AFS 2(2011): vol. 20 , 2 183-190 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 20(2011): 183–190. 183 © Agricultural and Food Science Manuscript received November 2010 Impact of temperature and germination time on the success of a C4 weed in a C3 crop: Amaranthus retroflexus and spring barley Terho Hyvönen MTT Agrifood Research Finland, Plant Production Research, FI-31600 Jokioinen, Finland e-mail: terho.hyvonen@mtt.fi Elevation in temperatures due to climate change could promote the invasion by C4 weed species of arable fields in the boreal region, which are dominated by C3 crops. The success of Amaranthus retroflexus L. (a C4 weed) in spring barley (a C3 crop) was studied at current and elevated temperatures (3 °C difference) in a greenhouse experiment in southern Finland. The competition treatments included no competition and four levels of competition with barley, differing in terms of germination time. The success of A. retroflexus was measured as growth (height and biomass) and seed production (number and biomass). Elevation in temperature enhanced seed production of A. retroflexus, but the impact on growth was minor (only dif- ference in plant height in one treatment). The growth and seed production of A. retroflexus in competition with barley was minimal although the growth of barley decreased with the rise in temperature. The results indicate that climate change could improve growth of a C4 weed such as A. retroflexus, but it is unlikely to succeed in spring barley. Key-words: arable weed, climate change, seed production, species invasion Introduction Climate change has been predicted to broaden the distribution areas of arable weed species (Patter- son 1995). A successful range expansion of arable weeds requires not only adaptation to the climatic conditions of a new region but also colonisation of cultivated fields. Therefore, crop-weed interac- tions must be considered in connection with range expansion of arable weeds as a consequence of global warming. The key issue to understanding crop-weed in- teractions with respect to climate change lies in the A G R I C U L T U R A L A N D F O O D S C I E N C E Hyvönen, T. Amaranthus retroflexus and spring barley 184 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 20(2011): 183–190. 185 different responses of C3 and C4 species to eleva- tion in temperature and CO2 concentration (Pat- terson and Flint 1980, Ziska 2001, 2003). C3 plants benefit from the elevated CO2 levels, whereas C4 plants benefit from the elevated temperature (Pear- cy et al. 1981, Sage and Kubien 2007). As a result climate change will affect crop-weed interactions disproportionately depending on the photosynthet- ic pathway of the crop and weed species (Ziska 2001, 2003). C3 and C4 species occupy a temporal niche in natural vegetation that is determined by temperature (see Sage and Pearcy 2000). This is also evident in arable weed communities since C4 weeds usually occur in C4 crops (e.g. Schroeder et al. 1993, Bunce and Ziska 2000), which are alike with respect to temperature requirements for germination and growth. Since the majority of the “world’s worst weeds” are C4 plants (Holm et al. 1977) and most crops are C3 plants (Pat- terson 1995), global warming can be assumed to strengthen the competition from these weeds in the future (Ziska 2003, Tungate et al. 2007). Regard- ing range expansion, the pressure on C4 weeds to invade northern regions, where they are currently rare, can be assumed to increase (Holm et al. 1997). This will require, however, the colonisation of C3 crops by C4 weeds. One of the most widely distributed C4 weeds in the world is Amaranthus retroflexus L., which is found in 60 crops in 70 countries (Holm et al. 1997, Costea et al. 2004). The main distribution area of A. retroflexus in Europe is in southern and central Europe, the northern border of the distribu- tion area being in the Nordic and Baltic countries. At the margins of the distribution area, including Finland and Estonia, A. retroflexus is regularly found as a casual alien (Hämet-Ahti et al. 1998, Kukk and Kull 2005), i.e. a species that cannot es- tablish permanent populations and is dependent on the regular supply of new seeds (Richardson et al. 2000). Based on its current distribution, A. retro- flexus can be regarded as one of the most probable C4 weed species to extend its distribution into the boreal region as