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Absorption, Translocation, and Metabolism of Mesotrione in Grain SorghumAuthor(s): M. Joy M. Abit and Kassim Al-KhatibSource: Weed Science, 57(6):563-566. 2009.Published By: Weed Science Society of AmericaDOI: http://dx.doi.org/10.1614/WS-09-041.1URL: http://www.bioone.org/doi/full/10.1614/WS-09-041.1

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Absorption, Translocation, and Metabolism of Mesotrione in Grain Sorghum

M. Joy M. Abit and Kassim Al-Khatib*

Studies were conducted under controlled growth conditions to determine whether differential absorption, translocation, ormetabolism was the basis for the differential response of grain sorghum hybrids to mesotrione. Mesotrione-tolerant(‘Dekalb DKS35-70’) and mesotrione-susceptible (‘Pioneer 84G62’) grain sorghum hybrids were treated with 14C-labeledmesotrione. At 1 d after treatment (DAT), absorption was 7% in both hybrids; at 7 DAT, however, absorption remainednearly steady in Pioneer 84G62 but increased to 12% in Dekalb DKS35-70. Translocation of 14C-mesotrione in sorghumhybrids was similar with less than 30% of the absorbed herbicide translocated out of the treated leaf by 7 DAT. A distinctmetabolite of 14C-mesotrione was separated in both hybrids at 3 DAT. The amount of mesotrione parent compound thatremained in Pioneer 84G62 and DKS35-70 was 72 and 65%, respectively. Dekalb DKS35-70 had significantly lessmesotrione at 3 DAT than Pioneer 84G62 did, but the amount of mesotrione was similar for both hybrids at 5 and 7DAT. Rapid metabolism of mesotrione may help explain the differential response of grain sorghum hybrids.Nomenclature: Mesotrione; sorghum, Sorghum bicolor (L.) Moench. SORBI.Key words: HPPD-inhibiting herbicides, hybrids.

Mesotrione is a selective, systemic, soil- and foliar-appliedherbicide that controls broadleaf and some grass weeds, suchas Palmer amaranth (Amaranthus palmeri S. Wats.) andcommon waterhemp (Amaranthus rudis Sauer), in corn (Zeamays L.), including weeds that are resistant to photosystem II,acetolactate synthase, protoporphyrinogen oxidase, and 5-enopyruvyl-shikimate-3-phosphate synthase herbicides (An-derson et al. 1996; Horak and Peterson 1995; Shoup et al.2003; Vencill et al. 2006). Mesotrione is a competitiveinhibitor of the enzyme p-hydroxyphenylpyruvate dioxygenase(HPPD), which catalyzes the conversion of tyrosine toplastoquinone and a-tocopherol (Mitchell et al. 2001; Norriset al. 1998), resulting in carotenoid biosynthesis reduction.Mesotrione is absorbed rapidly by susceptible speciesfollowing foliar application and is translocated acropetallyand basipetally (Mitchell et al. 2001).

Mesotrione injury symptoms in susceptible plants includebleaching followed by necrosis within 3 to 5 d (Senseman2007). Bleaching symptoms result from inhibition ofcarotenoid biosynthesis, coupled with destruction of chloro-phyll by light (photooxidation) and inhibition of chlorophyllbiosynthesis (Hess 2000; Kim et al. 2001). Under high lightintensities, rapidly growing species use # 50% of absorbedlight energy, and the remaining absorbed light is excess energy(Demmig-Adams et al. 1996). Plants have a natural ability todissipate this excess energy through photoprotection bycarotenoids (Taiz and Zeiger 2008). When chlorophyll iselectronically excited by absorbing light photons, it istransformed from a ground state, short-lived, singlet formto an excited state, longer-lived, triplet form (Hess 2000). Ifthe excited state of chlorophyll is not rapidly quenched, it canreact with molecular oxygen to form singlet oxygen. Theextremely reactive, singlet oxygen then reacts with, anddamages, many cellular components (Muller et al. 2001; Taizand Zeiger 2008). Carotenoids exert their photoprotectiveaction by rapidly quenching the excess energy of the tripletchlorophyll, which is especially generated under high lightintensity. If carotenoid synthesis is inhibited, chlorophyll andphotosynthetic membrane destruction occurs because of the

plant’s inability to quench the reactive, oxidative energy (Hess2000).

