Consequences of metolachlor induced inhibition of gibberellin biosynthesis in sorghum seedlings

Abstract

Forage sorghum [ Sorghum bicolor (L.) Moench] cultivars were field evaluated for response to seed protectants CGA 92194, CGA 133205, and flurazole for ability to reverse the activity of metolachlor (2.8 kg a.i./ha) and alachlor (3.36 kg a.i./ha). Funk “G102F” was “tolerant” to metolachlor and alachlor (∼17% decrease in forage production), R. C. Young “Red Top Kandy” was “intermediate” (37–50% inhibition), and AgraTech “GK 83S” was “susceptible” (60–65% inhibition). Average plant heights were decreased 0, 26, and 32% for “G102F,” “Red Top Kandy,” and “GK 83S,” respectively, while numbers of plants/4.57-m row were decreased 19, 70, and 90%, respectively. Resultant average individual plant fresh weights (g/plant) were 104, 200, and 410% of the untreated plants for G102F, Red Top Kandy, and GK 83S, respectively. Thus, metolachlor and alachlor lethal responses were exhibited at a very early stage of sorghum seedling development (<1 true leaf), and any plant which survived that stage of growth ultimately developed to the maximum attainable under extant competitive growing conditions. The seed applied protectants essentially reversed the growth inhibition by metolachlor and alachlor at this stage of development and forage production was not inhibited by metolachlor and alachlor when CGA 92194, CGA 133205, or flurazole were utilized as seed protectants. Greenhouse evaluation of the critical seedling stage of growth hypothesis demonstrated a massive shoot growth inhibition by metolachlor and alachlor for all three cultivars during the first 5 days after inhibition; the growth inhibition was reversed by all three seed protectants. Metolachlor inhibited the growth of sorghum root (96-hr bioassay) and acetate reversed that inhibition in Funk G-522 DR roots. Grass seedling (<1 true leaf) production of acetate is by the utilization of seed stored products via α-amylase and isocitric lyase, and the production of both of these enzyme systems was inhibited by metolachlor. The inhibition of α-amylase production by metolachlor was reversed by CGA 92194 and exogenous gibberellic acid (GA 3). These data support the concept that alachlor and metolachlor inhibit GA biosynthesis in seedling tissue which results in greatly decreased utilization of seed food reserves and failure of the juvenile seedling to grow sufficiently to become an independent plant. Alternatively, metolachlor inhibition of acetyl-coenzyme A synthesis results in decreased GA biosynthesis which induces a decreased α-amylase and isocitric lyase synthesis. Additionally, sorghum cv G-522 DR seeds were germinated (64 hr) in GA 3 (0, 2, 4, 6, 8, or 10 ppmw) in the dark. Coleoptiles were immersed in [ 14C]metolachlor (5 μ M metolachlor), GA 3 (≥4 ppmw) induced a decrease (DPM/g fresh wt) in total 14C absorption by G-522 DR sorghum coleoptiles. Conversion of [ 14C]metolachlor → [ 14C]metolachlorglutathione conjugate was increased as the GA concentration in the germination medium increased. GA induces the synthesis of enzymes requisite to starch degradation, β-oxidation of triglycerides, as well as influencing metolachlor uptake and GS conjugation.