Research Article Response of Herbicide-Resistant Palmer Amaranth (Amaranthus palmeri) Accessions to Drought Stress

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1 Agronomy Volume 2013, Article ID , 8 pges Reserch Article Response of Herbicide-Resistnt Plmer Amrnth (Amrnthus plmeri) Accessions to Drought Stress Amn Chndi, 1 Dvid L. Jordn, 1 Aln C. York, 1 Jim Burton, 2 Susn R. Mill-Lewis, 1 Jn Spers, 1 Jred R. Whitker, 1 nd Rndy Wells 1 1 Deprtment of Crop Science, North Crolin Stte University, Cmpus Box 7620, Rleigh, NC , USA 2 Deprtment of Horticulture Science, North Crolin Stte University, Cmpus Box 7609, Rleigh, NC , USA Correspondence should be ddressed to Dvid L. Jordn; dvid jordn@ncsu.edu Received 6 November 2012; Revised 15 Jnury 2013; Accepted 15 Jnury 2013 Acdemic Editor: Robert J. Kremer Copyright 2013 Amn Chndi et l. This is n open ccess rticle distributed under the Cretive Commons Attribution License, which permits unrestricted use, distribution, nd reproduction in ny medium, provided the originl work is properly cited. Plmer mrnth is very problemtic weed in severl crops in the southern USA due to its competitive bility nd resistnce to herbicides representing different mechnisms of ction. Vrition in growth nd subsequent interference of North Crolin Plmer mrnth ccessions hs not been exmined. A greenhouse experiment determined response of 15 North Crolin Plmer mrnth ccessions to drought stress beginning 15 dys fter seedling emergence (DAE) for durtion of 3, 5, 7, nd 9 dys. Following exposure to drought, plnts were grown under optiml moisture conditions until hrvest t 30 DAE. Five ccessions ech of glyphoste-resistnt (GR), cetolctte synthse inhibitor-resistnt (ALSR), nd cetolctte synthse inhibitor-susceptible nd glyphoste-susceptible (ALSS/GS) were compred. Vrition in response to drought stress, bsed on height nd dry weight reduction reltive to nonstressed controls, ws noted mong ccessions. Stress for 3 or more dys ffected height nd dry weight. Height nd dry weight of GR nd ALSR ccession groups were reduced less by drought thn the ALSS/GS ccession group. Results suggest possible reltionship between herbicide resistnce nd bility of Plmer mrnth to withstnd drought stress nd thus possible competitive dvntge for resistnt ccessions under limited moisture vilbility. 1. Introduction The bility of crops nd weeds to extrct wter from soil nd their response to moisture stress re key fctors in determining the outcome of crop-weed interference under drought [1 6]. Ability to bsorb wter from soil under limited wter vilbility, wter use efficiency, nd trnspirtion vry mong crop nd weed species [7 11]. For exmple, wter use efficiency of genotypes of vegetble mrnth (Amrnthus tricolor L., A. blitum L., nd A. cruentus L.) ws not ffected by drought stress. However, stress significntly reduced totl plnt dry mss nd lef re per unit root dry mss nd incresed root dry mss rtio differently in genotypes [12]. Under limited wter conditions, plnts respond differently ndshowwiderngeofdroughttolerncemechnisms both in terms of morphology nd physiology [13]. In nother experiment involving vegetble mrnth, significnt vrition existed mong genotypes for trnspirtion nd stomtl conductnce which ws positively correlted with reltive decrese in dry weight cross four genotypes [14]. The criticl period for crop-weed interference nd the extent of crop losses to weed competition cn be influenced by soil moisture vilbility [3]. In some studies [15, 16], yield loss to weeds ws less in yers with more rinfll or irrigtion while the opposite ws noted in other reserch [17, 18]. A weed-free period of 2 weeks ws sufficient to void yield loss when soyben (Glycine mx (L.) Merr.) competed with common rgweed (Ambrosi rtemisiifoli L.)indryyer, but 4-week weed-free period ws required in yers with dequte moisture [19]. In contrst, Jckson et l. [20] nd Hrrison et l. [21] reported the need for longer weed-free period when soyben ws grown under limited soil moisture conditions. These vrying results suggest weed interference is species nd environment specific.

