What do we really know about alien plant invasion? A

Planta DOI 10.1007/s00425-016-2510-x

REVIEW

What do we really know about alien plant invasion? A review of the invasion mechanism of one of the world’s worst weeds Ali Ahsan Bajwa1,2 • Bhagirath Singh Chauhan2 • Muhammad Farooq3,4,5 Asad Shabbir6 • Steve William Adkins1,2

•

Received: 13 January 2016 / Accepted: 26 March 2016 Ó Springer-Verlag Berlin Heidelberg 2016

Abstract Main conclusion This review provides an insight into alien plant invasion taking into account the invasion mechanism of parthenium weed (Parthenium hysterophorus L.). A multi-lateral understanding of the invasion biology of this weed has pragmatic implications for weed ecology and management. Biological invasions are one of the major drivers of restructuring and malfunctioning of ecosystems. Invasive plant species not only change the dynamics of species composition and biodiversity but also hinder the system productivity and efficiency in invaded regions. Parthenium weed, a well-known noxious invasive species, has invaded diverse climatic and biogeographic regions in more than 40 countries across five continents. Efforts are under way to minimize the parthenium weed-induced environmental, agricultural, social, and economic impacts. However, insufficient information regarding its invasion mechanism & Ali Ahsan Bajwa [email protected]; [email protected] 1

School of Agriculture and Food Sciences, The University of Queensland, Gatton, QLD 4343, Australia

2

The Centre for Plant Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Toowoomba, QLD 4350, Australia

3

Department of Agronomy, University of Agriculture, Faisalabad 38040, Pakistan

4

The UWA Institute of Agriculture, The University of Western Australia, Crawley, WA 6009, Australia

5

College of Food and Agricultural Sciences, King Saud University, Riyadh 11451, Saudi Arabia

6

Department of Botany, University of the Punjab, Lahore 54590, Pakistan

and interference with ecosystem stability is available. It is hard to devise effective management strategies without understanding the invasion process. Here, we reviewed the mechanism of parthenium weed invasion. Our main conclusions are: (1) morphological advantages, unique reproductive biology, competitive ability, escape from natural enemies in non-native regions, and a C3/C4 photosynthesis are all likely to be involved in parthenium weed invasiveness. (2) Tolerance to abiotic stresses and ability to grow in wide range of edaphic conditions are thought to be additional invasion tools on a physiological front. (3) An allelopathic potential of parthenium weed against crop, weed and pasture species, with multiple modes of allelochemical expression, may also be responsible for its invasion success. Moreover, the release of novel allelochemicals in non-native environments might have a pivotal role in parthenium weed invasion. (4) Genetic diversity found among different populations and biotypes of parthenium weed, based on geographic, edaphic, climatic, and ecological ranges, might also be a strong contributor towards its invasion success. (5) Rising temperatures and atmospheric carbon dioxide (CO2) concentrations and changing rainfall patterns, all within the present day climate change prediction range are favorable for parthenium weed growth, its reproductive output, and therefore its future spread and infestation. (6) Parthenium weed invasion in South Asia depicts the relative and overlapping contribution of all the above-mentioned mechanisms. Such an understanding of the core phenomena regulating the invasion biology has pragmatic implications for its management. A better understanding of the interaction of physiological processes, ecological functions, and genetic makeup within a range of environments may help to devise appropriate management strategies for parthenium weed.

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Keywords Biological invasion
Climate change
Ecology
Parthenium hysterophorus L.
Weed management Abbreviations Cd Cadmium Co Cobalt CO2 Carbon dioxide cp-DNA Chloroplast DNA Cr Chromium Cu Copper ISSR Intersimple sequence repeats ITS Internal transcribed spacer K Potassium N Nitrogen Na Sodium NaCl Sodium chloride Ni Nickel P Phosphorus Pb Lead ppm Parts per million RAPD Randomly amplified polymorphic DNA ROS Reactive oxygen species USA United States of America Zn Zinc

Introduction Parthenium weed (Parthenium hysterophorus L.) is one of the most problematic invasive plant species across the globe (Adkins and Shabbir 2014; Tanveer et al. 2015). It

Fig. 1 A worldwide map of parthenium weed distribution. Parthenium weed is invasive in the countries shaded or circled in red. Blue-shaded countries represent transient populations of the weed in the Europe. Countries shaded green are considered to be within its native range (map prepared by A. Shabbir)

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has invaded vast areas in several countries across five continents and continues to spread rapidly (Fig. 1). It is a member of the Asteraceae family and has been recognized as a serious threat to environmental safety, agricultural sustainability, human and animal health, and ecosystem integrity in many developing and developed countries (Kohli et al. 2006). Having a neo-tropical origin, this invasive weed has spread to almost all the continents either accidently or due to unchecked trade and transportation across borders (Adkins and Navie 2006; Adkins and Shabbir 2014). Since its introduction in many parts of the world, parthenium weed has invaded large areas under variable management practices and geographic climatic conditions (Shrestha et al. 2015). Parthenium weed is not only responsible for the degradation of grasslands, pastures, peri-urban landscapes, and wastelands but has also infested numerous cropping systems in different countries (Tamado and Milberg 2004; Shabbir and Bajwa 2007; Dhileepan 2009; Tadesse et al. 2010; Shabbir et al. 2013; Safdar et al. 2015). It affects the biodiversity and species richness in its invaded range (Timsina et al. 2011). The suppression of forage and grain crops due to competitive and allelopathic interference has reduced livestock production and challenged the sustainability of many farming systems (Chippendale and Panetta 1994; Hanif 2014; Belgeri and Adkins 2015). The direct hazards to human and livestock health further exacerbate the situation (Javaid et al. 2007; Nasim and Shabbir 2012). Deleterious impacts of parthenium weed on numerous cropping systems, the socio-economic setup, and native ecosystems are well known (Bajwa et al. 2004; Kohli et al. 2006; Shabbir and Bajwa 2006; Batish et al. 2012; Aslam et al. 2014; Safdar et al. 2015). The magnitude of the problem caused by parthenium weed has been exclusively

