temperature effects on germination and growth of redroot pigweed (amaranthus retroflexus), palmer...

Guo and Al-Khatib: Temperature effect on germination and growth • 869

Weed Science, 51:869–875. 2003

Temperature effects on germination and growth of redrootpigweed (Amaranthus retroflexus), Palmer amaranth(A. palmeri), and common waterhemp (A. rudis)

Peiguo GuoDepartment of Agronomy, Kansas State University,Manhattan, KS 66506

Kassim Al-KhatibCorresponding author. Department of Agronomy,Kansas State University, Manhattan, KS 66506;[email protected]

Experiments were conducted to determine the effects of temperature on seed ger-mination and growth of redroot pigweed, Palmer amaranth, and common water-hemp. At 15/10 C day and night temperature, respectively, no seed germination wasobserved in any species. Seed germination increased gradually as temperature in-creased. Germination peaked at 25/20 C in common waterhemp and at 35/30 Cin redroot pigweed and Palmer amaranth. Seed germination of all three speciesdeclined when temperatures increased above 35/30 C. All three species producedless biomass at 15/10 C than at 25/20 C and 35/25 C. Redroot pigweed andcommon waterhemp biomass were similar at 15/10 C and higher than that of Palmeramaranth. However, Palmer amaranth produced more biomass than redroot pigweedand common waterhemp at 25/20 and 35/30 C. At 45/40 C, redroot pigweed,common waterhemp, and Palmer amaranth plants died 8, 9, and 25 d after initiationof heat treatment, respectively. The largest root volume among the three species wasin Palmer amaranth grown at 35/30 C, whereas the smallest root volume was pro-duced by Palmer amaranth grown at 15/10 C. Potential quantum efficiency (Fv/Fmax) of Palmer amaranth was higher than that of redroot pigweed and commonwaterhemp at higher temperature. The greater growth of Palmer amaranth at highertemperatures may be attributed in part to its extensive root growth and greaterthermostability of its photosynthetic apparatus.

Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; Palmeramaranth, A. palmeri S. Wats. AMAPA; redroot pigweed, A. retroflexus L. AMARE.

Key words: Temperature, chlorophyll content, chlorophyll fluorescence, germi-nation, heat resistance, root activity, Rubisco.

Amaranthus species are among the most troublesomeweeds in many cropping systems in the United States(Bridges 1992; Holm et al. 1977). There are at least 10Amaranthus species that cause serious problems in the Mid-west (Wax 1995; Wetzel et al. 1999). In Kansas, commonwaterhemp, Palmer amaranth, and redroot pigweed are spe-cies prevalent in corn (Zea mays), soybean (Glycine max),sorghum (Sorghum bicolor), and sunflower (Helianthus an-nuus) (Barkley 1986; Mayo et al. 1995). Palmer amaranthoccurs mainly in the southern half of the United States,common waterhemp occurs mainly in the eastern half, andredroot pigweed occurs throughout the United States andCanada (Mayo et al. 1995; Weaver and McWilliams 1980).Mixed populations of these three species are common inplaces where their distribution overlaps, such as Kansas.

Aggressive growth habit and prolific seed production al-low Amaranthus species to compete strongly with crops forlight, water, and nutrients (Barkley 1986; Knezevic et al.1997; Murphy et al. 1996). They reduce yield, quality, andalso harvest efficiency of cultivated crops (Klingaman andOliver 1994; Knezevic et al. 1997; Rowland et al. 1999).In addition, Amaranthus species have been shown to haveallelopathic chemicals that reduce seedling vigor of severalcrops and weeds (Menges 1987, 1988).