2 2 Agronomy Plmer mrnth (Amrnthus plmeri S. Wts.) is n economiclly importnt weed to mnge in crop production systems in the southern United Sttes [22] due to its competitive bility, C 4 photosynthesis, nd higher wter use efficiency nd growth rte thn mny other weeds. Resistnce to herbicides representing different mechnisms of ction, including 5-enolpyruvylshikimte-3-phosphte synthse inhibitors, inhibitors of polymeriztion of microtubules during mitosis, cetolctte synthse inhibitors, nd photosynthetic inhibitors, contributes to the difficulty in mnging this weed in severl gronomic crops [23 26]. Growth nlysis of Plmer mrnth, common wterhemp (Amrnthus rudis Suer), redroot pigweed (Amrnthus retroflexus L.), nd tumble pigweed (Amrnthus lbus L.) showed tht Plmer mrnth ws the tllest, possessed the gretest volume nd the gretest mount of brnching, hd the gretest reltive growth rte nd net ssimiltion rte, nd produced the gretest lef re, dry mtter, specific lef re, nd lef re rtio mong the four species [26]. The wter requirement to fix one grm of CO 2 is lower for C 4 plnts compred to C 3 plnts, nd s result of these differences C 4 weeds crry out photosynthesis more efficiently nd cn be more competitive thn C 3 weeds, especilly under hot climtes [27]. Smooth pigweed (Amrnthus hybridus L.), C 4 plnt, hd the gretest net photosynthesis rtes, net ssimiltion rtes, nd wter use efficiency bsed on whole plntswellsonsinglelefbsiswhencompringsoyben nd seven weed species under greenhouse conditions [11]. Ability of prticulr genotype to produce vible offspring reltive to other genotypes in popultion is known s the reltive fitness of tht biotype [28]. Previous reserch suggests tht in some cses herbicide-resistnt weed biotypes hve fitness penlty compred with nonresistnt wild types. In popultion, lleles tht crry lrger fitness penlty re less frequent thn those with smller ssocited fitness penlty. It is generlly ssumed tht herbicide resistnce lleles re infrequent in popultions in the bsence of selection pressure exerted by herbicide, possibly becuse of fitness penlty [29, 30]. Pederson et l. [31] reportedtht GR rigid ryegrss (Lolium rigidum Gudin) produced seeds tht were lrger in size but fewer in number compred with glyphoste-susceptible plnts. Segregting F 2 popultions of rigid ryegrss generted by crossing resistnt nd susceptible plnts resulted in decrese in glyphoste resistnce in plnts [32, 33]. Mternlly inherited trizine resistnce resulted in reduced erly seedling emergence, erly growth, mid-seson lef number, nd totl bove-ground biomss compred with trizine-susceptible popultions [34, 35]. These results suggest tht fitness is plstic nd cn depend on environmentl fctors [34 36]. Plmer mrnth is prticulrly troublesome in crops grown on drought-prone soils in North Crolin. This my be becuse of greter drought tolernce of Plmer mrnth reltive to other common weed species, llowing Plmer mrnth to compete more effectively with other weeds nd estblish monocultures [37]. It is commonly observed tht Plmer mrnth is less negtively ffected by drought thn gronomic crops, nd the Plmer mrnth cn better recover once drought conditions re relxed. It is lso on these drought-prone soils where GR biotypes of Plmer mrnth were first discovered nd where the gretest problems with resistnt biotypes occur, leding to the question of whether or not glyphoste resistnce nd drought tolernce my be relted.abilityofalsrorgrplmermrnthpopultions to recover from drought stress during erly seson growth when interference with crops is criticl hs not been reported. A greenhouse experiment ws conducted to compre the effect of vrious durtions of imposed drought stress on 15 Plmer mrnth ccessions from North Crolin expressing vrition in resistnce to glyphoste nd cetolctte synthse-inhibiting herbicides. 2. Mterils nd Methods Seeds of Plmer mrnth were rndomly collected from 290 fields in North Crolin during the fll of 2005 nd were screened for resistnce to glyphoste nd thifensulfuron [38]. From this seed collection, 15 ccessions were selected for the current study (Figure 1). Five ccessions were GR, five were ALSR, nd five were ALSS/GS. Seeds were plnted in excess in round plstic pots (10 cm dimeter by 12 cm depth) contining commercil potting mix (Ffrd 4P potting mix, Conrd Ffrd Inc., Agwm, MA, USA) mking sure tht ll pots contined the sme weight of potting mix. Seedlings similr in height nd number of leves were thinned to one per pot 8 DAE. Plnts were fertilized with 25 ml pot 1 of 4.6gL 1 fertilizer solution (Scotts Strter Fertilizer, The