Effects of Pyrazon on Growth, Photosynthesis, and Respiration

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Abstract
The herbicide 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) had a ten-fold greater inhibitory effect on the growth of common lambsquarters (Chenopodium album L.) than on sugar beets (Beta vulgaris L.), Production of dry matter was inhibited by accumulations of pyrazon in the tissues. When these accumulations reached a level approximately twice that which affected dry matter production, plants wilted, collapsed, and died. Pyrazon failed to affect the structural integrity of either chlorophyll or the chloroplast, but inhibited the Hill reaction of chloroplasts isolated from both sugar beets and common lambsquarters. The I50 value was 6 × 10−6 M pyrazon. There was no difference between sugar beets and common lambsquarters in the degree of inhibition in oxygen evolution by leaf discs placed in pyrazon. When plants were cultured in vivo with pyrazon, the rate of oxygen evolution was correlated with herbicide accumulations in the leaf tissues. Sugar beet leaves contained only one-third to one-fifth the level present in common lambsquarters. The rates of respiration of roots and leaf discs of both species were not affected by pyrazon treatments in vivo or in vitro.

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