Preplant- and preemergence-applied herbicides, such aspendimethalin, metolachlor, acetochlor, dimethenamid, tri-fluralin, atrazine, isoxaflutole, imazaquin, and imazethapyr,

are used extensively to control Amaranthus species in cornand soybean (Mayo 1995; Regehr et al. 2002). However,lack of soil moisture at planting time may reduce the efficacyof soil-applied herbicide. Therefore, postemergence (POST)herbicides may be needed to provide greater weed controlunder dry conditions. In addition, POST-applied herbicideshave an advantage over soil-applied herbicides because theycan be applied after weeds have emerged and been identi-fied, thereby allowing the herbicide to target the weed spe-cies present. However, differences in weed emergence andgrowth rate may influence POST-herbicide efficacy (Coetzeret al. 2002). Late-emerging weeds may escape injury whennonresidual herbicides are applied. Furthermore, POST her-bicides are more effective on smaller weeds, and increasedherbicide rates may be required to control large weeds(Coetzer et al. 2002; Lee and Oliver 1982). Therefore,POST-herbicide application on a mixture of weed speciesthat differ in sizes may result in inconsistent efficacy.

Common waterhemp, redroot pigweed, and Palmer am-aranth growth and development may differ under variousenvironmental conditions. Temperature is prominent amongthe cardinal ecological factors that determine species growthand productivity. Habitats where Amaranthus species growdiffer dramatically, and temperature ranges of 20 to 35 Care not uncommon. Wright et al. (1999) showed that Palm-er amaranth responds negatively to low temperature but pos-itively to high temperature. Furthermore, McLanchlan et al.

870 • Weed Science 51, November–December 2003

(1993) observed a linear increase in leaf-appearance ratewith increased temperature. However, little is known aboutthe effects of temperature on physiology and relative growthrate of redroot pigweed, common waterhemp, and Palmeramaranth.

The ability to predict Amaranthus species seed germina-tion under different environmental conditions is essential forpredicting optimum timing for POST-herbicide application.Unfortunately, this prediction is difficult because of differ-ences in seed germination within and between weed species(Ghorbani et al. 1999; Weaver et al. 1987). Other studieshave reported the effects of environmental conditions ongermination and emergence of redroot pigweed (Forcella etal. 1997; Frost and Cavers 1974; Gallagher and Cardina1998; Oryokot et al. 1997; Weaver 1984), green pigweed(Amaranthus powellii) (Frost and Cavers 1974; Oryokot etal. 1997), Palmer amaranth (Wright et al. 1999), andsmooth pigweed (Amaranthus hybridus) (Gallagher and Car-dina 1998). Maximum Amaranthus species seed germinationis relatively temperature insensitive, but germination rateshave precise temperature optima (Weaver and Thomas1986). Empirical observations indicated that green pigweedgerminated at a lower temperature than redroot pigweed,but the germination rate of redroot pigweed was much fasteras temperature increased (Oryokot et al. 1997). Ghorbaniet al. (1999) found that the minimum temperature for red-root pigweed germination was greater than 5 C, whereasmaximum germination occurred between 35 and 40 C.

The objective of this research was to study the effects oftemperature on seed germination, photosynthetic activity,and shoot and root growth of redroot pigweed, Palmer am-aranth, and common waterhemp.

Materials and Methods

Seed Germination Study

Seeds of redroot pigweed, Palmer amaranth, and commonwaterhemp were collected from Kansas State University Ash-land Botton Research field near Manhattan, KS, in 1999.Seeds were sterilized with 0.5% (v/v) sodium hypochloritesolution for 10 min, and then seeds were rinsed three timeswith distilled water. Thirty seeds were germinated on moistfilter paper placed in 9-cm petri dishes. Day and night (d/n) temperature treatments were 15/10, 25/20, 35/30, 45/40, and 50/45 C with 14:10 h d/n period, respectively.Seeds were considered germinated when length of theemerged radicle exceeded 1 mm. Germinated seeds werecounted and removed daily for 10 d.

Growth Study

Redroot pigweed, Palmer amaranth, and common water-hemp were grown in 15-cm-diam containers filled with 500g of a 1:1 (v/v) mixture of soil and sand. The soil mix wasa Morril loam (fine loamy-mixed mesic typic Argiudolls),with pH 7.0 and 1.7% organic matter. Plants were fertilizedweekly with a commercial fertilizer1 containing 3 mg L21

N, 2.5 mg L21 P, and 2.2 mg L21 K. Seedlings were thinnedto one plant per pot. Greenhouse conditions were 23/20 62 C d/n temperatures and 14/10 h d/n periods. The relativehumidity (RH) was 50 6 5% during the day and 60 6 5%

during the night. The supplemental light intensity was 80mmol m22 s21 photosynthetic photon flux (PPF).