Scotts Compny LLC, Mrysville, OH, USA) t 10 nd 24 DAE to ensure optimum plnt growth. Ech pot received the sme volume of wter on dily bsis designed to bring soil to sturtion. The greenhouse ws mintined t 35 ± 5 C, nd nturl lighting ws supplemented for 14 hours dily with metl hlide lmps (Hubbell Lighting, Inc., Greenville, SC, USA) delivering 400 μmol m 2 s 1. Beginning 15 DAE, wter ws withheld in order to induce drought stress for 3, 5, 7, or 9 dys. These intervls were selected keeping in mind tht weed mngement decisions re criticl during 3-4 weeks fter plnting. Soil ws brought bck to full sturtion fter completion of the stress regimes nd ws mintined t moisture sttus to ensure optimum growth for the reminder of the experiment. Plnt height nd bove-ground dry weight were recorded 30 DAE. Percent reduction in plnt height nd bove-ground dry weight 30 DAE were clculted reltive to the nonstressed control for ech ccession. Percent reduction in these prmeters ws used to llow sttisticl comprisons of Plmer mrnth ccessions tht vried considerbly in ctul plnt height nd dry weight. Actul plnt height nd dry weight for Plmer mrnth controls re presented in Tble 1. The experimentl design ws the rndomized complete block with 10 replictions. The experiment ws repeted immeditely fter hrvesting the first run. Dt were subjected to nlysis of vrince (Sttisticl Anlysis Systems, version 9.1, SAS Institute Inc., SAS Cmpus Drive, Cry, NC, USA) pproprite for fifteen (Plmer mrnth ccessions) by four (drought stress periods) fctoril tretment rrngement to test min effects nd interctions. In second nlysis,

3 Agronomy 3 Tble 1: Height nd dry weight of Plmer mrnth controls. Height (cm) At hrvest Accession County Resistnce Dys of drought stress Height Dry weight (cm) (gplnt 1 ) Individul Plmer mrnth ccessions b 1 Mrtin ALSS/GS Hrnett ALSS/GS Lenoir ALSS/GS Mrtin ALSS/GS Edgecombe ALSS/GS Wyne GR Hoke GR Robeson GR Cumberlnd GR Scotlnd GR Robeson ALSR Johnston ALSR Robeson ALSR Smpson ALSR Edgecombe ALSR Plmer mrnth ccessions grouped by response to herbicide c 1 ALSS/GS GR ALSR Dt re pooled over experiments. b Consists of fifteen Plmer mrnth ccessions. c Consists of group of 5 cetolctte synthse-inhibitor susceptible nd glyphoste-susceptible (ALSS/GS), group of 5 cetolctte synthse inhibitor-resistnt (ALSR), nd group of 5 glyphoste-resistnt (GR) Plmer mrnth ccessions. GS/ALSS (Edgecombe, Hrnett, Lenoir, nd Mrtin counties) GR (Cumberlnd, Hoke, Scotlnd, nd Wyne counties) ALSR (Edgecombe, Johnston, Robeson, nd Smpson counties) Figure 1: Loctions of Plmer mrnth ccessions collected during fll of 2005 used in the present study. Plmer mrnth ccessions were grouped bsed on confirmed resistnce to glyphoste (GR), resistnce to cetolctte synthse inhibitors (ALSR), nd susceptibility to both of these mechnisms of ction (ALSS/GS). Dt were subjected to nlysis of vrince considering the fctoril rrngement of three levels of ccession grouping nd four levels of drought stress durtion. Due to lck of interction, the dt were pooled over the two runs of the experiment. Mens of significnt min effects nd interctions were seprted using Fisher s protected LSD test t P Percentreductionin height nd dry weight were lso regressed ginst durtion of stress in SigmPlot 12.0 using qudrtic eqution y = x 2 +bx+c,wherey = percent reduction in height or dry weight,, b,ndc re constnts, nd x = durtion of drought stress in dys. Furthermore, liner regression ws fit to the percent reduction in height of 15 Plmer mrnth ccessions

4 4 Agronomy nd 3 ccession groups over vrying periods of drought stress forllthereplictionsinsas.theslopesthusobtinedwere compred using nlysis of vrince. In seprte experiment crried out longside the previously described one, photosynthetic ssimiltion (μmol CO 2 m 2 s 1 ) nd stomtl conductnce (μmol H 2 Om 2 s 1 ) were determined for one ALSS/GS ccession following 3, 5, 7, nd 9 dys of drought stress initited 15 DAE. These mesurements were recorded to document the physiologicl stress ssocited with the drought stress regimes nd to relte the physiologicl stress with ssocited height nd dry weight reductions in order to provide n estimte of the degree of drought stress imposed by tretments. Recording these vlues ws not possible for ll combintions of Plmer mrnth ccessions nd drought tretments. The fourth fully expnded lef from the top of ech plnt ws used for mesurements which were mde between 10 : 00 nd 14 : 00 hours EST using portble photosynthesis system (LI-6400 Portble Photosynthesis System, LI-COR Biosciences, P.O. Box 