Currently, sorghum growers rely on PRE applications ofmesotrione to control Amaranthus species that are resistant toseveral herbicide chemistries and to control many other weedscommonly found in grain sorghum; however, withoutsufficient moisture to activate mesotrione, weed control maynot be adequate (Armel et al. 2003). POST application ofmesotrione consistently controlled weeds but caused bleachingand chlorosis in grain sorghum (Abit et al. 2009; Horky andMartin 2005). Research has demonstrated, however, thatseveral grain sorghum hybrids are tolerant to POSTapplications of mesotrione. Abit et al. (2009) reported thatamong 85 sorghum hybrids evaluated, 23 were susceptible, 45were intermediate, and 17 were tolerant to mesotrione.Furthermore, the mesotrione rate that cause 50% sorghuminjury ranged from 121 to184 g ha21 and from 64 to 91 gha21 for tolerant and susceptible hybrids, respectively. Ingeneral, tolerant hybrids showed less injury and recoveredmore rapidly from mesotrione injury than susceptible hybrids.In corn, mesotrione tolerance has been attributed to lowerabsorption and increased cytochrome P450-mediated metab-olism compared with susceptible weed species (Bartlett andHall 2000; Mitchell et al. 2001). However, no research hasbeen conducted to examine foliar absorption, translocation,and metabolism of mesotrione in grain sorghum. Therefore,the objective of this study was to determine whetherabsorption, translocation, or metabolism was the basis forthe differential response of grain sorghum hybrids tomesotrione.

Materials and Methods

Plant Materials. Mesotrione-tolerant (‘Dekalb DKS35-70’)and mesotrione-susceptible (‘Pioneer 84G62’) grain sorghumhybrids (Abit et al. 2009) were planted in separate 11-cm-diam containers with sand : Morrill loam (fine-loamy, mixed,mesic Typic Argiudolls) soil (1 : 1 by vol) with pH 6.5 and2% organic matter. Plants were grown under growth-chamberconditions of 30/25 C day/night temperatures and a 16-hphotoperiod with supplemental light intensity of 250 mmolm22 s21 photosynthetic photon flux density. Plants werewatered as needed and fertilized weekly with a commercial

DOI: 10.1614/WS-09-041.1* Department of Agronomy, Kansas State University, Manhattan, KS 66506.

Corresponding author’s E-mail: khatib@ksu.edu

Weed Science 2009 57:563–566

Abit and Al-Khatib: Mesotrione in grain sorghum N 563

fertilizer1 solution containing 1.2 g L21 total nitrogen, 0.4 gL21 phosphorus, and 0.8 g L21 potassium. After emergence,grain sorghum hybrid seedlings were thinned to 2 plants pot21.

Absorption and Translocation. At the four-leaf stage, plantswere treated with 10, 1-ml droplets of 14C-labeled mesotrione[phenyl-U-14C]-mesotrione, with specific activity of 781MBq g21, on the upper surface of the third leaf of bothmesotrione-susceptible and mesotrione-tolerant plants. Asingle 1-ml droplet contained 87 Bq of 14C-mesotrione.Unlabeled mesotrione was added to the radioactive solution toobtain 105 g ai ha21, in a carrier volume of 187 L ha21. Cropoil concentrate (COC)2 was added at 1% v/v to enhancedroplet-to-leaf surface contact. Plants were harvested at 1, 3,and 7 d after treatment (DAT) and were divided into sixsections: treated leaf, leaves above the treated leaf, stem abovethe treated leaf, leaves below the treated leaf, stem below thetreated leaf, and roots. Treated leaves were washed with 15 mlof a 75% methanol (by vol) solution for 20 s to remove anyunabsorbed herbicide. Radioactivity in the leaf rinsate wasmeasured by using liquid scintillation spectrometry (LSS).3

Plant sections were dried at 45 C for 48 h and thencombusted by using a biological oxidizer.4 Radioactivityrecovered for each plant part was measured by using LSS.Herbicide absorption was calculated by dividing the radioac-tivity recovered in the entire plant by the total radioactivityapplied to the plant. Herbicide translocation was calculated bydividing the radioactivity recovered in each plant part by thetotal radioactivity absorbed in the plant (Schuster et al. 2007).

