The mode of action of RH-1965: a new phenylpyrimidinone bleaching herbicide

RH-1965 is a new bleaching herbicide which causes newly developing leaf tissue to emerge devoid of photosynthetic pigments. Mode-of-action studies revealed that RH-1965 inhibited the accumulation of both total chlorophyll and β-carotene. Concomitantly, it induced the accumulation of the β-carotene precursors, phytoene, phytofluene and, in particular, ξ-carotene. Inhibition of chlorophyll accumulation by RH-1965 is attributed to the photo-oxidative destruction of chlorophyll in the absence of β-carotene because RH-1965 blocked chlorophyll accumulation to a greater extent under high light (50–330 µE m−2 s−1) than under low light (0.8 µE m−2 s−1) conditions. Radish (Raphanus sativus L) and barnyardgrass (Echinochloa crus-galli (L) Beauv) were very senstive to RH-1965. Under high light (330 µE m−2 s−1), the I50 values for inhibition of chlorophyll accumulation were 0.10 and 0.15 µM, respectively. Wheat (Triticum aestivus L), on the other hand, was much less sensitive to RH-1965 (I50 = 1.4 µM). It is concluded that the mode of action of RH-1965 involves the inhibition of ξ-carotene desaturation.