Weeds : An Introduction and their Responses to ...

Trends in Biosciences 10(12), Print : ISSN 0974-8431, 2123-2129, 2017

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Weeds : An Introduction and their Responses to Climate Change SAURABH PAGARE1,2*MANILA BHATIA1, NIRAJ TRIPATHI2, SONAL PAGARE3 AND BHUMESH KUMAR2 1

Department of Biological Science, Rani Durgavati Vishwavidyalaya, Jabalpur, Madhya Pradesh Directorate of Weed Science Research, Jabalpur, Madhya Pradesh 3 NTPC Hospital, Korba, Chattisgarh * email : [email protected] 2

ABSTRACT Weeds are one of the main threats to agriculture and the environment. Their management remains a significant financial, logistical and research challenge. The main drivers for climate change impacts on plants, including weeds, will be changed temperatures and rainfall, altered frequency and intensity of extreme weather events and increasing concentrations of carbon dioxide (CO2) in the atmosphere. Climate change will require revisiting what we deem appropriate for weed control to keep current and future management strategies efficient and effective. Climate change will exacerbate both the threat to biodiversity and the cost to agriculture of weeds. This is because new and changed levels of weed impacts on the environment will arise, requiring new or significantly altered adaptation responses to reduce negative impacts. Likewise, the risk of negative impacts from weeds due to extreme weather events, such as prolonged drought, heat waves, floods and cyclones that occur under today’s climate, are similar to the risks associated with average climate change conditions. Thus future climates may further favour weed invasions, which could increase the risk of negative impacts from these species. However, it also means that adaptation responses to reduce the potential impact of weeds may also be just as appropriate to implement today. The present review develops an outline of vision, strategy and weed management plan that is required to address climate change adaptation and the effect of climate change on crop-weed interaction. Key words

climate change, CO2, weed, global warming, interaction, temperature

A weed is simply a plant that is not wanted where it is found. This means that plants are identified as weeds based on a value judgment and within a human context. This also means that a plant may be considered a weed by someone but not by others. The study of weed plants, their abundance and allocation is useful in the determination of how a plant population altering their behavior over time in response to different climatic conditions. It is important for managing agricultural land both for productivity and for biodiversity (Nkoa et al., 2015). The physiological plasticity of weeds and their greater intra specific genetic variation compared with most crops could provide weeds with a competitive advantage in a changing environment. Rising CO2 may be a selection factor in weed species dominance Climate change means more extreme weather events, greater

stresses on native species and ecosystems, and climatedriven activities, such as the introduction of new, hardier pasture and garden plant varieties. Agriculture provides human society with food, fiber and energy, and for many people in developing countries it is the main source of income. Agriculture usually takes place under the open sky and while human beings have gained a certain control over agricultural production, un-expected climatic changes and changes in weather events could always endanger a harvest. With climate change agriculture will change. (Fischer et al., 2005) projected the most significant negative changes for developing countries in Asia, where agricultural production declines of about - 4% to -10% are anticipated under different socio- economic and climate change scenarios. Plants exposed to elevated CO 2 often show increased growth and water use efficiency (Allen and Amthor 1995). Elevated atmospheric CO2 attracts biological scientists, especially ecologists and plant physiologists, because of the potential biological impacts from CO2 induced global warming and from direct effects of elevated CO2 on vegetation that are independent of global warming. The ability of plants to respond to future elevated CO2 levels will undoubtedly hinge upon physiological characteristics such as sink strength, efficiency of nitrogen and water use and photosynthetic pathway. The exclusive use of gas exchange data to predict plant success has been over-valued and over-represented in literature addressing plant response to elevated CO 2. Plant species respond elevated CO 2 in different ways starting right from germination up to maturity depending upon their growing habits and photosynthetic pathways and several other metabolic processes. Crops and weeds co-exist in an agricultural system and certainly a point of focus among scientists, agriculturists and environmentalists. Numerous studies can be found in literature regarding the crops-weeds interaction in field conditions suggesting that weeds pose a very serious and potential threat to agricultural production resulting approximately one third yield loss by virtue of their competitiveness with special mention to “noxious” or “invasive” weeds like Euphorbia geniculata which outplay the crop plants in almost every aspect and led to big loss to crop production in India. Recently, agriculture scientists have started paying more attention towards crop-weed competitiveness in a high CO2 environment and it has been suggested that possibly recent increases in atmospheric CO2 during the 20th century may have been a factor in the selection of weed species and a contributing factor of invasiveness of weed species. Rising CO2 may also affect

