Abstract:
In onions, sensitive cultivars diminish the usefulness of the selective
herbicide bromoxynil. Tolerant cultivars can overcome this limitation. A better
understanding of the biochemistry, physiology and genetics of bromoxynil
tolerance could lead to improved weed control programs in onions.
Onion cultivar response to bromoxynil was studied under greenhouse
conditions. Tolerant cultivars were identified using survival and fresh weight of
surviving plants as indicator variables. Cultivars 'Utah Yellow Sweet Spanish'
(YSS) and 'White Bermuda' were designated as tolerant and sensitive,
respectively. Bromoxyml rates eliciting minimum and maximum responses in
percent kill and fresh weight of the surviving plants were determined. Cultivar
'Winner' was identified as tolerant from the screening of cultivars with 5.0 kg
ae/ha bromoxynil.
Mechanisms that confer cultivar tolerance to bromoxynil in onions were
studied by examining the effects of ethofumesate, piperonyl butoxide and
hydrolysis of cyano ¹⁴C bromoxynil in Utah YSS and White Bermuda.
Ethofumesate did not enhance bromoxynil phytotoxicity to Utah YSS. Piperonyl
butoxide did not modify bromoxynil phytotoxicity to either Utah YSS or White
Bermuda. Hydrolysis of cyano ¹⁴C bromoxynil in Utah YSS and White Bermuda
yielded no significant differences in the amount of unextractable radioactivity
from the plant residues or in the amount of 3,5-dibromo-4-hydroxybenzonitrile
formed. Hydrolysis of the cyano moiety of bromoxynil occurs in both Utah YSS
and White Bermuda. It is not the mechanism that confers tolerance to
bromoxynil in the onion cultivar Utah YSS.