One week after seedling emergence, the containers ofeach species were divided into four groups. These groupswere randomly placed in separate growth chambers with d/n temperature regimes of 15/10, 25/20, 35/30, or 45/40 C.RH was 50 6 10%, and light intensity was 550 mmol m22

s21 PPF, with 14 h of daylight.Chlorophyll fluorescence was measured 3 wk after tem-

perature treatment (WAT) on the abaxial surface of the sec-ond leaf of each plant. Initial (F0) and maximum (Fmax)fluorescence were measured with a Plant Efficiency Analyz-er.2 Before fluorescence measurements, the leaf spot that wasused for fluorescence measurements was held in the dark for10 min with leaf clips. Variable fluorescence (Fv) was cal-culated as Fv 5 Fmax 2 F0 (Krause and Weis 1984), andpotential quantum efficiency was calculated as FV/Fmax(Hipkins and Baker 1987).

Rubisco activity was measured as described by Zhu(1990). Leaves were cut into 0.5-cm sections 3 WAT. Onegram of sectioned leaves were homogenized in solution con-taining 40 mM Tris–Cl buffer (pH 7.6), 5 mM 2-mercap-toethanol, 10 mM MgCl2, and 0.25 mM ethylenediamine-tetraacetic acid (EDTA) in chilled mortar with pestle. Thehomogenate was filtered through four layers of cheeseclothand centrifuged for 15 min at 15,000 3 g. The supernatantfraction that included the crude Rubisco extract was storedat 4 C until use. Rubisco carboxylation was measured usinga coupled spectrophotometric assay adapted from Lilley andWalker (1974). Rubisco (100 ml crude extract) was equili-brated at 30 C for 10 min in an assay buffer containing100 mM Tris–Cl (pH 7.8), 12 mM MgCl2, 0.4 mMEDTA, 13 mM NaHCO3, 3.3 mM phosphocreatine, 3.3mM adenosine triphosphate, 0.33 mM nicotinamide ade-nine dinucleotide (NADH), 3.3 mM dithiothreitol, 10 unitsml21 3-phosphoglycerate kinase, 10 units ml21 glyceralde-hyde-3-phosphate dehydrogenase, and 10 units ml21 crea-tine phosphokinase. The reaction was initiated with 0.83mM ribulose bisphosphate. Reduction of NADH was mon-itored at 340 nm with a Hitachi U-1100 spectrophotome-ter.3 Protein content was determined as described by Brad-ford (1976), and chlorophyll content was estimated accord-ing to Arnon (1949).

Four WAT, root activity was measured as described byZhang et al. (1980), and root volume was measured usingthe water replacement method. Root activity was measuredby submerging roots in a solution containing 0.2 mmol L21

methyl blue for 3 min, and then solution was brought tothe original volume. Roots were submerged again in a newsolution of methyl blue for 1.5 min, then roots were re-moved, and solution was brought to the original volume.The root–solution ratio was 1:10 (v/v). One milliliter ofeach solution was added to the tubes containing 9 ml ofdistilled water, and absorbance was determined at 665 nm.3The standard curve was established with different concen-trations of methyl blue (0.001 to 0.006 mg ml21) and wasused to calculate the concentration of methyl blue in thetwo solutions. The reduction in methyl blue concentrationin the first solution per unit root volume indicated the totalabsorption, whereas the decline in methyl blue concentra-tion in the second solution per unit root volume represented


FIGURE 1. Percent seed germination of redroot pigweed, Palmer amaranth,and common waterhemp as affected by temperature.

FIGURE 2. Seed germination rate of redroot pigweed, Palmer amaranth, andcommon waterhemp as affected by temperature.

active absorption. Percent active absorption was determinedas:

% active absorption of roots

5 (root active absorption/root total absorption)100

Plant height and primary stem diameter at soil level weremeasured weekly. Plants were harvested 4 WAT, and totaldry biomass was determined.