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growth and combustibility of many invasive weeds changing fire ecology. Increased CO 2 may select for invasiveness within assemblage of plants. Climate change directly affects the geographic range of species, the timing of species life cycle (phenology), the population dynamics of species, the decline and extinction of some species and the invasion of other species. Plants with C 3 photosynthetic pathways are expected to benefit more than C4 from CO2 enrichment. However, rising global temperature may give competitive advantage to C4 plants than C3. This differential response of C3 and C4 plants will alter crop weed interaction because of the fact that majority of weeds are C4 and most of the food grain crops are C3. Higher levels of carbon dioxide could stimulate the growth of some weed species and greater production of rhizomes and tubers in perennial weeds making them difficult to control. Warmer temperatures will accelerate the rate at which day degrees accumulate, so the life cycles of some plant species may accelerate. As a result weeds are likely to mature and start to decay earlier (Singh et al., 2011).

Challenges Many questions have to be answered in context of weeds and weed management under the regime of climate change. Some are: 1. How an individual factor will affect crop, weeds and associated micro-organisms? 2. How multifactor climate change (i.e. CO2, ozone, UV radiation and other greenhouse gases, temperature) will affect the relative competitiveness of crop, weeds and microbes? Who will dominate whom? 3. Weed dynamics under climate conditions. 4. How will a change in precipitation (seems to be almost certain) effect weed growth? 5. What are the physiological, biochemical and molecular basis and mechanism of dominance? 6. What are ways to sustain/ increase the productivity of crops in changing climate? 7. How we can predict the possible losses of crop yields in futuristic climate change conditions? In any one location, weather can change very rapidly from day to day and year to year, even with an unchanging climate. These changes are influenced by shifts in temperature, precipitation, winds, and clouds. In contrast to weather, climate is generally influenced by slow changes in features like the ocean, the land, the orbit of the earth about the sun, and the energy output of the sun. Climate change, on the other hand, refers to a fundamental shift in the mean state of the climate (i.e., longer-term trends – decades, centuries). Another point of possible confusion rises from the terms: greenhouse effect, global warming and climate change, which are related – but not the same. Greenhouse effect is a descriptive term that helps explain how greenhouse gases like carbon dioxide trap heat in the atmosphere. Global warming refers to the increased temperatures that are expected to result with more greenhouse gases in the atmosphere. Temperature is the primary element of climate and increased temperatures will alter precipitation patterns, wind speed and direction, and air pressure.

Impact of Climate Change on Agriculture Climate change has already had an effect on agriculture estimate that in the time span 1981-2001, changes in precipitation and increased temperatures have already resulted in annual combined losses of wheat, maize and barley of roughly 40 million tons per year. While the scientists consider these losses relatively small in comparison to the technological yield gains over the same period, the results demonstrate the negative impacts of climate change already occurring on crop yields at a global scale. Increased CO2 concentrations could have a direct effect on the growth-rates of individual crop plants and weeds and also cause vegetation communities to change; CO2 induced climate changes may alter temperature, rainfall patterns and amounts of radiation received in different parts of the world; this will influence the productivity of natural Ecosystems or agricultural landscapes with significant regional variations; Some perspectives are provided as to how the changing climate of the world may affect the growth of crops and weeds and their interactions. A better understanding of potential changes in both crops and weeds is crucial to enable adapting to future climate changes, and sustain our ability to manage weed populations effectively. Agriculture represents the core part of India economy, representing 35% of crops and thus is central in the country development. At the country level, there has been no consistent trend in monsoon rainfall during the last century, although there are some regional patterns. Areas with increasing monsoon rainfall are the west coast, north Andhra Pradesh and northwest India. Areas with decreasing monsoon rainfall are east Madhya Pradesh and adjoining areas, northeast India and parts of Gujarat and Kerala (–6 to –8% of normal over 100 years). Most of the temperature trend studies in India focus on the analysis of annual and seasonal temperature data for a single station or a group of stations. Such studies date back to at least 50 years. The magnitude of warming was higher in the post-monsoon and winter seasons. The monsoon temperature did not show a significant trend in any major part of the country, except for a significant negative trend over northwest India. A significant average warming of 0.4°C has been recorded over the last century (1901–2000). A significant warming has been observed along the west coast, in central India and the interior peninsula and over north-east India. A cooling has been observed in the northwest and parts of southern India. No significant long term trend in the frequency of droughts or floods has been recorded over the past 130 years. Projected climate change for all of India indicates that by 2100 there will be an increase of 15–30% in rainfall across the country and the mean annual temperature will increase by 3–6°C, with the maximum increase over northern India. This is supported by work from the Indian Agricultural Research Institute which projects that for every 1°C rise in temperature there is an associated loss of up to five million tons in wheat production, assuming irrigation continues at today’s level.