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attributed to its rapid and effective invasion mechanism (Adkins and Navie 2006; Dhileepan 2012; Tanveer et al. 2015). However, a detailed account of its invasion mechanism and associated features is lacking. Effective management strategies will require a clear understanding of the invasion mechanism of parthenium weed. Moreover, information about the interaction and responses of the weed with several biological and physical constraints in invaded regions is also required for a better ecological understanding. In this review, all established, presumed, and possible elements to the invasion mechanism of parthenium weed are discussed. Interplay of multiple coexisting biotic and abiotic processes governing the dynamics of parthenium weed invasion at different tiers of the ecosystem are analyzed. For instance, the interaction of morpho-physiological attributes of parthenium weed, the ecological diversity, and biogeographic variations are discussed to better understand the overlapping components of the invasion mechanism. Concerning the eco-biological interface, parthenium weed has several characteristics including phenotypic plasticity (Navie et al. 1996, 2004), an efficient reproductive and seed dispersal mechanism, seedbank persistence (Navie et al. 1996), efficient resource acquisition mechanisms (Tamado et al. 2002a; Annapurna and Singh 2003), biological life cycle plasticity (Adkins and Shabbir 2014), allelopathic suppression of coexisting species (Batish et al. 2007; Singh et al. 2003; Aslam et al. 2014), and tolerance to a range of abiotic stresses (Sharma et al. 2014), which make it a remarkable invader. In addition to reviewing the mechanism for parthenium weed invasion and its interaction with environmental signals, a detailed account of the potential research gaps will be presented. For instance, the detailed investigation of the allelopathic potential of parthenium weed in the invaded than its native range may lead to confirmation or rejection of the ‘‘novel weapon hypothesis’’ for its invasion (Callaway and Aschehoug 2000). As parthenium weed invasion is likely to increase in the future (McConnachie et al. 2011; Shabbir et al. 2014; Khan et al. 2015), preventive measures should be adopted to protect land areas predicted to be suitable for its invasion. The contribution of genetic diversity in the weed’s invasion mechanism is another possibility, but the extent of genetic variation is yet to be determined (Tang et al. 2009; Hanif et al. 2012; Hanif 2014; Jabeen et al. 2015). The remarkable invasive potential of parthenium weed is beyond doubt; however, an understanding of all the possible distinct and overlapping biological, physiological, genetic, and ecological processes contributing towards its invasion mechanism is crucial. Here, all these perspectives are discussed in context to spatial and temporal changes at global level.

Eco-biological perspective Certain biological features of an invasive plant species have a very close relationship to their ecological behavior and adaptive success (Sakai et al. 2001). Parthenium weed is one of the most successful invader in terms of having a unique biology (Annapurna and Singh 2003; Adkins and Shabbir 2014; Shrestha et al. 2015). The superior botanical features make it perfect to sustain and adapt to a wide range of biogeographic conditions. Similarly, other features and/or responses of biological, ecological, and ecobiological nature play a vital role in parthenium weed invasion. Morphological attributes Parthenium weed is an annual or short-lived perennial herbaceous plant with an erect growing habit and a tap root system providing strong soil anchorage. It has a unique set of botanical features, which makes it highly invasive (Table 1; Fig. 2). A unique rosette growth habit, an erect and rigid stem, a tall stature, multiple branching, high leaf production, presence of trichome hairs on leaves and stems, and an extensive flowering capacity resulting in a massive seed production, all play an important role in its successful growth and adaptability (Navie et al. 1996; Adkins and Shabbir 2014; Tanveer et al. 2015). The rosette stage is an important growth stage of parthenium weed, with the plant adjusting the timing of all subsequent stages according to the prevailing environmental conditions which in return is thought to aid invasion (Adkins and Shabbir 2014). In areas where winter temperature drops to a sub-zero levels, the rosette stage also helps juvenile plants that develop in late autumn to over-winter. Although all these morphological characteristics enable parthenium weed to be an aggressive invasive species, the relative contribution of each trait is not known. In a recent review, Adkins and Shabbir (2014) proposed that the superior morphology plays a key role in its invasiveness along with other ecological and physiological mechanisms. Previously, Hanif et al. (2012) also stated that strong morphological characteristics provide a significant contribution towards invasiveness across different parthenium weed populations. Nevertheless, this is not the only set of attributes contributing to invasiveness. Biological life cycle plasticity Parthenium weed is a resilient plant that can grow in wide range of soils and climatic conditions. In general, it has the characteristics of an annual plant but may behave more like a short-lived perennial in certain locations (Shrestha et al. 2015). It may complete vegetative growth within

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Planta Table 1 Morphological attributes of parthenium weed contributing towards its invasiveness Attribute

Function/mechanism facilitated

Impact

References

Tap root system

Involved in deep penetration in wide range of soil profiles

Provides strong anchorage, water and nutrient uptake, stress tolerance, and regrowth ability

Gnanavel (2013), Shrestha et al. (2015)

Rosette growth stage

A resting phase, possibly C4 photosynthesis

Seedling chocking of other species emerging after parthenium weed, flexible growth, protection from stressful environments

Adkins and Shabbir (2014)

Erect stem growth

Upright vertical growing stature facilitating competition with other species

Competitive advantage over creeping or horizontally growing herbs, improved ability to spread seeds

Adkins and Shabbir (2014)

Angular woody stem

Able to withstand mechanical injury and to support a large canopy

Resist any physical damage, Increase chances to complete the life cycle due to vigorous growth, aids flower and seed production

Tanveer et al. (2015)

Tall stature

Suppression of understory or coexisting plants through shading, space occupation, and senescence

Improved growth and better light capture, aids flower and seed production and seed dispersal

Dhileepan (2012)

Trichomes/ pubescence

Tolerance to herbivory, reduces transpiration losses, and physical damage

Avoid loss of green leaves during active vegetative growth, Improve physiological efficiency

Hanif et al. (2012)

Multiple branching Extensive flowering

Improved plant canopy structure and indirectly enhances leaf area Clustering of inflorescences enables plants to produce a large number of seeds