Heat Resistance Study

Redroot pigweed, Palmer amaranth, and common water-hemp seedlings were grown as described earlier. Three weeksafter emergence, 20 pots of each species were placed in agrowth chamber with d/n temperature of 45/40 C for 30d. RH was 50 6 10%, and light intensity was 550 mmolm22 s21 PPF with 14 h of daylight. Heat resistance wasdetermined by counting daily the number of plants dead ineach temperature treatment. Percent plant survival was cal-culated by dividing the number of surviving plants by thetotal number of plants.

Experimental Designs and Data Analyses

Treatments were arranged in a randomized completeblock design in all experiments. Treatments were replicatedfour times, and the germination study was repeated fivetimes. The growth and heat resistance studies were replicat-ed four times, and experiments were repeated two times.The data were subjected to analysis of variance, and meanswere separated by Fisher’s Protected LSD at the 5% level.Nonlinear regression was used to analyze the heat resistancestudy (Gomez and Gomez 1984).

Results and Discussion

Seed Germination Study

Seed germination of redroot pigweed, Palmer amaranth,and common waterhemp differed in its response to temper-atures (Figure 1). At 15/10 C, no seed germination wasobserved in any of the Amaranthus species, indicating thatgermination is very susceptible to low temperatures. How-

ever, seed germination increased gradually as temperatureincreased and peaked at 25/20 to 35/30 C for commonwaterhemp and at 35/30 C for redroot pigweed and Palmeramaranth. In general, seed germination of all three speciesdeclined at temperatures above 35/30 C. However, the re-duction was more severe in common waterhemp than inredroot pigweed and Palmer amaranth. Keeley et al. (1987)reported that germination rate of Palmer amaranth seed waslow at 16/10 C and gradually increased as temperature in-creased, with maximum germination at 38/32 C. At 50/45C, no seed germination was observed in any of the Ama-ranthus species. These results are in agreement with earlierresults that showed that seed germination of Amaranthusspecies is inhibited by high temperatures (Keeley et al. 1987;Kigel 1994; Wright et al. 1999). In general, seed germina-tion of redroot pigweed was higher across temperatures thanthat of Palmer amaranth and common waterhemp. At 25/20 and 35/30 C, Palmer amaranth seed germination wasthe least among the three species. These results are not sur-prising because earlier reports showed wider optimum ger-mination temperature for redroot pigweed (Kigel 1994;Schonbeck and Egley 1980).

Redroot pigweed and Palmer amaranth had higher ger-mination rates at 35/30 C than at 25/20 or 45/40 C. How-ever, common waterhemp germination rates were equal at


FIGURE 3. Total plant biomass of redroot pigweed, Palmer amaranth, andcommon waterhemp at three temperature regimes.

25/20 and 35/30 C and reduced at 45/40 C (Figure 2). Inaddition, redroot pigweed germination rates at 35/30 and45/40 C were higher than those of Palmer amaranth andcommon waterhemp. However, at 25/20 C, common wa-terhemp germination rate was the highest among the threespecies, whereas that of Palmer amaranth was the lowest.These results indicate that redroot pigweed seeds have highergermination rate over a wider range of temperatures thanseeds of common waterhemp and Palmer amaranth.

Growth StudyRedroot pigweed, Palmer amaranth, and common water-

hemp growth responded differently to temperatures (Figure3). At 15/10 C, all three species produced less biomass thanat either 25/20 or 35/25 C. Redroot pigweed and commonwaterhemp biomass were similar at 15/10 C, and biomassof both was higher than Palmer amaranth biomass. How-ever, Palmer amaranth produced more biomass than redrootpigweed and common waterhemp at 25/20 and 35/30 C.No differences were observed between total biomass of red-root pigweed and common waterhemp at 25/20 C, whereascommon waterhemp produced more biomass than redrootpigweed at 35/30 C. At 45/40 C, redroot pigweed, commonwaterhemp, and Palmer amaranth plants died 8, 9, and 26d after exposure to heat treatment, respectively (data notshown).