4425, Lincoln, NE, USA) nd lef chmber fluorometer equipped with version 6.2 softwre. Ech mesurement ws tken over 30-second period of time. There were five replicte plnts per tretment nd six mesurements were tken per plnt. The mesurements were verged over plnts. The experiment ws repeted immeditely fter completion of the first run. Dt for photosynthetic ssimiltion nd stomtl conductnce were subjected to nlysis of vrince, nd mens of significnt min effects were seprted using Fisher s protected LSD test t P Percentreductionin photosynthetic ssimiltion nd stomtl conductnce were regressed ginst durtion of stress in SigmPlot 12.0 using liner eqution y = bx+c,wherey =percentreductionin photosynthetic ssimiltion or stomtl conductnce, b nd c re constnts, nd x = durtion of drought stress in dys. 3. Results nd Discussion The min effects of ccession nd durtion of drought stress nd the interction of these fctors were significnt for percent reduction in height nd dry weight when ll 15 Plmer mrnth ccessions were nlyzed individully (Tble 2). Considerble differences in response to vrying periods of drought existed mong ccessions (Tble 3). For exmple, with 9 dys of drought stress, height reduction rnged from 8% to 25% while dry weight reduction rnged from 9% to 47%. With ll of the ALSS/GS ccessions, dry weight reduction ws similr regrdless of durtion of drought except Mrtin nd Edgecombe t 9 dys of drought stress. With the exception of the Hoke GR ccession, ll herbicide-resistnt ccessions hd similr nd smll dry weight reductions following exposure to 3 to 5 dys of drought. Reduction in dry weight incresed with 7 to 9 dys of drought for ll GR ccessions except for GR ccession from Wyne County. This suggested tht the herbicide-resistnt ccessions could better withstnd short-term drought stress thn the susceptible ccessions. When pooled over the durtion of drought stress, reduction in height nd dry weight for the Plmer mrnth ccessions t hrvest rnged from 5% to 15% nd 12% to 29%, respectively (Tble 4). The slopes of regression lines for the Tble 2: P>Ffor percent reduction in height nd dry weight of 15 Plmer mrnth ccessions 30 dys fter emergence. Tretment fctors Height P vlue Individul Plmer mrnth ccessions b Dry weight Accession < < Drought stress < < Accession drought stress < Coefficient of vrition (%) Plmer mrnth ccessions grouped by response to herbicide c Accession group Drought stress < < Accession group drought stress Coefficient of vrition (%) Dt re pooled over experiments. b Consists of fifteen Plmer mrnth ccessions. c Consists of group of 5 cetolctte synthse-inhibitor susceptible nd glyphoste-susceptible (ALSS/GS), group of 5 cetolctte synthse inhibitorresistnt (ALSR), nd group of 5 glyphoste-resistnt (GR) Plmer mrnth ccessions. percentge of reduction in height were similr for 13 of the 15 Plmer mrnth ccessions. This suggested similr rte of reduction in height for mjority of the ccessions of Plmer mrnth used in this study. Differences mong ccessions for these vribles my well be expected given the degree of phenotypic vrition often observed mong Plmer mrnth biotypes [39]. The primry objective of this experiment ws to determine the reltionship between herbicide resistnce expression nd response to drought stress. When ccessions were segregted into three groups ccording to their resistnce chrcteristics (GR, ALSR, nd ALSS/GS), the interction of ccession group by durtion of drought stress ws not significnt for height nd dry weight reductions (Tble 1). However, the min effect of ccession grouping ws significnt for percent reduction in height (P = ) nd dry weight (P = ). Incresing durtion of drought stress resulted in greter reductions in Plmer mrnth height nd dry weight. Additionlly, drought stress hd greter effect on dry weight thn on height (Figure2). Averged over resistnce groupings, drought stress for 3, 5, 7, nd 9 dys cused 6%, 8%, 12%, nd 18% reduction in height nd 13%, 16%, 22%, nd 32% reduction in dry weight, respectively Differences mong ccession groupings were noted for reduction in height nd dry weight (Tble 5). Both height nd dry weight of the ALSR nd GR groups were less ffected by drought stress thn the ALSS/GS group. Percentge of dry weight reduction for GR nd ALSR groups ws 20% compred with 23% for the ALSS/GS group. Percentge of reduction in height of ALSR nd GR ws 10% compred with 12% for the ALSS/GS group. Lck of significnt differences in the slopes of reduction in height mong the three ccession groups suggested similr rtes of height reduction over durtion of drought stress.