a consequence of climate change. Previous studies showed that elevation in tem- perature increases growth of A. retroflexus (Guo and Al-Khatib 2003) but decreases seed produc- tion (Weaver 1984), indicating opposing respons- es regarding success as a consequence of climate change. Seed production is lowered in competition with a crop (McLachlan et al. 1995). The success of A. retroflexus as a competitor with C3 cereals has not been established experimentally, but field observations suggest it is poorer than with C4 crops (Costea et al. 2004). The aim of the present study was to explore the growth response and the seed production of A. retroflexus at an elevated temperature with and without competition from a C3 crop (spring bar- ley). The study was conducted in Finland, where the dominant C3 crop species is spring barley and which represents the northern distribution limit of A. retroflexus. Since the timing of germination is a critical factor affecting crop-weed competition (Knezevic et al. 1997) and the optimum germina- tion temperature for A. retroflexus is high compared with that for barley (Ghorbani et al. 1999, Guo and Al-Khatib 2003), which indicates later germination and emergence times, several germination dates for A. retroflexus were included in the study. I expected the elevation in temperature to enhance growth but decrease seed production of A. retroflexus. Material and Methods Plant material The seeds of Amaranthus retroflexus were collected from southern Sweden (55°45´ N; 13°19´ E) in Sep- tember 2004. In summer 2008, the seeds were sown in pots and grown both outdoors and in a greenhouse in Jokioinen, southern Finland (60°49’ N; 23°29’ E) in order to produce seeds for the experiment. The seeds were stored at +4 °C but kept at -18 °C for two days in October 2008. The seeds of spring barley, Hordeum vulgare L., cultivar Rolfi, were obtained from a local agricultural market. The plants for each treatment (see below) were grown in 3.5 l pots (diameter 18 cm) in a com- mercial peat-sand mixture (‘Kylvöseos’ by Kek- kilä Oyj, Mellilä, Finland; pH 6.0, N: 70 mg l-1, P: A G R I C U L T U R A L A N D F O O D S C I E N C E Hyvönen, T. Amaranthus retroflexus and spring barley 184 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 20(2011): 183–190. 185 21 mg l-1, K: 140 mg l-1) and fine sand (grain size 0.2-1 mm) at a ratio of 2:1. The pots were watered regularly and fertilised (a mixture of 2.5 kg ferti- lization [N-P-K content 14-5-21 %, respectively] with 25 litres water mixed with irrigation water in a proportion of 1:50) 7 times during the experi- ment (total amount of fertiliser-water mixture per pot was 2.7 l). The sowing density of barley was 12 seeds per pot sown in two rows (row width 7 cm), the same sowing density as in field cropping, and one seed or seedling of A. retroflexus placed in the middle of the pot. The seeds of A. retroflexus were either sown in the pot (treatments 1 and 2) or planted as 1–2 cm long seedlings (treatments 3–5). The seed- lings were germinated on Petri dishes in a growth chamber at a temperature regime of 35/10 °C (16/8 h) with full light. If a seedling of A. retroflexus died within two weeks after the sowing date it was replaced by a new seedling of the same age grown in the same greenhouse. Seventy-seven (32.1% of all plants in the experiment) such replacements were made during the experiment. At the end of the experiment, 12 A. retroflexus individuals were dead of which 9 were replacement seedlings. Two dead barley plants were also replaced with new seedlings after 10 days from the beginning of the experiment. Experimental design The greenhouse experiment was conducted in Jokio- inen between 10th February and 8th May 2009. The experiment was conducted in four greenhouses of which two had a temperature regime of 20/10 °C (18/6 h) (hereafter, current) and the other two 23/13 °C (18/6 h) (hereafter, elevated). The temperature regimes were randomized among greenhouses. The temperature regime with lower values was in line with the long-term (1971–2000) average minimum and maximum temperatures for the summer months (June–August) recorded at Jokioinen meteorological station (Drebs et al. 2002). The 3 degree elevation in temperature conforms to the predicted median temperature value for summer months (June-August) for the time period 2040–2069 according to four climate scenarios (Jylhä et al. 