Five varieties of 3-month-old citrus rootstocks were grown for 1 month in nutrient solution containing 0, 10-3, 10-4, or 10-5 M of norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3 (2H)-pyridazinone] to assess the effect of this herbicide on growth and chlorophyll levels. The uptake, translocation, and metabolism of root-applied 14C-norflurazon was also examined in these rootstocks. Norflurazon treatment reduced chlorophyll level and lowered overall growth in sour orange (Citrus aurantium), Swingle (Poncirus trifoliata × C. paradisi), Milam (C. jambhiri), and Cleopatra mandarin (C. reshni) rootstocks. Herbicide treatments had little or no effect on chlorophyll levels or growth in Carrizo (C. sinensis × P. trifoliata). In Cleopatra, unlike in other rootstocks, leaf chlorosis was evident only along the midrib and major veins. Herbicide treatment reduced leaf flush (leaf production) in Swingle but not in other rootstocks. By the end of a 4-week treatment period, total absorption of 14C-norflurazon was greater in Swingle than in the other rootstocks. 14C-norflurazon detected in the roots (excluding taproot) of Swingle was greater than in the other rootstocks. The amount of 14C-norflurazon translocation to the foliage was limited in all rootstocks. Stem and taproot also contained only a small amount of 14C-norflurazon. Leaf extracts of all five rootstocks contained no parent compound, but up to 90% of recovered 14C was identified as 14C-norflurazon in root extracts. Differences in the uptake, translocation, and metabolism of norflurazon among citrus rootstocks were small and did not appear to explain their differential sensitivity to this herbicide.