5 Agronomy 5 Tble 3: Percent reduction in height nd dry weight of 15 Plmer mrnth ccessions t hrvest (30 DAE) s ffected by the interction of ccession nd durtion of drought stress,b. Height Dry weight Accession County Resistnce Durtion of drought stress (dys) Durtion of drought stress (dys) % 1 Mrtin ALSS/GS 3 op 7 k p 12 g l 20 b e 15 g p 21 f k 24 d h 16 g o 2 Hrnett ALSS/GS 7 k p 14 e j 20 b e 16 c h 18 f n 23 d i 22 e j 22 e j 3 Lenoir ALSS/GS 4 n p 9 i o 13 f k 20 b e 14 h q 21 f k 17 g o 21 f k 4 Mrtin ALSS/GS 6 l p 4 n p 10 h n 19 b f 17 g o 15 g p 15 g p 12 j q 5 Edgecombe ALSS/GS 10 h n 8 j p 9 i o 24 b 24 d h 17 g o 16 g o 9 m q 6 Wyne GR 11 g m 8 j p 11 g m 22 bc 15 g p 11 k q 15 g p 13 i q 7 Hoke GR 7 k p 16 c h 11 g m 22 c 21 f k 23 d i 28 c f 47 8 Robeson GR 3 op 5 m p 11 g m 16 c h 8 n q 10 l q 28 c f 33 b d 9 Cumberlnd GR 3 op 5 m p 5 m p 8 j p 5 pq 10 l q 13 i q 21 f k 10 Scotlnd GR 5 m p 2 p 15 d i 17 c g 11 k q 4 q 25 d g 33 b d 11 Robeson ALSR 9 i o 8 j p 12 g l 21 b d 16 g o 16 g o 19 f m 38 c 12 Johnston ALSR 8 j p 7 k p 11 g m 25 b 5 pq 5 pq 16 g o 38 c 13 Robeson ALSR 5 m p 7 k p 8 j p 17 c g 15 g p 19 f m 32 b e 39 b 14 Smpson ALSR 8 j p 11 g m 6 l p 17 c g 15 g p 14 h q 10 l q 24 d h 15 Edgecombe ALSR 4 n p 9 i o 14 e j 14 e j 13 i q 20 f l 16 g o 24 d h Dt re pooled over experiments. b Mens within prmeter followed by the sme letter re not significntly different ccording to Fisher s Protected LSD test t P Tble 4: Percent reduction in height nd dry weight 30 dys fter emergence nd the slopes of regression lines for percentge of reduction in height of 15 Plmer mrnth ccessions. Accession County Resistnce b Height Dry weight Slope % reduction plnt 1 % reduction dy 1 1 Mrtin ALSS/GS 11 b d 27 b Hrnett ALSS/GS b 4.5 bc 3 Lenoir ALSS/GS 12 d 21 d 3.2 c 4 Mrtin ALSS/GS 10 cd 16 e g 3.5 c 5 Edgecombe ALSS/GS 12 d 22 cd 3.3 c 6 Wyne GR 13 c 14fg 4.0bc 7 Hoke GR 14b bc 8 Robeson GR 9 d 20 de 3.4 c 9 Cumberlnd GR 5e 12g 3.2c 10 Scotlnd GR 10 cd 18 d f 3.3 c 11 Robeson ALSR 13 c 22 cd 4.7 bc 12 Johnston ALSR 13 c 16 e g 5.1 b 13 Robeson ALSR 9 d 26 bc 3.6 bc 14 Smpson ALSR 11 b d 16 e g 3.3 c 15 Edgecombe ALSR 11 b d 18 d f 3.8 bc Dt re pooled over experiments nd durtion of drought stress. Mens within prmeter followed by the sme letter re not significntly different ccording to Fisher s Protected LSD test t P b Abbrevitions: ALSS/GS: cetolctte synthse-inhibitor susceptible nd glyphoste-susceptible ccession; ALSR: cetolctte synthse inhibitor-resistnt ccession; GR: glyphoste-resistnt ccession. Photosynthesis rte nd stomtl conductnce were mesured in seprte experiment conducted longside the bove mentioned study. These mesurements were tken on one ALSS/GS ccession of Plmer mrnth t 3, 5, 7, nd 9 dys fter initition of drought stress to quntify the effect of drought stress on physiologicl processes tht could result in reduced growth. Although more informtive, recording these vlues for ll 15 ccessions would hve been difficult under the series of drought tretments. Drought stress of 3 dys or more reduced photosynthesis nd stomtl conductnce significntly (Tble 6). However, visully wilting ws observed only with5ormoredysofdroughtstress.similrtotheresultsfor