Mesotrione Metabolism. To detect all metabolites, highermesotrione radioactivity was used in this study compared withthe absorption and translocation study. Ten 1-ml dropletscontaining 2,183 Bq of 14C-mesotrione were applied to theupper surface of the four largest leaves on each plant in acontainer. Unlabeled mesotrione was mixed with 14C-mesotrione to reach the desired application rate as describedfor the foliar absorption and translocation study. Herbicidesolution included COC, as previously described.

Treated leaves were harvested at 3, 5, and 7 DAT. Theleaves were washed with 15 ml of 75% methanol to removeany unabsorbed mesotrione. Plant tissues were then frozenwith liquid nitrogen and ground with a mortar and pestle.Subsamples of the ground tissue were weighed and oxidized,and captured 14CO2 was determined by using LSS to assessthe amount of radioactivity in the plant tissue. Leaf tissueswere stored at 280 C until radioactivity was extracted.

Frozen leaf tissues were homogenized with 20 ml of 75%methanol (by vol) and shaken for 1 h. Samples were filtrated,and the supernatant was saved. The leaf tissues wereresuspended twice in 15 ml of 50% methanol and shakenfor an additional hour. Samples were filtered, and supernatantwas added to the first and second supernatant. The remainingleaf tissues were resuspended in 15 ml of 100% methanol andshaken for 6 h. Samples were filtered, and the supernatant wasadded to the total supernatant. To determine the amount ofradioactivity not extracted into the supernatant, the remainingplant residue and filter paper were oxidized, and radioactivitywas measured (14C extraction efficiency 5 95.3 6 0.2).Supernatant was then evaporated at 35 C to 0.5 ml byusing a centrivap.5 Solution was then filtered with a 0.2-mm filter6 and stored at 220 C until use.

Extracts were injected into a Beckman high-performanceliquid chromatograph7 equipped with a Zorbax ODS end-capped Sb-C18 column,8 (4.6 by 250 mm, 5 mm particle size)with a mobile phase of water with 0.1% formic acid andmethanol at a flow rate of 0.5 ml min21 and an injectionvolume of 50 ml. The elution profile was as follows: step 1,40% methanol isocratic gradient for 6 min; step 2, 40 to 75%methanol linear gradient for 2 min; step 3, 75 to 100%methanol linear gradient for 2 min; step 4, 100% methanolisocratic gradient for 3 min; step 5, 100 to 40% methanollinear gradient for 3 min; and step 6, 40% methanol isocraticgradient for 7 min. Fractions were sequentially collected at0.5-min intervals, and radioactivity was measured by usingLSS. A mesotrione standard was included to determineherbicide retention time.

Experimental Design and Data Analysis. The experimentaldesign for all studies was a randomized complete block.Treatments were blocked by harvest time. Foliar absorptionand translocation treatments were replicated four times, andthe experiment was conducted three times. In the metabolismstudy, treatments were replicated four times, and theexperiment was repeated. There were no interactions amongruns for both studies; therefore, data were pooled over runs.Data from both studies were analyzed using ANOVA, andmeans were separated by using standard errors at P # 0.05(Schuster et al. 2007).

Results and Discussion

Absorption. Absorption of 14C mesotrione was low in bothgrain sorghum hybrids (Table 1) and lower than mesotrioneabsorption in corn reported by others (Armel et al. 2004). Thelow foliar mesotrione absorption in sorghum may be due tothe presence of a large number of prickle hairs (trichomes withswollen bases and sharp tips) and higher amount of looselybound leaf wax (Cannon and Kummerow 1957; Traore et al.1989). For example, wax concentration in sorghum leaves was0.6% but was only 0.35% in corn (Cannon and Kummerow1957). At 1 DAT, both mesotrione-tolerant (DKS35-70) andmesotrione-susceptible (84G62) hybrids absorbed 7% of thetotal applied mesotrione. Mesotrione absorption in DKS35-70 increased over time but peaked 3 DAT in 84G62. At 3 and7 DAT, DKS35-70 absorbed 9 and 12%, respectively,whereas 84G62 absorbed only 8% at both harvest times.Other researchers have reported similar amounts of foliarabsorption of other POST HPPD herbicides in other species(Young and Hart 1998). The tolerant sorghum hybrid hadslightly higher absorption than the susceptible hybrid, likelybecause there was less mesotrione injury to the toleranthybrid. As a consequence, tolerant tissue would continueabsorbing mesotrione over time, whereas susceptible tissue

