FORMATION OF CHLOROPHYLL BIOSYNTHESIS INTERMEDIATES IN BEAN, CUCUMBERAND BARLEY SEEDLINGS TREATED WITH 5-AMINOLEVULINIC ACID AND HOMOCYSTEINE

Porphyrin accumulation was investigated in intact green bean (Phaseolu s vulgaris), cucumber (Cucumis sativus) and barley (Hordeum vulgare) s eedlings that were placed for 17 h into darkness after having been spr ayed with solutions of natural metabolites of 5-aminolevulinic acid (A LA), a specific porphyrin precursor, and homocysteine, an inhibitor of S-adenosyl-L-methionine:Mg-protoporphyrin IX methyltransferase. At co ncentrations of 3-15 mM, homocysteine in combination with ALA, in cont rast with the action of ALA alone, strengthened both the porphyrinogen esis in the dicotyledons investigated and the porphyrin-induced photod amage that resulted in plant death after 4 d of continuous irradiation of previously darkened seedlings. Further increase of the homocystein e concentrations resulted in a gradual decrease of the stimulation of porphyrin accumulation with a loss of herbicidal activity. Primary lea ves of barley as monocotyledons were much less sensitive to the new he rbicide compositions than the dicotyledons investigated.