PAGARE et al., Weeds : An Introduction and their Responses to Climate Change

It is expected that a major impact of climate change will be on rain fed crops (other than rice/wheat) which contribute approximately 60% of the cropland area. Using historical records and crop-growth modeling, increasing temperature has been projected to have varying negative impacts on the yields of barley, chickpea, mustard and wheat in northwest India. A 2–3ºC increase in temperature will reduce yields in the majority of the wheat growing areas (Aggarwal and Sinha 1993), and this yield reduction will be greater in non-irrigated (and thus water stressed) crops due to rainfall variability. So, the warmer regions will suffer crop losses. A minor increase of only 0.5ºC during the winter is expected to decrease wheat yield by 0.45 t/ha. The situation is similar for sorghum and pearl millet which are exposed to extreme high temperatures in Rajasthan. An increase in temperature of 2–4ºC is expected to reduce yields of rice. An increase of 2°C may decrease the rice yield by 0.75 t/ha in high yield areas (Sinha and Swaminathan 1991). Weeds are no strangers to man. In the world there are 30,000 weed species, out of these 18,000 sps cause damage to the crops. Jethro Tull first coined the term weed in 1931 in the book “Horse Hoeing Husbandry”. Weeds are the plants, which grow where they are not wanted. Weeds are unwanted or undesirable plants compete with crops for water, soil nutrients, light and space (i.e. CO2) and thus reduce crop yields. They have been there ever since farmer started to cultivate crops about 10,000 BC and undoubtedly recognized as a problem from the beginning. Any plant in the field other than his crop became weed. Again the characters of certain weed species are very similar to that of wild plants in the region. Some of the crops for example including the wheat of today are the derivatives of wild grass. Man has further improved them to suit his own taste and fancy. Even today they are crossed with wild varieties to transfer the desirable characters such as drought and disease resistance. So the weeds are to begin with essential components of native and naturalized flora but in course of time these plants are well placed in new environment by the conscious and unconscious efforts of man. Weeds may be classified under following categories: I.

Indigenous weeds: All the native weeds of the country are coming under this group and most of the weeds are indigenous. Example: Acalypha indica, Abutilon indicum, Sorghum halepense, Cynodon dactylon and Echinochloa colonum

II.

Introduced or Exotic weeds or Alein weeds: These are the weeds introduced from other countries. These weeds are normally troublesome and control becomes difficult. Example: Parthenium hysterophorus, Acanthospermum hispidum, Eichhornia crassipes, Argemone mexicana, Lantana camara and Croton bonplandianus.

III.

Noxious weeds: These weeds are arbitrarily defined as being undesirable, troublesome and difficult to control. They have immense capacity of reproduction and high dispersal capacity. They adopt tricky ways to defy man efforts to remove them. These weeds are also known as special problem weeds. Example:

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Cyperus rotundus. Cynadon dactylon, Circium arvense, Parthenium, Eichhornea crassipes, Lantana camara, Saccharum spontaneum, Imperata cylindrical and Striga spp. IV.