6 6 Agronomy Percent reduction in height nd dry weight Durtion of drought stress (dys) Height y = 0.25x 2 1x+6.7; r 2 = 0.99 Dry weight y = 0.44x 2 2.1x ; r 2 = 0.99 Figure 2: Percent reduction in plnt height nd dry weight s ffected by the durtion of drought stress in dys. Dt pooled over ccessions. Points re mens ± S.E. Tble 5: Percent reduction in height nd dry weight 30 dys fter emergence nd slopes of regression lines for percentge of reduction in height of 15 Plmer mrnth ccessions when ccessions were grouped s cetolctte synthse inhibitor-susceptible nd glyphoste-susceptible (ALSS/GS), cetolctte synthse inhibitorresistnt (ALSR), nd glyphoste-resistnt (GR). Accession grouping Height Dry weight Slope % reduction plnt 1 % reduction dy 1 ALSS/GS GR 10 b 20 b 4.5 ALSR 10 b 20 b 4.1 Dt re pooled over experiments nd durtion of drought stress. Mens within prmeter followed by the sme letter re not significntly different ccording to Fisher s protected LSD test t P height nd dry weight reductions (Figure 2), incresing durtion of drought stress progressively reduced photosynthesis rte nd stomtl conductnce (Figure 3). Drought stress for 3, 5, 7, nd 9 dys cused 14%, 37%, 60%, nd 83% reduction in photosynthesis nd 10%, 40%, 70%, nd 99% reduction in stomtl conductnce, respectively. Reductions in rte of photosynthesis nd stomtl conductnce ppered to be closely correlted with reductions in height nd dry weight. 4. Conclusions The number of ccessions in ech resistnce grouping ws limitedtofiveinthisstudy.amorecomprehensivescreening of lrger number of ccessions with drought stress extended beyond 9 dys would be more informtive in defining reltionships between herbicide resistnce nd bility to recover from drought stress. However, no other reserch is published in the peer-reviewed literture ddressing this topic for Plmer mrnth. This experiment ws conducted during the first 30 dys fter weed emergence when weeds, including Plmer mrnth, hve the gretest potentil to Tble 6: P>Ffor influence of drought stress on photosynthesis nd stomtl conductnce of n cetolctte synthse inhibitor-susceptible nd glyphoste-susceptible Plmer mrnth ccession. Durtion of stress Dys Photosynthesis μmol CO 2 m 2 s 1 Stomtl conductnce mol H 2 Om 2 s 1 P vlue < Dt re pooled over experiments. impct growth, development, nd yield of crops [19 21, 40 43]. Differences observed with ccession groupings in this experiment, lthough smll, suggest possible reltionship between herbicide resistnce nd greter bility of Plmer mrnth to withstnd drought stress. This could result in competitive dvntge for Plmer mrnth resistnt to ALS inhibitors nd glyphoste. Previous reserch with limited ccessions of glyphoste resistnt nd glyphoste susceptible indictesbothdvntgesnddisdvntgesofglyphoste resistnce in terms of interference nd response to herbicides other thn glyphoste [44, 45]. Future reserch with much lrger pool of ccessions from more diverse geogrphicl re would provide clerer understnding of the impct of glyphoste resistnce on biology of Plmer mrnth. Use of sister lines from common prent popultion to compre potentil spects influencing fitness would lso be informtive. Conflict of Interests None of the uthors hs conflict of interests in terms of the products mentioned in the pper.