In general, redroot pigweed, Palmer amaranth, and com-mon waterhemp plant height and stem diameter responseto temperatures showed a similar pattern as total biomassresponse to temperatures (data not shown). At 15/10 C,Amaranthus species were stunted, and growth was generallyslow. Common waterhemp, redroot pigweed, and Palmeramaranth plant heights were reduced by 84, 88, and 92%,respectively, for plants grown at 15/10 C compared with25/20 C. Amaranthus species grown at 25/20 C were gen-erally taller than plants grown at 35/30 C. Redroot pigweed,Palmer amaranth, and common waterhemp plant heightswere reduced by 27, 9, and 8%, respectively, for plantsgrown at 35/30 C compared with 25/20 C. At 25/20 and35/30 C, redroot pigweed growth was slower than that ofPalmer amaranth and common waterhemp.

Palmer amaranth root volume was the largest among thethree species when plants were grown at 35/30 C. Thesmallest root volume was produced by Palmer amaranthgrown at 15/10 C (Figure 4). Similar results were obtainedby Wright et al. (1999), who showed that Palmer amaranthroots were susceptible to root zone temperature below 18C. In general, the effects of temperature on root volumeparalleled the effects on shoot growth.

Palmer amaranth and common waterhemp root volumeincreased when temperature increased from 15/10 to 25/20C. However, no further increase in root volume was ob-served when temperature increased from 25/20 to 35/30 C.Root volume of redroot pigweed grown at 35/30 C declineddramatically compared with that of plants grown at 25/20C, but the reduction was less than that reported in othercultivated plant species such as wheat (Triticum aestivum)(Guedira and Paulsen 2002; Kuroyanagi and Paulsen 1988).The lack of severe Amaranthus root injury by higher tem-peratures may be due in part to higher cell membrane sta-bility as shown in other plant species (Al-Khatib and Paulsen1999).

The effect of temperature on root active absorption wasless evident than the effect on root volume (Figure 4). Rootactive absorption of the three species slightly increased whentemperature increased from 15/10 to 25/20 C. However,root active absorption slightly decreased in redroot pigweedwhen temperature increased from 25/20 to 35/30 C. Inaddition, root activity increased in Palmer amaranth whentemperature increased from 25/20 to 35/30 C and was sim-ilar under both temperature regimes in common waterhemp.

Rubisco activity of Amaranthus species was greater at 15/10 C than at 25/20 and 35/30 C (Figure 5). The declinein Rubisco activity at higher temperatures was more severein redroot pigweed and common waterhemp than in Palmeramaranth. At 25/20 C, Rubisco activity of Palmer amaranth,common waterhemp, and redroot pigweed was reduced by2, 19, and 22%, respectively, compared with 15/10 C. At35/30 C, Rubisco activity of Palmer amaranth, redroot pig-weed, and common waterhemp was reduced by 8, 19, and23%, respectively, compared with 15/10 C.


FIGURE 4. Root volume and active root absorption of redroot pigweed,Palmer amaranth, and common waterhemp 4 wk after treatment with threetemperature regimes.

FIGURE 5. Rubisco carboxylase activities, ratio of Fv/Fmax, and total chlo-rophyll content of redroot pigweed, Palmer amaranth, and common wa-terhemp at three temperature regimes.

The lower Rubisco activity at higher temperature coin-cided with the increase in plant growth (Figure 5 vs. Figure3). This response may indicate that a slight decline in Rub-isco activity of Amaranthus plants at higher temperature maynot be a limiting factor for plant growth. This response isnot surprising because Amaranthus species are C4 plants(Weaver and McWilliams 1980). Plants that display C4 me-tabolism release CO2 at high rates in the vicinity of Rubiscoand thereby increase the ratio of Rubisco carboxylation (Lee-good 2002).