Captures most of the available resources due to extensive branching and leaves Large numbers of flowers ensures the full expression of the reproductive potential and large number of viable seeds sets to produce another generation of invasive plants

Navie et al. (1996)

Seeds with special features

Small but numerous, compact, equipped with appendages, light weight, persistent, and protected seeds with variation in color helps further fast dispersal and successful reproduction

Large numbers of seeds, easily dispersed by many vectors, ability to be buried easily, and ability to create large seedbanks that persist in the soil. Probably avoids predation

Navie et al. (1996), Tamado et al. (2002a); Kohli et al. (2006)

1–2 months after germination and may even sustain the reproductive growth phase for up to 3–4 or even 6–8 months, depending upon soil moisture, temperature, and relative humidity conditions (Gnanavel 2013; Tanveer et al. 2015). Although typically it is a warm season plant, it can expand or contract its life span, with the relative duration in any growth phase depending upon the environmental conditions (Navie et al. 1996; Adkins and Shabbir 2014). After germination, it can stay in the rosette stage for variable periods of time and similarly, after entering into the reproductive phase, it can prolong or reduce flowering time as well as the time for seed set (Adkins and Shabbir 2014). Due to such a flexible growth habit and life span, it can prevail under diverse conditions, especially within its introduced range (Tanveer et al. 2015). In a recent study from Nepal, Shrestha et al. (2015) reported that parthenium weed can be seen throughout the year in the Kathmandu valley, and at any given time at different stages of growth. A similar kind of growth pattern has also been observed in other parts of its introduced range including Pakistan (Adkins and Navie 2006; Javaid et al. 2007; Shabbir and Bajwa 2007), Bangladesh (Masum et al. 2013), India (Kohli et al. 2006), and to a lesser extent

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Chippendale and Panetta (1994)

Australia (Navie et al. 1996, 1998; Adkins and Shabbir 2014). Seed biology and dispersal Seed ecology plays a key role in the determination of the extent and size of many weed problems (Chauhan and Johnson 2010; Bajwa et al. 2015). Parthenium weed only reproduces via seeds and has great ability to produce a large number of seeds (up to 25,000 seeds per plant under ideal conditions; Navie et al. 1996). The time over which seeds are produced may range from 30 to 230 days after emergence depending upon temperature, soil moisture and other environmental conditions. Seed germination is also possible under a wide range of temperatures (9–35 °C), but the optimum temperature range is 21–25 °C (Navie 2002). Williams and Groves (1980) found that a 21/16 °C (day/ night) thermoperiod was best for its germination. Tamado et al. (2002a) reported that parthenium weed seeds were able to germinate at a wide range of fluctuating temperatures between 12/2 °C and 35/25 °C under illuminated conditions. Parthenium weed seeds can also germinate at soil moisture levels as low as 40–60 % of field capacity

Planta Fig. 2 Morphological features of parthenium weed making it a resilient invasive species: a pubescent leaves right from the seedling stage providing protection against insects and herbivores, b rosette stage enabling the plant to sustain harsh environmental conditions and to suppress the understory vegetation, c large number of expanded green leaves contributing to vigorous growth and containing allelochemicals, d hairy angular stem provides defense and upright stature to the plant, e multiple prominent flowers being produced over expanded period of time enables flower production under almost any conditions, and f large number of seeds produced on well-exposed inflorescence contributes to a higher reproduction and efficient seed dispersal (photos by A. A. Bajwa)

(Tanveer et al. 2015). Parthenium weed seeds may also exhibit innate dormancy, which may be removed by leaching of germination inhibitors from the fruit and testa tissues. Navie et al. (1998) found that seeds buried to a depth of 5 cm for 24 months only reduced viability by 30 % and the predicted half-life of seed bank was between 5 and 7 years. However, the quality of the seed when buried also affects the seedbank persistence (Nguyen 2011) with high-quality seed living longer than poor-quality seed. Tamado et al. (2002a) reported over 50 % viability after 26 months burial of parthenium weed seeds at a 5 cm depth. Navie et al. (2004) described the parthenium weed

seedbank at two sites in Central Queensland, Australia, to be large and consistent over time. In India, Joshi (1991) reported that its seedbank may be up to 200,000 seeds ha-1 within abandoned fields. Seedbank persistence is an important feature contributing toward invasiveness and further field-based studies with a special focus on this parameter are needed. The ability of parthenium weed to germinate and grow in a range of soil types is another distinct feature, aiding its vigorous invasion (Navie et al. 1996; Timsina et al. 2011; Adkins and Shabbir 2014). These features have important ecological implications also. Many invasive species

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introduced into disturbed land sites, quickly adjust, and then further proliferate. As suggested by McFadyen (1992), parthenium weed is an aggressive colonizing species of disturbed land sites in Australia. Parthenium weed seeds may be dispersed by domestic and feral animals, by vehicles and farm machinery, with the transportation of crop seeds and fodder, within irrigation and flood water, and by wind (Chippendale and Panetta 1994; Navie et al. 1996). Nguyen (2011) found parthenium weed seeds to be a dominant component within wash-down facilities used to clean vehicles of weed seeds in Central Queensland. Some human activities including the use of parthenium weed as an ornamental plant, placing it into floral bouquets, using its vegetative and reproductive parts as packaging, and its use as a green manure, have all contributed to its dispersal in the introduced range (Dhileepan 2012). Competitive ability Parthenium weed is an aggressive invader with a vigorous growth habit and is often stated as having a strong competitive ability (Gnanavel 2013; Adkins and Shabbir 2014). It may exhibit this competitiveness in grasslands, cropped areas, forests, and native ecosystems, depending upon where it is invading (Evans 1997; Kohli et al. 2006). Parthenium weed infestation in several cropping systems across the globe is threatening crop production, food security, and agricultural sustainability (Tanveer et al. 2015). The rapid and early emergence of parthenium weed as compared with crop plants is a major reason for its competitive ability (Tanveer et al. 2015). It has become an important weed in major field crops in more than forty different countries (Fig. 3). In Australia (Central Queensland), parthenium weed has caused substantial yield losses to sorghum (Sorghum bicolor L.) and sunflower (Helianthus annuus L.) crops due to its strong competition for resources (Parsons and Cuthbertson 1992). Similarly, up to 40 % losses in pastures in Australia have been reported (McFadyen 1992). In India, yield losses were 40 and 90 % in grain crops and pastures, respectively (Khosla and Sobti 1979; Nath 1981). Tamado et al. (2002b) reported up to 97 % yield loss in grain sorghum due to parthenium weed infestation at a lowland site. Even a very low parthenium weed density of 3 plants m-2 caused 69 % yield loss, which again highlighted its strong competitiveness. In a recent study from Pakistan, Safdar et al. (2015) reported that an increase in parthenium weed density from 0 to 20 plants m-2 in a maize (Zea mays L.) crop resulted in 145 and 433 % increase in relative competition index and parthenium weed biomass, respectively. Such a competitive profile shows another possible reason behind the successful invasion of parthenium weed.