7 Agronomy Percent reduction in photosynthesis nd stomtl conductnce Durtion of drought stress (dys) Photosynthesis y = x; r 2 = 0.99 Stomtl conductnce y = x; r 2 = 0.99 Figure 3: Percent reduction in photosynthesis nd stomtl conductnce of n cetolctte synthse-sensitive nd glyphoste-sensitive (ALSS/GS) ccession s ffected by durtion of drought stress in dys. Points re mens ± S.E. Acknowledgments Syngent Crop Protection nd Monsnto Compny provided prtil finncil support. Dr. Fitz Booker, USDA-ARS Plnt Physiologist nd Professor of Crop Science, North Crolin StteUniversity,USA,providedtheLI-6400PortblePhotosynthesis System nd vluble suggestions for the experiment. Erin Silv, Biologicl Science Technicin (USDA-ARS), ssisted with the instrumenttion. References [1] J. K. A. Blesdle, Studies on plnt competition, in The Biology of Weeds,J.L.Hrper,Ed.,pp ,BlckwellScientificPublishers, Oxford, UK, [2] A. G. Ogg Jr., R. H. Stephens, nd D. R. Gely, Interference between myweed chmomile (Anthemis cotul) ndpe (Pisum stivum) is ffected by form of interference nd soil wter regime, Weed Science, vol. 42, no. 4, pp , [3] D. T. Ptterson, Effects of environmentl stress on weed/crop interctions, Weed Science,vol.43, no.3,pp , [4] M.S.Riffle,D.S.Murry,J.F.Stone,ndD.L.Weeks, Soil-wter reltions nd interference between devil s-clw (Proboscide louisinic) ndcotton(gossypium hirsutum), Weed Science, vol.38,no.1,pp.39 44,1990. [5] A.F.WiesendC.W.VnDiver, Soilmoistureeffectsoncompetitive bility of weeds, Weed Science,vol.18,no.4,pp , [6] R. L. Zimdhl, Weed-Crop Competition A Review, Blckwell Publishers, Ames, Iow, USA, 2nd edition, [7] R.J.Aldrich,Weed-Crop Ecology: Principles in Weed Mngement, Breton Publishers, North Scitute, Mss, USA, [8] R.D.Geddes,H.D.Scott,ndL.R.Oliver, Growthndwter use by common cocklebur (Xnthium pensylvnicum)ndsoyben (Glycine mx) under field conditions, Weed Science, vol. 27,no.2,pp ,1979. [9] J. L. Hrper, Popultion Biology of Plnts, Acdemic Press, London, UK, [10] D. T. Ptterson, Responses of soyben (Glycine mx) ndthree C 4 grss weeds to CO 2 enrichment during drought, Weed Science,vol.34,no.2,pp ,1986. [11] D. T. Ptterson nd E. P. Flint, Comprtive wter reltions, photosynthesis, nd growth of soyben (Glycine mx)ndseven ssocited weeds, Weed Science,vol.31,no.3,pp ,1983. [12] F. Liu nd H. Stützel, Biomss prtitioning, specific lef re, nd wter use efficiency of vegetble mrnth (Amrnthus spp.) in response to drought stress, Scienti Horticulture,vol. 102,no.1,pp.15 27,2004. [13] A. Blum, Crop responses to drought nd the interprettion of dpttion, Plnt Growth Regultion,vol.20,no.2,pp , [14] F. Liu nd H. Stützel, Lef expnsion, stomtl conductnce, nd trnspirtion of vegetble mrnth (Amrnthus sp.) in response to soil drying, Journl of the Americn Society for Horticulturl Science,vol.127,no.5,pp ,2002. [15] F.L,Young,D.L.Wyse,ndR.J.Jones, Effectofirrigtionon quckgrss (Agropyron repens)interferenceinsoyben(glycine mx), Weed Science,vol.31,no.5,pp ,1983. [16] F. L. Young, D. L. Wyse, nd R. J. Jones, Quckgrss (Agropyron repens) interferenceincorn(ze mys), Weed Science, vol.32, no. 2, pp , [17] D. A. Mortensen nd H. D. Coble, The influence of soil wter content on common cocklebur (Xnthium strumrium) interference in soybens (Glycine mx), Weed Science,vol.37,no.1, pp.76 83,1989. [18] S. E. Wever, Fctors ffecting threshold levels nd seed production of jimsonweed (Dtur strmonium L.) in soybens (Glycine mx (L.) Merr.), Weed Reserch, vol. 26, no. 3, pp , [19] H. D. Coble, F. M. Willims, nd R. L. Ritter, Common rgweed (Ambrosi rtemisiifoli L.) interference in soybens (Glycine mx), Weed Science,vol.29,no.3,pp ,1981. [20] L. A. Jckson, G. Kpust, nd D. J. Schutte Mson, Effect of durtion nd type of nturl weed infesttions on soyben yield, Agronomy Journl,vol.77,no.5,pp ,1985.

