroles was a necessary, but not only reason for photodynamic damage of the plants. The degree of leaf damage was related to the amount and chemical nature of accumulated tetrapyrroles and to the greening group to which the investigated plant belongs. and V. Toneva, T. Gechev, I. Minkov.