Acetolactate synthase inhibitors increase ethylene production and cause fruit drop in citrus

The abscission action of two sulfonylureas and one imidazolinone was evalua ted in laboratory studies with harvested orange (Citrus sinensis L. cv, Val encia) 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 soluti ons 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-methy l solutions produced more ethylene than control fruit. Drop of treated frui t began when ethylene production was at a maximum, High temperatures (avera ge 33 degrees C) suppressed ethylene production and fruit drop of metsulfur on-methyl-treated fruit. The results indicate the importance of environment al conditions in evaluating the potential of sulfonylureas and imidazolinon es 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-pyridinecarb oxylic acid (imazameth); methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin- 2y l) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl); 1-(4-me thoxy-6-methyl -triazin-2-yl)-3- [2-(3,3,3-trifluoropropyl) phenylsulfonyl] urea (prosulfuron); N-(phosphonomethyl) glycine (glyphosate); 2-[4,5-dihyd ro-4-methyl-4-(1-methylethyl)-5- oxo-1 H-imidazol-2-yl] -3-quinolinecarboxy lic acid (imazaquin).