Parasitic weeds: Weeds which usually parasite the host crop partially or fully for their nourishment. Example: Cuscuta reflexa.

Weeds are highly persistent. Persistence is an adaptive measure of a weed that enables it to grow in any environment. The persistence of an organism refers to repeatedly invade an environment even when it is apparently removed from the scene by man (or any other agent). This should be differentiated from its hardiness, which refers to its ability to withstand all kinds of natural stresses at a given place. Weeds are both persistent and hardy. Persistence of weed is largely influenced by Climatic, Edaphic (Soil factors) and Biotic factors which determine the distribution, prevalence, competing ability, behavior and survival of weeds. Competition between crop plants and weeds is most severe when they have similar vegetative habit and common demand for available growth factors. Water, nutrient, light and space are the major factors for which usually competition occurs. Weeds appear much more adapted to agro-ecosystems than our crop plants. Without interference by man, weeds would easily wipe out the crop plants. Generally, an increase in on kilogram of weed growth will decrease one kilogram of crop growth. The period at which maximum crop weed competition occurs called critical period. It is the shortest time span in the life cycle/ontogeny of crop when weeding results in highest economic returns.

Differential Response of Weeds to Elevated CO2 Over the past three decades, many experiments have tested the effects of higher atmospheric CO2 on weeds with C3 and C4 photosynthetic pathways. Some examples from an early review by Patterson (1995) indicate significant variations in response to CO2 both within a species and between species, depending on experimental condition, such as temperature, light, availability of water and nutrients. While the variability in plant responses is large, C3 weeds generally increased their biomass and leaf area under higher CO2 concentrations compared with C4 weeds. In view of such results, it could be predicted that C3 weeds, like Parthenium (Parthenium hysterophous L.) and Chromalaena [Chromalaena odorata( L.) will be much more competitive under raised CO2 environment, independently of temperature and rainfall effects. The effect of elevated CO2 levels on the growth and biomass production of six C4 weeds (Amaranthus retroflexus L., Echinochloa crus-galli (L.) P. Beauv., Panicum dichotomiflorm Michaux, Setaria faberi Herrm., Setaria viridis (L.) P. Beauv., Sorghum halapens (L) Pers.) and four C4 crop species (Amaranthus hypochondriacus L., Saccharum officinarum L., Sorghum bicolor (L) Moench and Zea mays L.) compared. Eight of the ten studied C4 species showed a significant increase in photosynthesis. The largest and smallest increases

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observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at higher CO2, which also resulted in significant increases in whole plant biomass for four C 4 weeds (A. retroflexus, E. crusgalli, P. dichotomiflorm, S. viridis) relative to the ambient CO2 condition. Leaf water potential for three of the species (A.retroflexus, A. hypochondriacus Z. mays) indicated that difference in photosynthetic stimulation was not due solely to improved leaf water status. This study confirmed that C4 plants may respond directly to increasing CO2 in the atmosphere and in the case of some C 4 weeds (A. retroflexus), the photosynthetic increase could be similar to those published for C3 species. Of the 15 crops, which supply 90% of the world’s calories, 12 have the C3 photosynthetic pathways. In contrast, 14 of the 18 world’s worst weeds are C4 plants. The general consensus of the above and other similar studies is that the greater majority of weeds in the world, which are C3 plants, will benefit from increased CO2 levels under climate change, while most tropical grasses, increased growth in higher CO2. However, because C 4 plants are generally more tolerant of heat and moisture stress, the simple notion that climate change will only benefit C4 plants may not be accurate.

