Formation of chlorophyll biosynthesis intermediates in bean, cucumber and barley seedlings treated with 5-aminolevulinic acid and homocysteine

Porphyrin accumulation was investigated in intact green bean (Phaseolus vulgaris), cucumber (Cucumis sativus) and barley (Hordeum vulgare) seedlings that were placed for 17 h into darkness after having been sprayed with solutions of natural metabolites of 5-aminolevulinic acid (ALA), a specific porphyrin precursor, and homocysteine, an inhibitor of S-adenosyl-L-methionine:Mg-protoporphyrin IX methyltransferase. At concentrations of 3-15 mM, homocysteine in combination with ALA, in contrast with the action of ALA alone, strengthened both the porphyrinogenesis in the dicotyledons investigated and the porphyrin-induced photodamage that resulted in plant death after 4 d of continuous irradiation of previously darkened seedlings. Further increase of the homocysteine concentrations resulted in a gradual decrease of the stimulation of porphyrin accumulation with a loss of herbicidal activity. Primary leaves of barley as monocotyledons were much less sensitive to the new herbicide compositions than the dicotyledons investigated.