2004). The light/dark regime was 19/5 h for both temperature regimes, equivalent to the day length for Jokioinen in June. The experiment included the following treat- ments: 1) A. retroflexus seedling planted on the 1st day of the experiment, no barley, 2) A. retroflexus seedling planted and barley sown on the 1st day of the experiment, 3) A. retroflexus seedling planted on the 7th and barley sown on the 1st day of the ex- periment, 4) A. retroflexus seedling planted on the 14th and barley sown on the 1st day of the experi- ment, 5) A. retroflexus seedling planted on the 21st and barley sown on the 1st day of the experiment and 6) Barley sown on the 1st day of the experiment, no A. retroflexus. Treatments 1–5 were placed on the same table as a row column design with three replicates (10 cm between pots). Four such tables were placed in each greenhouse (n = 12 for each treatment per greenhouse). The distance between the tables was about 1 metre. Barley control (treat- ment 6) consisted of two pots placed in close prox- imity to each table (n = 8 per greenhouse). Measurements and data analyses At the end of the experiment, plant height, above ground biomass (air-flow drying at 40 °C) and number and weight of seeds of A. retroflexus were recorded. The height of each barley plant and the above ground biomass of 12 barley plants per pot were also measured. Prior to analyses, the heights of barley plants were averaged and the data were pooled for treatments 1–5. The averages for barley controls were calculated for above ground biomass and height. The data were analysed using linear mixed models. The treatment and greenhouse were fixed factors and the interaction of greenhouse and table a random factor in the models used for the analyses of all barley data sets and the height of A. retroflex- us. However, for the data sets of seed number and weight, as well as for the biomass of A. retroflexus, comparisons within treatment 1 only were possi- ble due to missing data (seeds) or highly skewed A G R I C U L T U R A L A N D F O O D S C I E N C E Hyvönen, T. Amaranthus retroflexus and spring barley 186 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 20(2011): 183–190. 187 data distributions (biomass). In these analyses, the greenhouse was a fixed factor and the interaction of greenhouse and table a random factor. In both anal- yses, the paired comparisons between temperatures within treatment or between treatments within tem- perature were made using two-sided t-tests. All the A. retroflexus data were square root-transformed and the height and biomass data for barley (treat- ments 1–5) log (x+1) transformed prior to analyses. The statistical analyses were performed using the MIXED procedure in SAS (Littell et al. 2006). Results Seed production of A. retroflexus was affected both by competition with barley and temperature. The impact of competition was so strong that only 9 A. retroflexus plants in treatment 2, 2 plants in treatment 3 and a single plant in treatments 4 and 5 produced seeds. The majority of these cases were in the greenhouses at an elevated temperature: only one plant produced seeds in competition with barley at the current temperature. Due to the scattered data for seed production, the impact of competition and Fig. 1. The number and biomass of Amaranthus retroflexus seeds per plant (mean and SD) at the two temperature regimes. 0 2000 4000 6000 8000 10000 Number of seeds T20/10 Seed number T23/13 0 1 2 3 4 Seed biomass T23/13T20/10 Biomass (g) the interaction of competition and temperature could not been verified using statistical testing. In the comparison of treatment 1 (i.e. no competition with barley) at different temperatures, a greater number (1655 ± 896 vs. 5465 ± 2711 seeds per plant, t = 4.3, df = 12, p < 0 .001) and weight (0.7 ± 0.4 vs. 2.1 ± 1 g per plant, t = 4.2, df = 12, p < 0.01) of seeds were associated with an elevated rather than a current temperature (Fig. 1). The biomass of A. retroflexus did not differ be- tween temperature treatments (treatment 1: t = 1.8, df = 12, p < 0.05). The impact of temperature on the height of A. retroflexus was dependent on the competition treatment (treatment and temperature interaction F4, 48 = 4.8, p < 0.01), the height be- ing greater (t = -5.61, df = 59.3, p < 0.001) only for treatment 2 (Fig. 2). For barley, the elevated temperature resulted in both lower biomass and re- duced height for all treatments (biomass: t = 3.6- 4.1, df = 46.2, P < 0.001; height: t = 5.7-6.9, df = 17.7, p < 0.001). The same pattern was detected in control barleys, where both height (63.3 ± 3.6 vs. 55.6 ± 2.8 cm) and biomass (34.8 ± 2.3 vs. 30.8 ± 1.8 g) were higher at current rather than at an elevated temperature (height: t = -5.2, p < 0.001; biomass: t = -4.8, P < 0.001). A G R I C U L T U R A L A N D F O O D S C I E N C E Hyvönen, T. Amaranthus retroflexus and spring barley 186 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 20(2011): 183–190. 