Porphyrin stability in plant supernatant fractions: implications for the action of porphyrinogenic herbicides

Photobleaching diphenyl ether herbicides act by inhibiting the chloroplast and mitochondrial enzyme which converts protoporphyrinogen to protoporphyrin, a step in the porphyrin synthesis pathway. This results in accumulation of the phototoxic compound protoporphyrin and subsequent light-dependent membrane damage. Recent findings suggest that protoporphyrin accumulation is a result of protoporphrin accumulation in the herbicide-inhibited chloroplast followed by export into the cytoplasm and subsequent oxidation to the phototoxic protoporphyrin by herbicide-resistant mechanisms associated with extraorganellar plant membranes. The present study indicates that accumulated protoporphyrinogen can be decomposed to nonporphyrin (and therefore nonphototoxic) products by factors in the soluble fraction of barley and mustard leaves. In contrast, protoporphyrin is quite stable in these fractions. The rate of protoporphyrinogen destruction is dependent upon the amount of soluble leaf fraction present and the time of incubation. Protoporphyrinogen destruction is suppressed in the presence of the reducing agent dithiothreitol (5 mM) and by ascorbic acid (0.5 mM). Boiling of the soluble extract also suppresses protoporphyrinogen destruction. Uroporphyrinogen I is not destroyed by soluble leaf extracts. These findings suggest that the protoporphyrinogen which accumulates outside of the chloroplast upon herbicide treatment may decompose to nonporphyrin (and therefore nontoxic) products if it is not rapidly oxidized to protoporphyrin by herbicide-resistant mechanisms in extraorganellar membranes. Protoporphyrinogen destruction may be a mechanism for providing protection from the toxic effects of protoporphyrin accumulation. Some evidence for this was provided by our observation that older mustard leaves are more active in protoporphyrinogen destruction than younger leaves. Other studies have shown that older weeds are more herbicide tolerant than younger weeds.