Influence of 1,10-phenanthroline, a photodynamic herbicide, on the ultrastructure of mesophyll cells and photosynthetic activity in greening pea seedlings

Abstract

We have examined the ultrastructure of mesophyll cells and photosynthetic activity induced by a photodynamic herbicide, 1,10-phenanthroline (Phe), in greening pea seedlings. Greening pea seedlings treated in darkness and subsequently illuminated were resistant to low doses (2 mM) of Phe. Pea plants treated with 10 or 20 mM Phe were susceptible to light, CO 2 evolution in Phe-treated plants proceeded at a much higher rate than the rate of CO 2 uptake. Net photosynthesis rate ( Pn) was extremely low in Phe-treated plants. Ultrastructure of mesophyll cells was the same in control, Phe 10 and Phe 20 plants before transfer of plants to light. Differences between control and Phe-treated plants became visible after 4 hr of illumination and increased during 9 hr of illumination. This difference was manifested in thylakoid swelling, dilation of endoplasmic reticulum (ER) cisternae, degeneration of internal mitochondrial membranes and disruption of chloroplast envelopes. There were starch grains in control but not in Phe-treated plant mesophyll cells. A standard photoinhibition mechanism might explain the destruction caused by phenanthroline and subsequent illumination. In the present paper we suggest another possible mechanism, also based on chelating properties of Phe, which could directly cause membrane depolarization and thus change membrane permeability. One of these mechanisms might explain the structural and functional effects of Phe action.