187 0 10 20 30 40 50 60 70 80 T3 T2T2 T2 T1T1 T1T2 Amaranthus heigth (cm) T1 0 5 10 15 20 25 30 35 40 45 T3 Amaranthus biomass (g) 0 10 20 30 40 50 60 70 80 T5 T5 T5 T4 T4 T4T5T4T3 Barley heigth (cm) Temperature regime 20/10 23/13 0 5 10 15 20 25 30 35 40 45 T3 Barley biomass (g) Fig. 2. The height and biomass of Amaranthus retroflexus and barley individuals (mean and SD). In T1, no barley included. The treatments T2-T5 differed in terms of sowing date of A. retroflexus. Discussion Contrary to expectations, elevation in the tem- perature enhanced seed production and had a minor impact on the growth of the C4 weed A. retroflexus. The growth and seed production of A. retroflexus in competition with a C3 crop, spring barley, was minimal although barley growth was decreased by the elevation in temperature. The results indicate that climate change could improve the success of A. retroflexus populations but success in spring barley is unlikely. A G R I C U L T U R A L A N D F O O D S C I E N C E Hyvönen, T. Amaranthus retroflexus and spring barley 188 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 20(2011): 183–190. 189 Previous studies showed C4 weeds to respond positively to elevation in temperature (Guo and Al- Khatib 2003). However, the temperature optimum for photosynthesis of C4 plants is typically high (Sage and Kubien 2007); for A. retroflexus over 35 °C (Pearcy et al. 1981). This suggests that the maximum temperature used in the present study (23 °C) was too low to give an advantage to A. retroflexus in terms of photosynthetic rate. Interest- ingly, the 3 degree elevation in temperature used in the present study was high enough to limit the growth of barley, in accordance with earlier find- ings (Peltonen-Sainio et al. 2010, 2011). This sug- gests a decline in the competitive ability of spring cereals against weeds, in a future climate. For C4 weeds this would mean potential improvement in the success in the crop rotations typical of boreal regions. The results of the present study, however, suggest that even these changes will not promote the success of A. retroflexus in competition with spring cereals in the boreal region. Previous competition experiments (e.g. Kneze- vic et al. 1997) showed considerable yield losses due to competition by A. retroflexus. In the present study, yield loss was not evident but A. retroflexus was unable to effect any decline in the biomass of barley either. Previous studies included crops other than spring cereals as study species. Under field conditions A. retroflexus is more common in such crops than in cereals (e.g. Costea et al. 2004), which suggests the growth patterns to be more alike with these species. One of the factors related to growth patterns is the germination tem- perature which determines the emergence patterns of seedlings. In the present study, the germination temperature (35 °C) was high compared with the day temperatures used during growth (20 or 23 °C). Under field conditions, A. retroflexus would have germinated later than barley. Therefore, the treatment used in the study benefited A. retroflexus. However, the seedlings of A. retroflexus were ger- minated on Petri dishes and the seedlings planted in the pots to ensure the same germination date for each individual within treatments. This procedure appeared not to be successful since one third of the seedlings had to be replaced with new seedlings during the experiment. This likely contributed to the poor success of A. retroflexus in competition with barley. Even though A. retroflexus did not affect de- crease in barley biomass, seed production showed a positive response to elevation in temperature. The finding was contradictory to previous findings by Weaver (1984), who reported an increase in seed production when the temperature regime was changed from 28/22 °C to 22/14 °C. The latter tem- perature regime is comparable with the elevated temperature regime used in this study. Therefore, the results are not entirely contradictory. The seed production level recorded in the present study was lower compared with those reported earlier (Weav- er 1984, Costea et al. 2004). The increase in seed production at the elevated temperature suggests an improvement in the success of establishment of permanent populations. The finding suggests that some individual plants could produce seeds even in competition with barley. However, the building up of the populations, which would effect consid- erable yield losses, would require a crop rotation including a crop species less competitive against A. retroflexus (e.g. sugar beet). Even though the present study focused only on seed production, neglecting many components of the population demography (e.g. Morse and Bazzaz 1994), the suggestion is that A. retroflexus could survive un- der these conditions. Range expansion of C4 weeds into the boreal re- gion of Europe as a consequence of global warming is of concern. Several C4 weed species comprise an important part of the weed community in central Europe, but C4 weed species typically occur in C4 crops (Schroeder et al. 1993). The results of the present study suggest that A. retroflexus is unlikely to take hold in spring cereals. This is confirmed by the field observations: none of the 188 weed species found in a Finnish weed survey (Salonen et al., 2001) was a C4 species. In addition, none of the C4 species belong to the most important weeds of C3 crops in Europe (Schroeder et al. 1993). The results suggest that even though the 3 degree eleva- tion in average temperature decreased the growth of a C3 crop and improved the seed production of a C4 weed, C3 crop species could act as a barrier to invasion by the “world’s worst weeds” into the A G R I C U L T U R A L A N D F O O D S C I E N C E Hyvönen, T. Amaranthus retroflexus and spring barley 188 A G R I C U L T U R A L A N D F O O D S C I E N C E Vol. 20(2011): 183–190. 189 boreal region. Furthermore, climate warming will advance the sowing times of C3 crops (Kaukoranta and Hakala 2008), thus reinforcing the competitive benefit of C3 crops in spring time. The invasion of C4 weeds cannot be expected to take place before the introduction of C4 crops (e.g. maize), which has been predicted to occur not before the end of this century in Finland (Peltonen-Sainio et al. 2009). Acknowledgements. I am indebted to Lars Andersson for providing Amaranthus seeds for the experiment, Timo Hurme and Lauri Jauhiainen for their help in planning the experimental design and conducting the statistical analyses, and to Auli Kedonperä and Tuula Viljanen for taking care of the greenhouse experiment. The study was financed by the Academy of Finland (project number 122477). References Bunce, J.A. & Ziska, L.H. 2000. Crop ecosystem respons- es to climatic change: crop/weed interactions. In: Reddy, K.R. & Hodges, H.F. (eds.). Climate change and global crop productivity. CABI Publishing, Oxford, p. 333–352. Costea, M., Weaver, S.E. & Tardif, F.J. 2004. The biology of Canadian weeds. 130. Amaranthus retroflexus L., A. powellii S. Watson and A. hybridus L. Canadian Jour- nal of Plant Science 84: 631–668. Drebs, A., Nordlund, A., Karlsson, P., Helminen, J. & Ris- sanen, P. 2002. Climatological statistics of Finland 1971- 2000. Climatic Statistics of Finland No. 2002:1. Finnish Meteorological Institute, Helsinki. Ghorbani, R., Seel, W. & Leifert, C. 1999. Effects of en- vironmental factors on germination and emergence of Amaranthus retroflexus. Weed Science 47: 505–510. Guo, P. & Al-Khatib, K. 2003. Temperature effects on ger- mination and growth of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and a com- mon waterhemp (A. rudis). Weed Science 51: 869-875. Hämet-Ahti, L., Suominen, J., Ulvinen, T. & Uotila, P. (eds.) 1998. Retkeilykasvio (Field Flora of Finland). Finnish Museum of Natural History, Botanical Museum, Hel- sinki. 656 p. Holm, L.G., Plucknett, D.L., Pancho, J.V. & Herberger, J.P. 1977. The World’s Worst Weeds. Distribution and Biol- ogy. The University Press of Hawaii, Honolulu. 