Table 1. Absorption of mesotrione in mesotrione-tolerant (DKS35-70) andrelatively mesotrione-susceptible (84G62) grain sorghum hybrids at 1, 3, and 7 dafter treatment (DAT).a

Hybrid 1 DAT 3 DAT 7 DAT

------------------------------------------------% absorbed -----------------------------------------------

DKS35-70 7 6 1 9 6 1 12 6 284G62 7 6 1 8 6 1 8 6 1

a Table values are means 6 standard error.

564 N Weed Science 57, November–December 2009

would be severely injured preventing further mesotrioneabsorption. (Devine et al. 1993).

Translocation. Mesotrione translocation out of the treatedleaf was similar in tolerant and susceptible sorghum hybrids ateach harvest time (P 5 0.99); therefore, data were averagedacross hybrids. Translocation of 14C mesotrione in sorghumwas relatively low (Table 2). A similar level of translocationwas reported when mesotrione was applied to corn andsoybean [Glycine max (L.) Merr] foliage (Armel et al. 2004;Mitchell et al. 2001; Schuster et al. 2007). Only 10 to 17% of14C mesotrione translocated to the rest of the foliage with 7 to11% to the stem and only 5% or less to the roots (Table 2).No more than 30% of the absorbed 14C mesotrionetranslocated out of the treated leaf by 7 DAT. At 7 DAT,most of the 14C mesotrione remained in the treated leaf.These results are in agreement with earlier research thatshowed that the bulk of the 14C mesotrione applied to Canadathistle [Cirsium arvense (L.) Scop] remained in the treated leaf,only 9 to 20% of 14C mesotrione translocated to the rest ofthe foliage, and 2% or less translocated to the roots (Armel etal. 2005). Mesotrione translocation to the different plantparts, however, was different between harvest timings. At 1DAT translocation of 14C mesotrione to the leaves above thetreated leaf was 8%, whereas at 3 and 7 DAT translocationwas 16%.

Mesotrione Metabolism. A distinct metabolite was isolatedin both hybrids at 3 DAT. At 5 DAT, two metabolites wereseparated in both hybrids, whereas three and two metaboliteswere segregated from the parent herbicide at 7 DAT inDKS35-70 and 84G62, respectively (Table 3). Previousmetabolism studies in plants and soil show degradates can beformed from mesotrione with MNBA [4-(methylsulfonyl)-2-nitrobenzoic acid] and AMBA [2-amino-4-(methylsulfo-nyl) benzoic acid] as the major metabolites (Alferness andWiebe 2002; Armel et al. 2005). The mesotrione metaboliteswere eluted at 7, 9.5, and 14 min during the elution profile.Based on the mobile phase gradient used, of the three

metabolites, the first two appear to be hydrophobic, and thethird appears to be hydrophilic. DKS35-70 had signifi-cantly less mesotrione at 3 DAT than 84G62 had. At 3DAT, 72% of mesotrione remained in 84G62; only 65%remained in DKS35-70. These results are similar to those ofWichert et al. (1999), who found that sweet corn cultivarsthat are more tolerant to POST applications can metabolizemesotrione more rapidly than susceptible cultivars. Al-though there was a considerable amount of mesotrionepresent at 3 DAT, previous study revealed that differencesin injury were still observed between the two hybrids (Abitet al. 2009). Considering the rate of absorption (9%) andtranslocation (30% of the absorbed mesotrione), thedifference in the amount of mesotrione retained (percentageof the translocated amount) in tolerant and susceptiblehybrids can cause significant differences in injury. Thegreater mesotrione metabolism in tolerant, rather than insusceptible, sorghum hybrids resulted in lower concentra-tions of mesotrione in plants, which led to earlier recoveryin the tolerant sorghum. The metabolism pattern ofmesotrione, however, was similar for both hybrids at 5and 7 DAT. At 5 DAT, 59 and 63% of the mesotrioneremained in DKS35-70 and 84G62 hybrids, respectively,whereas 36 and 43% of the mesotrione remained inDKS35-70 and 84G62, respectively, 7 DAT.

