Acetolactate Synthase Inhibitors Increase Ethylene Production and Cause Fruit Drop in Citrus

The abscission action of two sulfonylureas and one imidazolinone was evaluated in laboratory studies with harvested orange ( Citrus sinensis L. cv. Valencia) fruit and greenhouse studies with orange (cv. Hamlin) and grapefruit ( Citrus paradisi Macf. cv. Marsh) trees. Dipping harvested fruit in 90 mg·L –1 imazameth, 2 mg·L –1 metsulfuronmethyl, or 30 mg·L –1 prosulfuron solutions increased levels of internal ethylene. Internal ethylene concentration was higher when fruit were dipped in 2 mg·L –1 metsulfuron-methyl solutions at low pH. Fruit retained on trees and dipped in 2 mg·L –1 metsulfuron-methyl solutions produced more ethylene than control fruit. Drop of treated fruit began when ethylene production was at a maximum. High temperatures (average 33 °C) suppressed ethylene production and fruit drop of metsulfuron-methyl–treated fruit. The results indicate the importance of environmental conditions in evaluating the potential of sulfonylureas and imidazolinones as abscission agents for citrus. Chemical names used: ±-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1 H -imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid (imazameth); methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2yl) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl); 1-(4-methoxy-6-methyl-triazin-2-yl)-3-[2-(3,3,3-trifluoropropyl) phenylsulfonyl] urea (prosulfuron); N -(phosphonomethyl) glycine (glyphosate); 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1 H -imidazol-2-yl]-3-quinolinecarboxylic acid (imazaquin).