Climate Change May Cause Range Shifts in Weed Distribution and Abundance A body of research is emerging (Luo and Mooney 1999; Bunce 2000), which indicates that elevated CO2 levels are likely to increase the ability of plants to tolerate both high and low temperatures. However, the responses are linked with moisture availability through modified rainfall patterns and possibly other factors like nitrogen deposition. Most ‘colonising’ species have wide ecological amplitudes i.e. the capacity of a species to establish in various habitats along an environment gradient and are already adapted to a broad range of conditions under which they can thrive and perpetuate. This innate ability to tolerate varying and extreme conditions will enable weeds to benefit under climate change, at the expense of less ‘weedy’ species. The increased tolerance of low temperature under elevated CO2 for several chilling-sensitive plants of tropical or subtropical origin established. Possible reasons were: improved plant water balance, less severe wilting and less leaf damage under elevated CO 2 compared with ambient levels. Temperature is recognized as a primary factor influencing the distribution of weeds across the globe, particularly at higher latitudes. Increased temperature and precipitation in some parts of earth may provide suitable conditions for stronger growth of some species, which are usually limited by now temperatures. The distribution of some tropical and sub tropical C4 species could shift northwards. This would expose temperate zone agriculture to previously not known, aggressive tropical colonizers (Parry 1998), particularly C4 grasses.

Impact of Climate Change on Crop and Weed Interactions The agro-ecosystem must be understood as a

multitrophic system with human interference. For the farmer, the crop is the centre of this ecosystem, and for ecologists the plant is the food basis or primary producer for an entire food web (Price 2002). Crop plants live in a very complex ecosystem. They live in competition with neighboring plants including weeds. Both are supported and/ or attacked by viruses, bacteria, fungi, insects, mites, spiders, amphibian, birds, mammals etc. All of these species interact with each other. Pimentel (2009) estimates that globally 70,000 pest species, including 9,000 insect and mites, 50,000 plant pathogens and 8,000 species of weed exist. In addition, each ecosystem also depends on its non-living (abiotic) environment like soil, water, climate, and micro-climate. Small changes might have large impacts for the individual plant/ animal, which are not seen or understood by us. Why, for example, is one plant infested by aphids, but not the neighboring plant? Climate change will have an impact on our ecosystems, which we will never fully comprehend. The ability of current science to make predictions about the impact of global changes on ecosystem interactions is limited, because models that include multiple interactive effects of global change are still relatively rare (Emmerson et al., 2004). Ecosystems are very complex and our society changes many parameters (land use, land management, climate, and air quality) at the same time. We need to be very cautious when making predictions for the real world by basing our findings on laboratory experiments and computer models. However, science is of course not useless, it shows trends and directions, and good science always discusses limitations and results. Weeds compete with crops over nutrients, water and light and can considerably reduce yields and crop quality. In some cases weeds can pose a human health problem (poisonous plants, allergens) or inhibit harvest. Elevated CO2, changes in temperature and precipitation patterns may affect weeds as much as crops. Higher CO2 will stimulate photosynthesis and growth in C 3 weeds and C 3 crops (C 4 plants account for a small fraction of the total number of plant species (less than 1.000 out of 250.000) (Elmore and Paul 1983) and reduce transpiration and increase water use efficiency in both C3 and C4 weeds and crops. Higher temperatures can possibly offset some of the benefits of elevatedCO2 for both, weeds and crops. High temperatures sometimes limit reproductive development and global warming may decrease reproductive output in such situations despite an increase in CO2. It is unclear whether this is more likely to occur in C3 than C4 species, but if it were, it could alter weed community compositions and affect crop/weed interactions (Bunce and Ziska 2000). This would imply that weed and crops both benefit or lose on the same scale. However, weeds are usually already very competitive due to greater genetic variation and physiological plasticity, otherwise they would not cause yield losses. Hence they may gain more advantages from climate change than crops.

Combination of Crops and Weeds and their Response to Climate Change In temperate regions, global warming will affect the growth and marginally affect phenology, and influence the

PAGARE et al., Weeds : An Introduction and their Responses to Climate Change

geographical distribution of weeds. Weed species of tropical and subtropical origins, currently restricted to the southern regions, may expand northward (Patterson 1995). However, since climatic change, especially increased CO2 affects C3 and C4 plants differently, and different combinations must be investigated separately: •

C4 weeds in C3 crops,

•

C3 weeds in C3 crops,

•

C3 weeds in C4 crops and,

•

C4 weeds in C4 crops.