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Release from natural enemies According to the popular ‘‘enemy release hypothesis’’ of biological invasion (Keane and Crawley 2002), many invasive species flourish and establish widely in non-native regions because of the absence of their natural enemies that exist in their native range (Andonian and Hierro 2011). Parthenium weed has numerous insect pests and fungal pathogens that keep its population low in its native range (Dhileepan 2009). Some important insect herbivores of parthenium weed existing in the native range include the Mexican beetle (Zygogramma bicolorata Pallister), the stem-galling moth (Epiblema strenuana Walker), and the seed-feeding weevil (Smicronyx lutulentus Dietz) (Fig. 4). Some rust species have also been reported to be significant pathogens within its native range (Shabbir 2012). However, none of these natural enemies exist in its invaded range, and as a result it has established well in the absence of these agents. So, the escape from natural enemies (e.g., insect herbivores and plant pathogens) is likely to be another strong reason for parthenium weed invasion. While struggling to manage this emerging weed in the 1970s, Australian weed scientists embarked upon a search for effective natural enemies in its native range (Central and South America) (Shabbir 2012). A variety of arthropods and fungi natural enemies were found. However, only nine insects and two fungi were found to be suitable to Australian conditions and were subsequently released as classical biological control agents after rigorous evaluation (McClay 1980; Evans 1983, 1987). Even with good success, only a few countries (Ethiopia, India, Kenya, South Africa, Tanzania, Vanuatu) have followed Australia and introduced biocontrol agents against parthenium weed (Dhileepan 2009; McConnachie 2015). The Mexican beetle, stem-galling moth, and the stemboring weevil (Listronotus setosipennis Hustache) have shown the greatest effect against parthenium weed so far (Dhileepan and McFadyen 2001; Dhileepan 2009; Shabbir et al. 2013) while two biocontrol agents have been unable to adopt or reach the effective population sizes in Australia.

Physiological perspective Certain morphological features are known to relate to certain superior physiological features of parthenium weed (Kohli et al. 2006; Javaid et al. 2007). In the following sections, these physiological features that relate to parthenium weed invasion are discussed.

Planta Fig. 3 Parthenium weed infestation in different field crops causes substantial yield losses in different countries. Parthenium weed growing in a maize, b sugarcane, c wheat (Triticum aestivum L.), d Egyptian clover (Trifolium alexandrinum L.) and e watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] crops in Pakistan, f in sorghum in Ethiopia, and in g rice (Oryza sativa L.) and h peanut (Arachis hypogaea L.) fields in China [photos by A. Shabbir (a–e) and SW Adkins (f–h)]

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Planta Fig. 4 Natural enemies of parthenium weed in its native range but missing in the introduced range, a, b Mexican beetle, c, d stem-galling moth, e seed-feeding weevil, and f winter rust [Puccinia abrupta var partheniicola (Jackson) Parmelee]. Escape from such enemies in its introduced range is one of the major causes of parthenium weed invasion (photos by L. Strathie, K. Dhileepan, S. Navie and S. W. Adkins)

Photosynthesis Parthenium weed is able to compete well with neighboring vegetation as it has an efficient photosynthetic system. The plant is C3/C4 intermediate and, therefore, has lower photorespiration losses than neighboring C3 plants, even under harsh, arid climates (Tang et al. 2009). Parthenium weed rosette leaves possess a C4 Kranz anatomy and this is in contrast with its close Asteraceae relatives mariola (Parthenium incanum Kunth) and guayule (Parthenium argentatum A. Gray), which utilize the C3 mode of photosynthesis in all stages of growth (Tang et al. 2009). Being C4 photosynthesis in its rosette leaves, the CO2 compensation concentration is lower (20–25 lL L-1) in parthenium weed than that seen in mariola and guayule

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(55 lL L-1), which might help parthenium weed to tolerate heat and strong light conditions, which are often encountered at this early stage of growth (Moore et al. 1987). In addition, it has a lower sensitivity to oxygen concentration, which is also beneficial for photosynthetic efficiency (Moore et al. 1987). As the adult plant possesses C3 photosynthesis under elevated CO2 concentration, its growth and reproduction is greatly increased (Navie et al. 1996; Shabbir et al. 2014). Stress tolerance Parthenium weed can tolerate abiotic stresses, which enables it to invade, establish, and even flourish under a wide range of stressful conditions (Kohli et al. 2006).

