Interactions of Lipoidal Materials and a Pyridazinone Inhibitor of Chloroplast Development

J. L. Hilton^a, J. B. St. John^a and M. N. Christiansen^a

K. H. Norris^b

^a Plant Science Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705, ^b Instrumentation Research Laboratory, Market Quality Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705

Formation of chloroplast pigments was inhibited, and free fattyacids accumulated in mustard (Brassica juncea [L.] Coss.) cotyledonsand in barley (Hordeum vulgare L.) first leaves developed aftertreatment with 4-chloro-5- (dimethylamino)-2- ( ${alpha}$ , ${alpha}$ , ${alpha}$ -trifluoro-m-tolyl)-3 (2H) -pyridazinone. The inhibitor reduced the amount of fattyacids found in polar lipids (galactolipids) of barley chloroplastsand increased the amount in nonpolar lipids while having littleeffect on total content of bound fatty acids. The inhibitionof chlorophyll formation was circumvented by D- ${alpha}$ -tocopherol acetate,phytol, farnesol, and squalene, and by unsaturated fatty acidsand their methyl esters. The protective action can be explainedpartially by an interaction external to the plant whereby 4-chloro-5-(dimethylamino) -2- ( ${alpha}$ , ${alpha}$ , ${alpha}$ -trifluoro-m-tolyl) -3 (2H) -pyridazinonepartitioned out of the aqueous phase and into the lipid phase,thus limiting availability of the inhibitor to plants. However,the amount of inhibitor reaching the cotyledons of tocopherol-protectedmustard seedlngs was still in excess of the amount necessaryto cause white foliage, but it failed to produce the effect.Tocopherol treatment did not prevent the 4-chloro-5- (dimethylamino)-2- ( ${alpha}$ , ${alpha}$ , ${alpha}$ -trifluoro-m-tolyl) -3 (2H) -pyridazinone-induced buildupof fatty acids in mustard cotyledons but did partially circumventthe effect in barley leaves. The amount of linolenic acid relativeto linoleic acid was reduced in barley leaves and chloroplastsby 4-chloro-5- (dimethylamino) -2- ( ${alpha}$ , ${alpha}$ , ${alpha}$ -trifluoro-m-tolyl)-3 (2H) -pyridazinone action and this effect was circumventedby tocopherol.