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 metsulfuron-methyl, 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 degrees 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-1H-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).