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Tolerance to heat stress through different physiological adaptations is a key attribute of many successful invasive species. Parthenium weed expressed significantly higher amounts of boiling-stable antioxidant enzymes, peroxidase, and superoxide dehydrogenase upon exposure to high, rather than low temperatures (Sharma et al. 2014). Higher activities of those heat-expressed proteins were associated with the scavenging of reactive oxygen species (ROS) and, therefore, the better physiological regulation of plant function as well as protection against structural damage (Sharma et al. 2014). So, the physiological adaptation against heat stress may partly explain the invasion of parthenium weed in harsh tropical and sub-tropical environments. Parthenium weed also tolerates relatively high levels of salt stress, which may enable it to invade the salt-affected and coastal areas. Parthenium weed seedlings were able to survive up to a 1 % concentration of sodium chloride (NaCl) without any reduction in growth as compared to the control at 30 and 60 days after sowing (Upadhyay et al. 2013). At 2.0–2.5 % NaCl concentrations, parthenium weed increased seedling proline content by up to 68 % over the control, which aided osmotic adjustment and allowed some plants to survive. Similarly, parthenium weed plants did not show any reduction in chlorophyll content at high salinity level and this explains its physiological ability to cope with salt stress. Khurshid et al. (2012) revealed that parthenium weed had a relatively high salinity threshold in the laboratory (20–30 mM NaCl) and in field (10 dS m-1) as compared to many crop and weed species, which was considered to facilitate its establishment in salt-affected soils. The authors argued that parthenium weed can sustain its growth, chlorophyll content, and mycorrhizal associations in higher salinity than

other local plants. Parthenium weed can also grow under wide range of soil moisture levels. Nguyen (2011) reported that under a low soil moisture condition, one predicted under climate change, parthenium weed’s life span was reduced by up to 43 % while growth was enhanced by 20 % due to modifications in the plants vegetative and reproductive biology. In our on-going study, parthenium weed has shown the potential tolerance to soil moisture stress (Bajwa et al. unpublished data; Fig. 5). Parthenium weed can uptake and accumulate a range of heavy metals, including copper (Cu), cobalt (Co), lead (Pb), nickel (Ni), chromium (Cr), and Zn from soils contaminated with industrial waste (Malik et al. 2010). Parthenium weed was the most successful plant among 16 species tested, in terms of stabilization and extraction of Ni and Pb. Higher bioaccumulation and translocation rates were made possible due to the vigorous growth of parthenium weed plants on the contaminated soils. Hadi and Bano (2009) also found parthenium weed to be a successful bioremediation agent for Pb due to its efficient extraction mechanism. Parthenium weed plants and materials derived from them also showed a high adsorption capacity for cadmium (Cd) and Ni (Ajmal et al. 2006; Lata et al. 2008). The ability to cope with phytotoxic heavy metal stress makes parthenium weed introduction and establishment in disturbed environments more feasible. Parthenium weed is very successful in roadside communities that have become highly polluted with vehicle emissions, and fouled with dust and dirt. Therefore, it is highly likely that the weed possesses strong mechanism(s) to tolerate such conditions of air, soil, and water pollution. So, the pollution tolerance in parthenium weed should be studied to understand the expedited invasion even in highly contaminated soils.

Fig. 5 Parthenium weed seedlings growing at 100 % (left) and 50 % (right) of soil water holding capacity (Bajwa et al. unpublished data). The photographs were taken 10 days after transplanting the seedlings under the two soil moisture regimes in a glasshouse study. Seedlings

performed equally well under moisture deficit conditions at that early growth stage which demonstrates the ability of parthenium weed to endure moderate drought conditions (photos by A. A. Bajwa)

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The ability to tolerate the abiotic stresses through physiological regulation can also be considered as a strong tool for invasion. Evaluation of parthenium weed performance under different abiotic stress regimes may enable us to predict about its invasion potential for different climatic and ecological zones. Soil interactions The role of soil–plant interactions is very important to the success of weed invasion. Parthenium weed has a very unique set of soil interactions which enables it to not only colonize under a wide range of soil types but also thrive well under stress conditions. Soils amended with parthenium weed residues had a higher organic carbon and matter content, hydraulic conductivity, Na and K content, and phenolic acid content as compared with non-treated soil (Batish et al. 2002a). In another study comparing invaded soils, soils in transition of invasion, and the soils non-invaded by parthenium weed, organic matter and N contents were found to be higher in the invaded or transitional soils than in the non-invaded ones (Timsina et al. 2011). The impact of parthenium weed invasion on soil properties was thought to be the mechanism by which alterations in the species composition of grassland communities were occurring. Parthenium weed may alter its growth in response to changes in soil conditions. For instance, it has been reported that a soil with a high clay content could prolong the rosette stage and improve shoot growth, as compared to root growth in parthenium weed (Annapurna and Singh 2003). Parthenium weed seed mass was higher in clay soils than from coarse soils, while the opposite was true for seed production. In another study, parthenium weed showed contrasting physiological responses to pH changes as soluble sugars and peroxidase contents were maximum under slight acidic pH (5) to neutral pH (7) while malondialdehyde and proline contents were minimum in parthenium weed leaves (Liu et al. 2012). However, the opposite scenario was observed at a basic pH of 9. In that way, parthenium weed was able to regulate its growth under changing soil pH levels. Parthenium weed has strong influence upon the soil microorganism abundance and composition due to its allelopathic expression in soil, from its root exudates and leaf leachates and from decomposing residues. For instance, microorganism population in the weed’s rhizosphere was quite different to non-infested soils (Jeyalakshmi et al. 2011). In another study, parthenium weed has been shown to express an antifungal activity against Fusarium solani (Mart.) Sacc., Alternaria alternate (Fries) Keissler, Bipolaris oryzae (Breda de Haan), and many other species (Zaheer et al. 2012; Singh and Srivastava 2013; Bezuneh 2015). In addition, the plant has an

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antibacterial activity against Escherichia coli Migula, Bacillus subtilis (Ehrenberg) Cohn and many other species (Fazal et al. 2011; Bezuneh 2015). The root leachates from parthenium weed were shown to impair biological nitrogen fixation and to inhibit the growth of the nitrogen fixing (Rhizobium phaseoli and Azotobacter vinelandii) and nitrifying (Nitrosomonas) bacteria (Kanchan and Jayachandra 1981). It was established that the growth inhibition of these bacteria was in part due to parthenium weed allelochemicals such as parthenin, caffeic acid, vanillic acid, and anisic acid as these chemicals also caused partial growth inhibition when applied separately. Parthenium weed has the ability to manipulate invaded communities directly (through phytotoxic effects on neighboring plants roots) or indirectly (through inhibiting the growth of beneficial microorganisms), which could make its introduction and establishment possible in diverse edaphic regimes.