When solely looking at the benefit of elevated CO2 it would be possible to argue that C4 weeds such as barnyard grass (Echinochloa crus-galli) and redroot pigweed (Amaranthus retroflexus), which do not react to elevated CO 2 with more biomass production would be less competitive than C3 crops which grow better under increased CO2. And vice versa: in C4 crops like millets, sorghum, maize and sugarcane C4 weeds may become less competitive than C3 weeds. According to Holm et al. (1977), 14 of the world’s worst weeds are C 4 plants, while around 76% of the harvested crop area in 2000 were grown with C 3 crops (Monfreda et al., 2008). If the hypothesis is right that C3 crops would benefit more from elevated CO2 than C4 weeds, losses due to C4 weeds might decrease. In the early 1980s, experiments were conducted to prove this kind of hypotheses (Patterson and Flints 1980) and basically the hypothesis was supported (Coleman and Bazzaz 1992; Ziska 2003). However, more research has been done manipulating CO2 concentrations alone. Temperature increase or drought in combination with elevated CO2 was less investigated (Fuhrer 2003; Bunce and Ziska 2000). When including temperature increase, trends are not clear, and will depend on the local conditions. Optimal temperatures for growth in C4 plants are generally higher than optimal temperatures for C3 plants (Flint and Patterson 1983), but with higher CO2 the optimum temperature of many C3 plants also increases (Bunce and Ziska 2000). However, looking at photosynthesis and temperature alone might be insufficient. (Tang et al., 2009) recently showed that barnyard grass (Echinocloa crusgalli) in combination with a mycorrhiza also benefits from elevated CO2 levels. In drought situations C4 weeds might also have advantages over C3 crops under elevated CO2 (Ward et al., 1999). The benefit of elevated CO2 under sufficient water condition will lead to higher C3 weed competitiveness in C4 crops. An experiment with Sorghum, and a C3 and C4 weed showed what the potential implications increased CO2 level may have on the crops. Under ambient CO2 the presence of the C3 velvetleaf (Abutilon theophrasti [Medicus]) had no significant effect on either sorghum seed yield or total above ground biomass; however, at elevated CO 2, yield and biomass losses were significant. The additional loss in sorghum yield and biomass was associated with a threefold increase in velvetleaf biomass in response to increasing CO2 (Ziska 2003). Elevated CO2 alone might not only lead to an increase of pure biomass of C3 weeds. McPeek and Wang (2007) showed that dandelion (Taraxacum officinale)

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produced more fertile seeds and eventually larger seedlings. However, C 4 crops might out-compete better growing C3 weed in drought situations, and at higher temperatures utilizing mycorrhiza (Tang et al., 2009). Logic would imply that the same type of plants (with regards to photosynthesis) in the same ecosystem would react to changes in a similar way. This is only partly true, while C 3 crops and C 3weeds, both benefit from elevated CO2 it seems that the magnitude varies. Stimulation of biomass accumulation from CO2 doubling was estimated by one research team to be +31% in wheat, +30% in barley, +27% in rice, +39% in soybean, +57% in alfalfa, and +84% in cotton. In contrast, a survey of experimental results on 27 non-crop C3 species revealed that biomass accumulation increased from 79% to 272% compared to CO2 ambient (Patterson 1995). An experiment, which investigated seven C3 crop and three C3 weeds at 350ppm and 700ppm CO2 showed similar growth rates and mass of C3 crops and C3 weeds (Bunce 1997). Since all C4 plants (weeds and crops) have the same photosynthesis path they may react to changes in the same ecosystem in a similar way. However, research on impact of climate change in this combination has not been done.