Fv/Fmax fluorescence data, a measure of photosystem II(PSII) response to temperature, supported the differentialresponse of the three species to different temperatures. Fv/Fmax ratio of redroot pigweed and common waterhempgrown at 25/20 C was slightly higher than Fv/Fmax of plantsgrown at 15/10 and 35/30 C (Figure 5). Palmer amaranthhad the highest Fv/Fmax ratio at 35/30 C and the lowest Fv/Fmax ratio at 15/10 C. Because the ratio Fv/Fmax indicatesthe potential photochemical yield of PSII and quantum ef-ficiency (Hipkins and Baker 1987), the response of thyla-koid membranes to high temperatures, as shown by Fv/Fmaxratio, may suggest that redroot pigweed and common wa-terhemp possess higher potential photosynthetic activity at25/20 C and Palmer amaranth at 35/30 C.

Total chlorophyll content of redroot pigweed increased astemperature increased from 15/10 to 25/20 C, with no fur-ther increase when temperature increased to 35/30 C (Figure

6). However, total chlorophyll content slightly decreased inPalmer amaranth and common waterhemp when tempera-ture increased above 15/10 C.

Heat ResistanceHigh-temperature injury is a function of heat magnitude

and length of exposure (Levitt 1972). Plant growth maycease at higher temperatures that are not immediately fatal.However, long exposure to these temperatures eventuallymay kill plants. Therefore, the temperature of 45/40 C wasused to evaluate Amaranthus species for heat resistance.Palmer amaranth had higher heat resistance than redroot


FIGURE 6. Heat tolerance of redroot pigweed, Palmer amaranth, and com-mon waterhemp after exposure to 45/40 C.

pigweed and common waterhemp (Figure 6). Common wa-terhemp had a higher heat tolerance than redroot pigweed,surviving 12 d of 45/40 C. The number of days at 45/40C exposure in which 50% of the plants were killed were2.6, 3.7, and 10.7 for redroot pigweed, common water-hemp, and Palmer amaranth, respectively. The high levelsof heat resistance of Amaranthus species in this study maybe partially attributable to thermostability of the thylakoidmembrane (Figure 5). However, accelerated development,enhanced respiration, and numerous indirect effects com-plicate interpretation of high-temperature injury to plants(Al-Khatib and Paulsen 1999; Harding et al. 1990).

This study showed that growth of redroot pigweed wasgreater than that of Palmer amaranth and common water-hemp at the low temperature. However, Palmer amaranthand common waterhemp had higher growth rate than red-root pigweed at higher temperatures. This may be in partbecause of more extensive root growth and higher thermo-stability of the photosynthetic apparatus. Despite the lowergermination efficiency of Palmer amaranth and commonwaterhemp, seedlings could rapidly grow and compete withcrops at higher temperature. Therefore, POST-applied her-bicide timing to mixture of Amaranthus species need to bebased on the growth stage of the species that vigorouslygrow at temperature conditions before herbicide application.

Sources of Materials1 Miracle-Gro soluble fertilizer, Scotts Miracle-Gro Products,

Inc., Consumer Products Division, Port Washington, NY 11050.2 Hansatech Instruments Ltd., Narborough Road, Pentney,

King’s Lynn, Norfolk PE32 1JL, U.K.3 Hitachi U-1100 spectrophotometer, Hitachi Instruments, Inc.,

44 Old Ridgebury Road, Danbury, CT 06810.

Literature CitedAl-Khatib, K. and G. M. Paulsen. 1999. High-temperature effects on pho-

tosynthesis processes in temperate and tropical cereals. Crop Sci. 39:119–125.

Arnon, D. I. 1949. Copper enzymes in isolated chloroplasts. Polyphenoloxidase in Beta vulgaris. Plant Physiol. 24:1–15.

Barkley, T. M., ed. 1986. Flora of the Great Plains. Lawrence, KS: GreatPlains Flora Association, University of Kansas. pp. 179–184.

Bradford, M. 1976. A rapid and sensitive method for the quantitation ofmicrogram quantities of protein utilizing the principle of protein-dyebinding. Anal. Biochem. 72:248–254.

Bridges, D. C. 1992. Crop Losses Due to Weeds in Canada and UnitedStates. Champaign, IL: Weed Science Society of America Weed LossCommittee. 403 p.

