Alachlor influence on sorghum growth and gibberellin precursor synthesis

Abstract

Growth (14 days) of sorghum ( Sorghum bicolor L. cv G522 DR) from seed planted in sand into which alachlor [2-chloro-2′,6′-diethyl- N-(methoxymethyl)acetanilide] was uniformly incorporated (0, 0.07, 0.14, 0.28, 0.56, 1.12, 2.24, or 4.48 kg/ha) was reduced by 0.14 kg/ha and severely inhibited (88%) by 0.56 kg/ha while cellular water cotent was not greatly influenced by 0.56 kg/ha. When added into the nutrient solution bathing the roots of 96-hr sorghum seedlings, alachlor (0, 0.0156, 0.0312, 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64, or 128 ppmw) was not lethal to 14-day-old sorghum at rates up to 32 ppmw (92% survival); however, shoot and root lengths were reduced 43 and 58%, respectively. Alachlor inhibition of sorghum growth appears to be closely associated with inhibition of cell enlargement; the coleoptile is the most susceptible stage of sorghum growth to alachlor. This situation closely resembles growth where gibberellic acid (GA) synthesis is inhibited. [2- 14C]Mevalonic acid ([2- 14C]MVA) incorporation into terpenoid GA precursors was evaluated using a cell-free enzyme system from etiolated sorghum coleoptiles. Alachlor did not inhibit total 14C incorporation but incorporation of 14C into kaurenol and sterols was decreased ca 80 and 75%, respectively, by 10 −6 M alachlor. Analyses for [ 14C]geranylgeraniol (GG), [ 14C]farnesol, and [ 14C]geraniol contents showed accumulation of [ 14C]farnesol and [ 14C]GG, and decreased [ 14C]geraniol. When seeds to which CGA-43089 [α-(cyanomethoximino)-benzacetonitrile] was applied 8 weeks prior to planting were substituted for untreated seeds, incorporation of [2- 14C]MVA into [ 14C]kaurenol was increased by alachlor while [ 14C]GG and [ 14C]farnesol accumulated and [ 14C]geraniol was absent at 10 −6 M alachlor. Additionally, sterol content increased in “safened” systems but was still decreased by alachlor. These data demonstrate multiple sites of alachlor activity in the GA and terpenoid biosynthetic pathway.