Studies into the differential activity of the hydroxybenzonitrile herbicides: I. Photosynthetic inhibition, symptom development, and ultrastructural changes in two constrasting species

Abstract

Symptom development in Matricaria inodora and Viola arvensis is compared following field rate application of ioxynil and bromoxynil salts. M. inodora is moderately susceptible to ioxynil-Na and susceptible to bromoxynil-K, whereas V. arvensis is moderately resistant to ioxynil-Na and resistant to bromoxynil-K. Levels of protein, amino acid, and reducing sugars reflected the susceptibility of M. inodora and the transient growth cessation of V. arvensis after hydroxybenzonitrile treatment. Rapid development of chlorotic and necrotic symptoms in M. inodora corresponded with complete inhibition of CO 2 fixation within 4 days of treatment. Chloroplasts were swollen, and, after 7 days, ioxynil-Na induced ultrastructural changes typical of photooxidative processes including thylakoid swelling and vesicle formation in the chloroplast. Bromoxynil-K treated mesophyll cells exhibited general cellular disruption including chloroplast swelling and plasmalemma and tonoplast rupture. In contrast, CO 2 fixation remained at 35% of control levels in bromoxynil-K-treated V. arvensis. Chloroplast grana contained twice the number of thylakoids of untreated chloroplasts and there was a small reduction in the chlorophyll a:b ratio of treated foliage. Ioxynil-Na induced a similar response with additional chloroplast swelling reflecting the greater inhibition of CO 2 fixation by this herbicide.