Coetzer, E., K. Al-Khatib, and D. E. Peterson. 2002. Glufosinate efficacyon Amaranthus species in glufosinate-resistant soybeans. Weed Tech-nol. 16:326–331.

Forcella, F., R. G. Wilson, and J. Dekker. 1997. Weed seedbank emergenceacross the corn belt. Weed Sci. 56:67–76.

Frost, R. A. and P. B. Cavers. 1974. The ecology of pigweeds (Amaranthus)in Ontario. I. Interspecific and intraspecific variation in seed germi-nation among local collection of A. powellii and A. retroflexus. Can. J.Bot. 53:1276–1284.

Gallagher, R. S. and J. Cardina. 1998. Phytochrome-mediated Amaranthusgermination I: effect of seed burial and germination temperature.Weed Sci. 46:48–52.

Ghorbani, R., W. Seel, and C. Leifert. 1999. Effects of environmental fac-tors on germination and emergence of Amaranthus retroflexus. WeedSci. 47:505–510.

Gomez, K. A. and A. A. Gomez. 1984. Statistical Procedures for Agricul-tural Research. New York: Wiley-Interscience. pp. 407–409.

Guedira, M. and G. M. Paulsen. 2002. Accumulation of starch in wheatgrain under different shoot/root temperatures during maturation.Funct. Plant Biol. 29:495–503.

Harding, S. A., J. A. Guikema, and G. M. Paulsen. 1990. Photosyntheticdecline from high temperature stress during maturation of wheat: I.Interaction with senescence processes. Plant Physiol. 92:648–653.

Hipkins, M. F. and N. R. Baker. 1987. Spectroscopy. Pages 51–102 in M.F. Hipkins and N. R. Baker, eds. Photosynthesis Energy Transduction.Oxford, Great Britain: TRL.

Holm, L. G., D. L. Plunkett, J. V. Pancho, and J. P. Herberger. 1977. TheWorld’s Worst Weeds—Distribution and Biology. Honolulu, HI: Uni-versity Press of Hawaii. 606 p.

Keeley, P. E., C. H. Carter, and R. J. Thullen. 1987. Influence of plantingdate on growth of Palmer amaranth (Amaranthus palmeri). Weed Sci.35:199–204.

Kigel, J. 1994. Development and ecophysiology of amaranths. Pages 39–73 in O. Parede-Lopez, ed. Amaranth Biology, Chemistry, and Tech-nology. Boca Raton, FL: CRC.

Klingaman, T. E. and L. R. Oliver. 1994. Palmer Amaranth (Amaranthuspalmeri) interference in soybeans (Glycine max). Weed Sci. 42:523–527.

Knezevic, S. Z., M. J. Horak, and R. L. Vanderlip. 1997. Relative time ofredroot pigweed (Amaranthus retroflexus L.) emergence is critical inpigweed-sorghum (Sorghum bicolor (L.) Moench) competition. WeedSci. 45:502–508.

Krause, G. H. and E. Weis. 1984. Chlorophyll fluorescence as a tool inplant physiology: II. Interpretation of fluorescence signals. Photosynth.Res. 5:139–157.

Kuroyanagi, T. and G. M. Paulsen. 1988. Mediation of high-temperatureinjury by roots and shoots during reproductive growth of wheat. PlantCell Environ. 11:517–523.

Lee, S. D. and L. R. Oliver. 1982. Efficacy of acifluorfen on broadleafweeds. Time and method of application. Weed Sci. 30:520–526.

Leegood, R. C. 2002. C4 photosynthesis: principles of CO2 concentrationand prospects for its introduction into C3 plants. J. Exp. Bot. 53:581–590.

Levitt, J. 1972. Responses of Plants to Environmental Stress. New York:Academic. pp. 243–244.

Lilley, R. M. and D. A. Walker. 1974. An improved spectrophotometricassay for ribulosebisphosphate carboxylase. Biochem. Biophys. Acta358:226–229.

Mayo, C. M., M. J. Horak, D. E. Peterson, and J. E. Boyer. 1995. Dif-ferential control of four Amaranthus species by six postemergence her-bicides in soybean (Glycine max). Weed Technol. 9:141–147.