609 p. Holm, L., Doll, J., Holm, E., Pancho, J. & Herberger, J. 1997. World Weeds: Natural Histories and Distribution. John Wiley & Sons, Inc. New York. 1129 Jylhä, K., Tuomenvirta, H. & Ruosteenoja, K. 2004. Climate change projections for Finland during the 21st century. Boreal Environmental Research 9: 127–152. Kaukoranta, T. & Hakala, K. 2008. Impact of spring warm- ing on sowing times of cereal, potato and sugar beet in Finland. Agricultural and Food Science 17: 165–176. Knezevic, S.Z., Horak, M.J. & Vanderlip, R.L. 1997. Rel- ative time of redroot pigweed (Amaranthus retroflex- us L.) emergence is critical in pigweed-sorghum (Sor- ghum bicolor (L.) Moench) competition. Weed Science 45: 502–508. Kukk, T. & Kull, T. 2005. Atlas of the Estonian Flora. In- stitute of Agricultural and Environmental Sciences of the Estonian University of Life Sciences. Tartu. 527 p. Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D. & Schabenberger, O. 2006. SAS for Mixed Models. SAS Institute Inc. Cary, NC, USA. 814 p. McLachlan, S.M., Murphy, S.D., Tollenaar, M., Weise, S.F. & Swanton, C.J. 1995. Light limitation of reproduction and variation in the allometric relationship between re- productive and vegetative biomass in Amaranthus ret- roflexus (redroot pigweed). Journal of Applied Ecology 32: 157–165. Morse, S.R. & Bazzaz, F.A. 1994. Elevated CO2 and tem- perature alter recruitment and size hierarchies in C3 and C4 annuals. Ecology 75: 966–975. Patterson, D.T. 1995. Weeds in changing climate. Weed Science 43: 685–701. Patterson, D.T. & Flint, E.P. 1980. Potential effects of glob- al atmospheric CO2 enrichment on the growth and com- petitiveness of C3 and C4 weed and crop plants. Weed Science 28: 71–75. Pearcy, R.W., Tumosa, N. & Williams, K. 1981. Relation- ships between growth, photosynthesis and competi- tive interactions for a C3 and a C4 plant. Oecologia 48: 371–376. Peltonen-Sainio, P., Jauhiainen, L., Hakala, K. & Ojanen, H. 2009. Climate change and prolongation of growing season: changes in regional potential for field crop pro- duction in Finland. Agricultural and Food Science 18: 171–190. Peltonen-Sainio, P., Jauhiainen, L., Trnka, M., Olesen, J.E., Calanca, P.L., Eckersten, H., Eitzinger, J., Gobin, A., Kersebaum, C., Kozyra, J., Kumar, S., Marta, A.D., Micale, F., Schaap, B., Seguin, B. & Skjelvåg, A.O. 2010. Coincidence of variation in yield and climate in Europe. Agriculture, Ecosystems and Environment 139: 483–489. Peltonen-Sainio, P., Jauhiainen, L. & Hakala, K. 2011. Crop responses to precipitation and elevated temperatures in cool growing conditions at high latitudes according to long-term multi-location trials. The Journal of Agricul- tural Science 149: 49–62. Richardson, D.M., Pyšek, P., Rejmánek, M., Barbour, M.G., Panetta, F.D. & West, C.J. 2000 Naturalization and in- vasion of alien plants: concepts and definitions. Diver- sity and Distributions 6: 93–107. Salonen, J., Hyvönen T. & Jalli, H. 2001. Weeds in spring cereal fields in Finland – a third survey. Agricultural and Food Science in Finland 10: 347–364. Sage, R.F. & Kubien, D.S. 2007. The temperature response of C3 and C4 photosynthesis. Plant, Cell and Environ- ment 30: 1086–1106. Sage, R.F. & Pearcy R.W. 2000. The physiological ecology of C4 photosynthesis In: Leegood, R.C., Sharkey, T.D. & Von Caemmerer, S. (eds). Photosynthesis: physiolo- A G R I C U L T U R A L A N D F O O D S C I E N C E Hyvönen, T. Amaranthus retroflexus and spring barley 190 gy and metabolism. Kluwer Academic Publishers, Se- caucus, p. 497–532 Schroeder, D., Mueller-Schaerer, H. & Stinson, C.S.A. 1993. A European weed survey in 10 major crop sys- tems to identify targets for biological control. Weed Re- search 33: 449–458. Tungate, K.D., Israel, D.W., Watson, D.M. & Rufty, T.W. 2007. Potential changes in weed competitiveness in an agroecological system with elevated temperature. Envi- ronmental and Experimental Botany 60: 42–49. Weaver, S.E. 1984. Differential growth and competitive ability of Amaranthus retroflexus, A. powellii and A. hy- bridus. Canadian Journal of Plant Science 64: 715–724. Ziska, L.H. 2001. Changes in competitive ability between a C4 crop and a C3 weed with elevated carbon dioxide. Weed Science 49: 622–627. Ziska, L.H. 2003. Evaluation of yield loss in field sorghum from a C3 and C4 weed with increasing CO2. Weed Sci- ence 51: 914–918. Impact of temperature and germination time on the success of a C4 weed in a C3 crop: Amaranthus retroflexus and spring barley Introduction Material and Methods Plant material Experimental design Measurements and data analyses Results Discussion References