Because no differences in foliar absorption and transloca-tion were observed between hybrids, selectivity is probably notdue to differential absorption or translocation. Previousresearchers have identified herbicide metabolism as theprimary basis for differential response of crops to mesotrione(Barlett and Hall 2000; Mitchell et al. 2000). The tolerance tomesotrione treatment in the tolerant hybrid could result fromthe slightly more rapid metabolism in this hybrid. Tolerantspecies have the capacity to metabolize herbicide more rapidlyand extensively than susceptible species. Thus, rapid metab-olism may help explain the differential response of grainsorghum hybrids to mesotrione observed in this study.

Sources of Materials

1 Miracle-Gro soluble fertilizer, Scotts Miracle-Gro Products Inc.,1411 Scottslawn Road, Marysville, OH 43041.

2 Prime Oil, Terra International Inc., P.O. Box 6000, Sioux City,IA 51102-6000.

3 Tricarb 2100TR Liquid Scintillation Analyzer, Packard Instru-ment Co., 800 Research Parkway, Meriden, CT 06450.

4 R. J. Harvey Biological Oxidizer, Model OX-600, R. J. HarveyInstrument Co., 123 Patterson Street, Hillsdale, NJ 07642.

5 Centrivap, Labconco, 8811 Prospect, Kansas City, MO 64132.6 0.2-mm filter, Osmotics Inc., 5951 Clearwater Drive, Minne-

tonka, MN 55343.

Table 2. Translocation of mesotrione in grain sorghum hybrids at 1, 3, and 7 dafter treatment (DAT). Means are the average of two hybrids.a

Plant part 1 DAT 3 DAT 7 DAT

------------------------------------ % translocated -----------------------------------

Treated leaf 76 6 2 71 6 2 71 6 2Leaves above treated leaf 8 6 1 16 6 1 16 6 1Stem above treated leaf 2 6 0 2 6 0 2 6 0Leaves below treated leaf 2 6 1 1 6 0 1 6 0Stem below treated leaf 9 6 1 6 6 0 5 6 1Roots 3 6 2 4 6 0 5 6 1

a Table values are means 6 standard error.

Table 3. Mesotrione metabolites at 3, 5, and 7 d after treatment (DAT) in mesotrione-tolerant (DKS35-70) and mesotrione-susceptible (84G62) grain sorghum.a

Compound Retention time

DKS35-70 84G62

3 DAT 5 DAT 7 DAT 3 DAT 5 DAT 7 DAT

min --------------------------------------------------------------------------------------------------------------- % of total radioactivity --------------------------------------------------------------------------------------------------------------

Metabolite 1 7 35 6 2 29 6 2 41 6 3 28 6 2 28 6 1 43 6 4Metabolite 2 9.5 12 6 1 12 6 1 9 6 3 14 6 3Metabolite 3 14 11 6 9Mesotrione 15.5 65 6 2 59 6 2 36 6 8 72 6 2 63 6 2 43 6 3

a Table values are means 6 standard error.

Abit and Al-Khatib: Mesotrione in grain sorghum N 565

7 Beckman high performance liquid chromatograph, BeckmanCoulter Inc., Life Science Division, 4300 N. Harbor Boulevard,P.O. Box 3100, Fullerton, CA 92834-3100.

8 Zorbax ODS endcapped Sb-C18 column, Agilent Technologies,Chemical Analysis Group, 2950 Centerville Road, Wilmington, DE19808.

Acknowledgments

The authors thank Syngenta Crop Protection, Inc. for funding.Contribution 09-257-J from the Kansas Agricultural ExperimentStation, Manhattan.

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Received March 3, 2009, and approved June 23, 2009.

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