Allelopathic perspective Allelopathy is the positive or negative impact of secondary metabolites (allelochemicals) released from a plant which then affect the growth and development of other plants growing in its vicinity (Farooq et al. 2013; Bajwa 2014). Parthenium weed is thought to be one of the most allelopathic weed species and, thus, has the great tendency to suppress the growth of neighboring and competing plants (Aslam et al. 2014). Allelopathic potential Parthenium weed produces a variety of allelochemicals belonging to different chemical classes. In a recent review, Adkins and Shabbir (2014) presented a detailed account of these types of allelochemicals released by parthenium weed and its residues. Parthenin (a sesquiterpene lactone) has been recognized as one of the most common and influential allelochemical released by parthenium weed. In addition to parthenin, parthenium weed produces a range of water-soluble phenolics (caffeic, p-coumaric, vanillic, ferulic, anicic, and fumaric acids), which are the main cause of phytotoxicity of aqueous extracts made from parthenium weed (Kanchan and Jayachandra 1981; Das and Das 1995; Adkins and Sowerby 1996; Pandey 1997; Batish et al. 2002a; Aslam et al. 2014). A variety of other sesquiterpene lactones, flavonoids, and tannins have also been reported to be potential allelochemicals produced by parthenium weed (Tanveer et al. 2015). The scientific literature is also rich with studies reporting the allelopathic potential of parthenium weed against a large number of crops as well as weed species through the

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inhibition of a number of different physiological processes (Mersie and Singh 1987; Tefera 2002; Singh et al. 2003; Bajwa et al. 2004; Biswas et al. 2010; Shabbir and Javaid 2010; Aslam et al. 2014). In addition, parthenium weed has been shown to suppress the germination and growth of several pasture grass species (Belgeri and Adkins 2015). The detailed account of allelopathic suppression of different plant species by parthenium weed has been presented in recent reviews (Adkins and Shabbir 2014; Tanveer et al. 2015). The variable allelochemical concentrations produced in different plant parts had varying degrees of negative impact on other plant species. Moreover, parthenium weed plants from Argentina and Bolivia exclusively contained the sesquiterpene lactone, hymenin, which was different from the more common sesquiterpene lactone, parthenin present in Indian and North American populations (Towers et al. 1977). Such differences in allelopathic chemical composition within biogeographically different parthenium weed populations may have implications for its invasion success.

allelochemicals expression. This variety of allelopathic expression makes parthenium weed ecologically superior and, thus, a successful invader under a wide range of environmental conditions. The role of allelochemicals in the invasiveness of parthenium weed may be limited under some conditions. For instance, the degradation of parthenin in soil under enhanced microbial activity has been shown to reduce the phytotoxicity (Belz et al. 2007). Under these circumstances, only dense populations of parthenium weed, releasing large quantities of parthenin, will express a sustained allelopathic profile due to parthenin in the ecosystem (Belz et al. 2007, 2009). So, the role of allelopathy in parthenium weed invasion is complex and involves the overlapping mechanisms at the interface of competition and population biology. Little is known about relative significance of allelopathy in parthenium weed interference and this aspect requires detailed research.

Genetic perspective Multiple modes of allelochemical expression Parthenium weed releases several allelochemicals from its roots, stem, leaves, fruits, and flowers (Mersie and Singh 1988; Swaminathan et al. 1990; Kohli et al. 1993; Reinhardt et al. 2005; Bajwa et al. 2013). The modes of release of these chemicals are also different, depending upon the surrounding environments. For instance, parthenium weed leaves may release different sesquiterpene lactones, phenolics and volatile allelochemicals through leaching and volatilization (Maharjan et al. 2007; Biswas et al. 2010) when grown in different environments. The phytotoxic activity of leaf extracts was found to be greater than extracts made from all other plant parts due to the higher concentrations of allelochemicals in those extracts (Aslam et al. 2014). The roots of parthenium weed are thought to release different allelochemicals including an array of organic acids into the rhizosphere which in turn have severe negative effects on microbial activity and the growth of other plant roots (Kanchan and Jayachandra 1980; Kumari and Kohli 1987; Valliappan and Towers 1988). The leaching and decomposition of parthenium weed leaf litter and other plant residues is another unique mode of allelopathic expression (Mersie and Singh 1987; Shafique et al. 2013). Allelochemicals from these residues are effectively expressed above ground as well as when incorporated below ground through natural soil disturbance (Batish et al. 2002b; Singh et al. 2003, 2005; Gupta and Narayan 2010; Aslam et al. 2014). Parthenium weed litter suppressed the germination and growth of several crop and pasture species through these multiple modes of

The genetic makeup of any plant species will determine its principle morphological and physiological features and, therefore, have an important role towards its adaptation to the environment. Genetic diversity, regulating ploidy level, and the breeding system used, all play a vital role in the plant invasion process as adaptation to the new environment depends on it (Prentis et al. 2008). Evolution of crosspollination in alien plant species may help them to establish in a non-native environment as this pollination mechanism allows for the production of greater genetic diversity (Coyer et al. 2006). Hanif et al. (2012) reported that two Australian biotypes of parthenium weed, Toogoolawah and Clermont (named after the places of their first introductions), differed significantly in their invasiveness including aspects of the growth habit, floral morphology, and pollination mechanism. The highly invasive Clermont biotype had a very high tendency of cross-pollination as under bagged flowers treatments, it only produced 28 % filled seeds while the less invasive Toogoolawah biotype produced 73 % filled seeds under the same conditions. The variation in parthenium weed reproductive characteristics, especially number of seeds produced, was also associated with the plant size and biomass under natural conditions (Dhileepan 2012). The Clermont biotype of parthenium weed could be more invasive due to its vigorous and rapid growth, tall stature, and its tendency to cross-pollinate. However, such relations between reproductive biology and invasiveness must be tested for further populations keeping in view the biogeographic differences. Exploring the genetic basis for such characteristics will be helpful to understand the actual mechanism.