8 8 Agronomy [21] S. K. Hrrison, C. S. Willims, nd L. M. Wx, Interference nd control of gint foxtil (Setri fberi) in soybens(glycine mx), Weed Science,vol.33,no.2,pp ,1985. [22] T. M. Webster, Weed survey southern sttes, Proceedings Southern Weed Science Society,vol.62,pp ,2009. [23] C. C. Blck, T. M. Chen, nd R. H. Brown, Biochemicl bsis for plnt competition, Weed Science,vol.17,no.3,pp , [24] A. S. Culpepper, T. M. Webster, L. M. Sosnoskie, nd A. C. York, Glyphoste-resistnt Plmer mrnth in the United Sttes, in Glyphoste Resistnce in Crops nd Weeds: History, Development, nd Mngement, V.K.Nndul,Ed.,pp ,John Wiley & Sons, Hoboken, NJ, USA, [25] I. Hep, The Interntionl Survey of Herbicide Resistnt Weeds, 2012, [26] M. J. Hork nd T. M. Loughin, Growth nlysis of four Amrnthus species, Weed Science,vol.48,no.3,pp ,2000. [27] S.Rdosevich,J.Holt,ndC.Ghers,Weed Ecology: Implictions for Mngement, John Wiley & Sons, New York, NY, USA, 2nd edition, [28] C. Preston, A. M. Wkelin, F. C. Dolmn, Y. Bostmm, nd P. Boutslis, A decde of glyphoste-resistnt lolium round the world: mechnisms, genes, fitness, nd gronomic mngement, Weed Science,vol.57, no.4,pp , [29] M. A. Jsieniuk, A. L. Brûl-Bbel, nd I. N. Morrison, The evolution nd genetics of herbicide resistnce in weeds, Weed Science,vol.44,no.1,pp ,1996. [30] C. Preston nd S. B. Powles, Evolution of herbicide resistnce in weeds: initil frequency of trget site-bsed resistnce to cetolctte synthse-inhibiting herbicides in Lolium rigidum, Heredity, vol. 88, no. 1, pp. 8 13, [31] B. P. Pedersen, P. Neve, C. Andresen, nd S. B. Powles, Ecologicl fitness of glyphoste-resistnt Lolium rigidum popultion: growth nd seed production long competition grdient, Bsic nd Applied Ecology,vol.8,no.3,pp ,2007. [32] C. Preston nd A. M. Wkelin, Resistnce to glyphoste from ltered herbicide trnsloction ptterns, Pest Mngement Science,vol.64,no.4,pp ,2008. [33] A. M. Wkelin nd C. Preston, The cost of glyphoste resistnce: is there fitness penlty ssocited with glyphoste resistnce in nnul ryegrss, in Mnging Weeds in Chnging Climte, C.Preston,J.H.Wtts,ndN.D.Crossmn,Eds.,pp , Weed Mngement Society of South Austrli, Torrens Prk, South Austrli, [34] N. Jordn, Effects of the trizine-resistnce muttion on fitness in Amrnthus hybridus (smooth pigweed), Journl of Applied Ecology,vol.33,no.1,pp ,1996. [35] N. Jordn, Fitness effects of the trizine resistnce muttion in Amrnthus hybridus: reltive fitness in mize nd soyben crops, Weed Reserch, vol. 39, no. 6, pp , [36] R. Cousens nd M. Mortimer, Dynmics of Weed Popultions, Cmbridge University Press, West Nyck, NY, USA, [37] J. Ehleringer, Ecophysiology of Amrnthus plmeri,sonorn desert summer nnul, Oecologi, vol. 57, no. 1-2, pp , [38] J. R. Whitker, Distribution, biology, nd mngement of glyphoste-resistnt Plmer mrnth in North Crolin [Ph.D. disserttion], North Crolin Stte University, Rleigh, NC, USA, [39] T. E. Klingmn nd L. R. Oliver, Plmer mrnth (Amrnthus plmeri) interferenceinsoybens(glycine mx), Weed Science,vol.42,no.4,pp ,1994. [40] B. Bukun, Criticl periods for weed control in cotton in Turkey, Weed Reserch,vol.44,no.5,pp ,2004. [41] A. S. Culpepper nd A. C. York, Weed mngement in glyphoste-tolernt cotton, JournlofCottonScience,vol.2,no. 4, pp , [42] B.J.Fst,S.W.Murdock,R.L.Frris,J.B.Willis,ndD.S.Murry, Criticl timing of plmer mrnth (Amrnthus plmeri) removl in second-genertion glyphoste-resistnt cotton, Journl of Cotton Science,vol.13,no.1,pp.32 36,2009. [43] D. Ppmichil, I. Eleftherohorinos, R. Froud-Willims, nd F. Grvnis, Criticl periods of weed competition in cotton in Greece, Phytoprsitic, vol. 30, no. 1, pp , [44] A. Chndi, D. L. Jordn, A. C. York et l., Interference of selected Plmer mrnth (Amrnthus plmeri) popultions in soyben (Glycine mx), InterntionlJournl of Agronomy,vol. 2012, Article ID , 7 pges, [45] A. Chndi, D. L. Jordn, A. C. York et l., Interference nd control of glyphoste-resistnt nd -susceptible Plmer mrnth (Amrnthus plmeri) popultions under greenhouse conditions, Weed Science, Article ID WS-D , 2013.

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