Effect of 2,2'-bipyridyl, a photodynamic herbicide, on chloroplast ultrastructure, pigment content and photosynthesis rate in pea seedlings

The effect of a foliar application of 2.2'-bipyridyl (2.2'-B) at concentrations of 5, 10 or 30 mM on ultrastructure of mesophyll cells, chlorophyll (Chl) and carotenoid (Car) contents, photosynthetic activity rate (Pn) and dark respiration rate (R) and chlorophyll fluorescence in pea seedlings was examined. Pea plants were resistant to a low dose (5 mM) with respect to morphology, pigment level and structure of mesophyll cells. However, 10 and 30 mM 2.2'-B and illumination caused a loss of turgor of treated plants and photooxidative perturbations at the structural and functional level in chloroplasts. Photosystem II (PS II) was the primary target of 2.2'-B action. 2.2'-B treatment and subsequent illumination caused direct and secondary reactions. The former, caused by processes which took: place even prior to light exposure, was first expressed and monitored by changes of Chl fluorescence and Pn. The latter, subsequent slow reactions, were induced by light, required longer illumination time and led to destructive changes in mesophyll cell ultrastructure including dilation of thylakoid membranes and endoplasmic reticulum (ER) cisternae and disruption of the chloroplast envelope as well as a decrease in Chl and Car contents. These slow reactions were the result of the rapid ones as well as the result of light-induced photooxidative mechanism based on the process of porphyrin photosensitizing.