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High genetic diversity helps plants to establish rapidly in new regions (Sakai et al. 2001). Only a few studies have been conducted to evaluate the role of genetic diversity in parthenium weed invasion. The use of internal transcribed spacer (ITS) markers showed Australian biotypes to be more closely related to Mexican biotypes but significantly different from Indian biotypes (Adkins et al. 1997). The use of randomly amplified polymorphic DNA (RAPD), intersimple sequence repeats (ISSR), chloroplast DNA (cpDNA), and microsatellite markers to assess the genetic diversity of parthenium weed has also been reported in the literature (Adkins et al. 1997; Tang et al. 2009; Qian et al. 2012). The issues of lack of reproducibility, high costs, and the complex nature of genetic studies limit the progress. Following genetic lineages can help in identifying the point of origin of a specific invader. From such studies it has been shown that most invasive species have multiintroductions and each introduction may have different genetic features (Chandra and Dubey 2009). For instance, morphologically similar parthenium weed biotypes in China had contrasting genetic makeup and, thus, were attributed to independent introductions (Tang et al. 2009). Hanif (2014), while working on the genetic diversity of two Australian biotypes of parthenium weed, Toogoolawah and Clermont, reported that the Toogoolawah and the Clermont biotypes were introduced independently in Australia possibly from the northern and southern Texas races, respectively. There was a significant genetic variation between the two biotypes explaining their contrasting morphological and physiological characteristics. The invasiveness of the Clermont biotype was partly attributed to its higher genetic diversification than the Toogoolawah biotype. Recently, Jabeen et al. (2015) studied the genetic structure of 11 populations (95 individuals) from across Pakistan including the two populations (10 individuals) from Australia using ISSR fingerprinting. About 18 % genetic diversity existed among the populations, while 82 % existed within the populations. The genetic diversity was highest among the Pakistani populations; however, gene flow was limited. The genetic heterogeneousity (crosspollination) was attributed to be one cause of parthenium weed invasiveness. It was suggested that such a wide range of genetic variations might have appeared due to multiple introductions into Pakistan.

Climate change perspective Climate change is usually confined to the rising temperature and CO2 levels across the globe. The changes in global climate have significant impacts upon biological invasion. Certain invasive species having a specific set of morphological and physiological traits have been recognized as

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beneficial under a changing climate (Dukes and Mooney 1999). The C3 invasive weed species are supposed to increase their photosynthetic rates, and thus biomass and seed production under an elevated CO2 and temperature regime (Sheppard and Stanley 2014). Parthenium weed has shown the ability to grow excellently under a changing climate. Despite having C4 rosette leaves, the main vegetative part of the parthenium weed is C3 and shows significant improvement in growth and biomass production under a higher CO2 concentration (480 parts per million, ppm) as compared with the ambient concentration (360 ppm) (Navie et al. 2005). It not only explains its invasive potential under the present climatic conditions but also warns about the growing severity of the issue in the future. Parthenium weed has been shown to have an improved photosynthetic rate, water use efficiency, and better growth under a high CO2 concentration (700 ppm) and a temperature of 25–35 °C (Pandey et al. 2003). The increase in photosynthetic rate due to elevated CO2 was compromised by an increase in transpiration rates at very high temperature (47 °C). Parthenium weed biomass was significantly increased at a high CO2 concentration (550 ppm) when it was grown alone and in combination with other grass and legume species (Khan et al. 2015). Recently, Shabbir et al. (2014) also reported a substantial increase of 52, 55, 62, 120, 94, and 400 % in plant height, biomass, branching, leaf area, photosynthesis, and water use efficiency of parthenium weed, respectively, at an elevated CO2 concentration (550 ppm) than at the ambient CO2 concentration (380 ppm). A remarkable increase in parthenium weed growth has been recorded under water-deficit conditions together with high temperature and elevated CO2 levels (Nguyen 2011; Belgeri 2013). So, parthenium weed may become more aggressive and grow more rapidly under elevated temperature and CO2 conditions as well as reduced soil moisture in the future. Predictive modeling studies have been conducted in recent years to estimate the potential spread parthenium weed in different parts of the world and under a changing climate (Shabbir 2012; Kriticos et al. 2015; Mainali et al. 2015). McConnachie et al. (2011) estimated the existing and potential geographical distribution of parthenium weed in Africa using CLIMEX model. It was suggested that subSaharan African countries were the most at risk of parthenium weed invasion in future. The model also predicted that most parts of the Asian-Pacific region and some European countries were at risk including Portugal, Italy, Spain, and France. Later, Shabbir (2012) also predicted through CLIMEX modeling that parthenium weed could invade further parts of Australia, Pakistan, and other South Asian countries especially under a changing climate of ?3 °C. Shabbir (2012) also applied an irrigation scenario

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(winter, 0.5 mm day-1 and summer, 1.0 mm day-1) to the CLIMEX, which predicted that the threat of parthenium weed invasion was higher than expected for southern regions of Pakistan where extra moisture is available through canal water irrigation and flooding in the monsoon season. The frequent flooding in the Indus River basin of Pakistan was determined to be a major cause for further parthenium weed spread in this region. Although the CLIMEX modeling predictions are not precise, they still give a good idea of which landscapes will be at threat from parthenium weed invasion.

South Asia: a case study Parthenium weed has successfully invaded most of the South Asian countries including, Bangladesh, Bhutan, India, Nepal, Pakistan, and Sri Lanka over the past six decades (Fig. 6). Although India is the most affected country, the weed has also spread furiously in Pakistan and Nepal during the last two decades. The most important parthenium weed introduction to India was from seed contained within a wheat seed lot imported from the USA in 1956 (Kohli et al. 2006). The dispersal throughout India and to other neighboring countries is thought to have

happened through the movement of vehicles carrying parthenium weed seed and the trade of contaminated seed lots and livestock (Shrestha et al. 2015). Genetic and ecological studies have indicated that in Nepal and Pakistan, parthenium weed has been introduced on numerous occasions from India (Shabbir and Bajwa 2006; Javaid et al. 2007; Hanif et al. 2012; Jabeen et al. 2015; Shrestha et al. 2015). After invading disturbed landscapes, including noncropped areas, roadsides, and arid regions, parthenium weed starts to spread into agricultural areas, especially those that are irrigated (Tanveer et al. 2015). Appropriate rainfall patterns and temperatures during its growing season, unchecked transportation of contaminated agricultural commodities within and across the landscape, dispersal through floods, suitable genetic adaptations under varying geographical conditions, lack of natural enemies, and lack of management are the primary causes of remarkable invasion of parthenium weed in South Asia (Kohli et al. 2006; Javaid et al. 2007; Tanveer et al. 2015). Due to competition, allelopathic interference, and its role as an alternate host for other plant pests, it has claimed substantial losses to botanical and soil microorganism diversity, crop and pasture yields, ecosystem functioning and human and animal health throughout the region (Table 2).