Weed/Crop Competition will be Altered by Climate Change The differential responses of C 3 and C 4 plants to increasing CO 2 are especially relevant to weed-crop competition in agro-ecosystems. However, studies on competition outcomes between C3 crops and C4 weeds, or vice versa, are limited in the literature. In general, elevated CO2 levels would stimulate the growth of major C3 crops of the world; the same effect is likely to also increase the growth of the both C3 and C4 weeds. In all probability, this would lead to increased weed-crop competition, negating some of the otherwise beneficial effects of CO2 ‘fertilization’ of the C3 crops and their yields. Carter and Peterson (1983) found that festuca elatior L., a C3 grass, out-competed Sorghum halepense (L.) Pers., a C 4 grass, in mixed culture, under both ambient CO2 levels and elevated CO 2, even under temperature unfavourable to C 3 photosynthesis (Between 25 and 40 0C). The author predicted that global CO 2 enrichment would alter the competitive balance between C3 and C4 plants and this may affect seasonal nich separation, species distribution patterns and net primary production within mixed communities. Ziska (2000) evaluated the outcome of competition between ‘Round-up Ready’ Soybean (Glycine max L.) and a C 3 weed (Common Lanbsquarter, Chenopodium album L.) and a C 4 weed (Redroot Pigweed, Amaranthus retroflexus), grown at ambient and enhanced CO2 (ambient+250 ìL L-1). In a weedfree environment, elevated CO 2 resulted in increased soybean growth and yield, compared to the ambient CO2 condition. However, soybean growth and yield were significantly reduced by both weed species at both levels of CO2. With lambs’ quarter, at elevated CO2, the reduction in soybean seed yield relative to the weed-free control increased from 28-39%. Concomitantly, the dry weight of

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lambs quarter increased by 65%. Conversely, for pigweed, soybean seed yield losses diminished with increasing CO2 from 45 to 30%, with no change in weed dry weight. This study suggests that rising CO2 could alter yield losses due to competition from weeds, and that weed control will be crucial in realizing any potential increase in the yield of crops, such as soybean, as climate change occurs. (Alberto et al., 1996), studied competition outcomes between rice and Echinochloa glabrescens L., which is a C4 weed, using replacement series mixture at two different CO 2 concentrations (393 and 594 ìLL-1 ) under day/night temperature of 27/210 C and 37/290 C. Increasing the CO2 concentration, at 27/210 C, resulted in a significant increase in above ground biomass (+47%) and seed yield (+55%) of rice, averaged over all mixtures. For the C4 weed, higher CO2 concentration did not produce a significant effect on biomass or yield. When grown in mixture , the proportion of rice biomass increased significantly relative to that of the C 4 weed in all mixture at elevated CO 2 indicating increased ‘competitiveness’ of rice. However, under elevated CO2 level and the higher temperature regime, competitiveness and reproductive stimulation of rice was reduced compared to the lower growth temperature, suggesting that while a C3 crop like rice may compete better against a C 4 weed at elevated CO2 alone, simultaneous increase in CO2 and temperature could still favor a C 4 species. As systems for simulating future atmospheric composition and/or climates have become more sophisticated over greater spatial scales, is it fair to state that the results from such newer Systems represent (or are closer to representing) the ‘true’ response of crop yields to projected changes in CO2 /climate? In a recent hypothesis first published in the Philosophical Transactions of the Royal Society (Long et al., 2005), and later reiterated in. Science, (Long et al., 2006). New and innovative strategies to simulate and control a range of projected environments using methodologies that include control of abiotic/biotic uncertainties over large spatial scales are crucial in bettering our understanding of the underlying plant processes likely to be affected by projected changes in CO 2/climate. Understanding of such processes, in turn, will allow us to extrapolate experimental data to improve model scenarios for a wide range of crop yields with respect to global climate change.

CONCLUSIONS The growing literature suggests that a lot has been predicted about the future climate, a lot has been worked out regarding its impact on the agro ecosystem and still there is lot that is hidden behind the curtain. Since the starting of cultivation/agriculture farmers are facing the various nature calamities to their crops and they themselves find out the way to overcome, indeed they are the primary innovator/ experimentalists/ modelers. As we know farmers have find out the treatment for biotic stress (pests/ disease/ weeds) but abiotic stress i.e. climate change beyond their understandings. Most stress has been given over the weeds

and scientists have suggested various methods for its management by understanding weeds biology its persistence and its superiority over crops. Regarding the climate change, suggested by the scientist because of anthropogenic activity (elevation CO2) we are helpless as has been becoming a non modifiable factor; until we depends on the bio fuel for our need.

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Received on 10-03-2017

Accepted on 16-03-2017