McLanchlan, S. M., S. F. Weise, C. J. Swanton, and M. Tollenaar. 1993.Effect of corn induced shading and temperature on rate of leaf ap-pearance in redroot pigweed (Amaranthus retroflexus L.). Weed Sci. 41:590–593.

Menges, R. M. 1987. Allelopathic effects of Palmer amaranth (Amaranthuspalmeri) and other plant residues in soil. Weed Sci. 35:339–347.

Menges, R. M. 1988. Allelopathic effects of Palmer amaranth (Amaranthuspalmeri) on seedling growth. Weed Sci. 36:325–328.

Murphy, S. D., Y. Yankuba, S. F. Weise, and C. J. Stanton. 1996. Effect


on planting patterns and inter-row cultivation on competition betweencorn (Zea mays) and late emerging weeds. Weed Sci. 44:856–870.

Oryokot, J.O.E., S. D. Murphy, A. G. Thomas, and C. J. Swanton. 1997.Temperature- and moisture-dependent models of seed germinationand shoot elongation in green and redroot pigweed (Amaranthus pow-ellii, A. retroflexus). Weed Sci. 45:488–496.

Regehr, D. L., D. E. Peterson, P. D. Ohlenbusch, W. H. Fick, P. W. Stahl-man, and R. E. Wolf. 2002. Chemical Weed Control for Field Crops,Pastures, Rangeland, and Noncropland. Manhattan, KS: Kansas Ag-ricultural Experiment Station Report of Progress 867, Kansas StateUniversity.

Rowland, M. W., D. S. Murray, and L. M. Verhalen. 1999. Full-seasonPalmer amaranth (Amaranthus palmeri) interference with cotton (Gos-sypium hirsutum). Weed Sci. 47:305–309.

Schonbeck, M. W. and G. H. Egley. 1980. Redroot pigweed (Amaranthusretrofluexus) seed germination responses to after-ripening, temperature,ethylene, and some other environmental factors. Weed Sci. 28:543–547.

Wax, L. M. 1995. Pigweeds of the Midwestern states—distribution, im-portance and management. Proc. Integr. Crop Manag. 7:239–242.

Weaver, S. E. 1984. Differential growth and competitive ability of Ama-ranthus retroflexus, A. powellii and A. hybridus. Can. J. Plant Sci. 64:715–724.

Weaver, S. E. and E. L. McWilliams. 1980. The biology of Canadian weeds.Amaranthus retroflexus L., A. powellii S. Wats. and A. hybridus L. Can.J. Plant Sci. 60:1215–1234.

Weaver, S. E., C. S. Tan, and P. Brain. 1987. Effect of temperature andsoil moisture on time of emergence of tomato and four weed species.Can. Plant Sci. 68:877–886.

Weaver, S. E. and A. G. Thomas. 1986. Germination responses to tem-perature and atrazine-resistant and susceptible biotypes of two pigweed(Amaranthus) species. Weed Sci. 34:865–870.

Wetzel, D. K., M. J. Horak, and D. Z. Skinner. 1999. Use of PCR-basedmolecular markers to identify weedy Amaranthus species. Weed Sci.47:518–523.

Wright, S. R., H. D. Coble, C. D. Raper, Jr., and T. W. Rufty, Jr. 1999.Comparative responses of soybean (Glycine max), sicklepod (Senna ob-tusifolia), and Palmer amaranth (Amaranthus palmeri) to root zone andaerial temperatures. Weed Sci. 47:167–174.

Zhang, Z., Z. Shen, J. Zhao, L. Wang, and T. Hu. 1980. The Measurementof Roots Activity—Method of Methyl Blue Absorption. ExperimentalGuide to Plant Physiology. Beijing: Renmin Education. Pp. 71–73.

Zhu, G. 1990. The Measurement of Rubisco Carboxylase Activity. PlantPhysiological Experiment. Beijing: Beijing University Press. Pp. 73–76.

Received August 23, 2002, and approved April 7, 2003.

temperature effects on germination and growth of redroot pigweed (amaranthus retroflexus), palmer...

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