Fig. 6 The current distribution of parthenium weed in South Asia. The administrative areas within each country shaded in red are those where weed is present (map prepared by A. Shabbir)

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Planta Table 2 Status and impact of parthenium weed invasion in South Asia Country

Introduction and invasion status

Impact

Management strategies prevalent

References

India

Introduced through contaminated food grains imported from USA in 1956, first reported in Pune, Maharashtra state. Some reports also indicated an earlier introduction in the 1800 s. Parthenium weed invasion continued after 1956 and presently it is present in all states of India

Substantial yield losses (up to 40 %) in grain crops. Yield (up to 90 %) and quality reduction of forage crops has also been recorded. Moreover, negative impact on biodiversity, livestock and especially human health. Land value is also significantly affected

Manual uprooting, pre- and post-emergence herbicide application, and mechanical control are common. One biological control agent has also been released and now offering varying degree of control. However, preventive measures, quarantine control, and legislative check are still lacking

Rao (1956), Khosla and Sobti (1979), Nath (1981), Kohli et al. (2004)

Pakistan

First reported in Gujrat district of Punjab province in 1980. Another possible introduction in the 1990s from India is also documented. Almost certainly introduced to Pakistan through road transportation from India. Major expansion in invasion occurred in the 1990s. It has now invaded cropping systems of north, central and even southern parts especially where irrigation water is available

Loss of biodiversity and fodder production in northern parts of country. Substantial yield losses in maize crops have been documented. Other major crops like wheat, cotton, and sugarcane (Saccharum officinarum L.) are also infested. Serious health hazard in the form of dermatitis due to parthenium weed is also recorded

Manual, mechanical, and chemical control is common. Efforts to optimize biological control are possible as one agent has spread from India. Some suppressive fodder species have also been identified for parthenium weed control. Preventive and quarantine measures are still lacking

Razaq et al. (1994), Shabbir and Bajwa (2006), Javaid et al. (2007), Hanif et al. (2012), Khan et al. (2012), Safdar et al. (2015)

Nepal

First reported in Trishuli Valley, Central Nepal in 1967. The open border with India likely resulted in the movement of parthenium weed seeds upon vehicles. Several introduction are suspected during the 1960s. Major expansion occurred in the 1990s. It has invaded urban areas, national parks, grasslands, and some crops

Severe losses to native grassland species due to competition and allelopathic suppression. Deterioration of pastures have also been observed which indirectly is affecting livestock production. Parthenium weed invasion in maize, mustard (Brassica juncea L.), and sugarcane fields have been confirmed with anticipated yield losses

Uprooting, burning, herbicide application, and biological control (one agent naturally spreading into Nepal from India) of parthenium weed are prevalent. Focus has also been diverted towards public awareness and preventive measures. The quarantine laws are yet to be devised and implemented

Mishra (1990), Shrestha et al. (2011, 2015), Timsina et al. (2011)

Bangladesh

First identified in Mymensingh in 2008 presumably introduced from India. It has been found to invade roadsides, alongside railway tracks, and barren lands. Still under primary phase of colonization but now present in about half of the provinces

Suppression of associated grass species and land degradation in many districts of Bangladesh. Crops like onion, cucurbit and potato are also infested Potential health hazards have also been recognized

Public awareness campaign is going on to prevent the spread of weed through vehicles, animals, and flower bouquets. Optimization of cultural and biological control measures is part of current strategies and future endeavors

Akter and Zuberi (2009), Karim (2009), Masum et al. (2013)

Bhutan

The presence of parthenium weed in Bhutan was reported in early the 1990s but exact time and possible mode of introduction are not clear. It has invaded areas at a wide range of altitudes (200–2250 m)

Parthenium weed has been observed to infest roadsides, wastelands, and some perennial crops with varying densities in 12 districts including Punakha, Tongsa, Mongar, and Tashigang

Parthenium weed has been recognized as threat to rich biodiversity of mountainous areas. Information about management practices is lacking at this point in time

Parker (1992), Biswas and Das (2007)

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Planta Table 2 continued Country

Introduction and invasion status

Impact

Management strategies prevalent

References

Sri Lanka

First introduced in 1987 through goats carried by Indian army during military operation. Later on the import of onion and chili seed lots contaminated with parthenium weed seeds from India were also reported. It has invaded the vegetable producing northern province as well as several other provinces

Over 1.3 million ha of vegetable production area in northern province have been infested by parthenium weed. It causes serious yield losses to tomato through competition, allelopathic suppression, and serving as host of tomato leaf curl virus. Health hazards to humans and livestock also exists

Declared as weed of national significance by Sri Lanka Council for Agricultural Research Policy. Herbicide application and cultural operations are common; however, integrated management through herbicides, mulching, and harrowing has also been found successful

Jayasuriya (1999, 2005), Marambe et al. (2001), Bambaradeniya (2002), Dhileepan and Senaratne (2009), Nishanthan et al. (2013)

Countries are arranged in order of invasion magnitude based on the literature available

Predictions about the further spread of parthenium weed in South Asia are also alarming as it will become even more widespread a bigger threat to agricultural sustainability (Shabbir 2012). However, the efforts to understand the invasion process in this part of the world and possible causes are also lacking. The focus on some conventional management strategies without considering the eco-biological mechanisms and serious legislative measures seems to be unaccomplished.

Conclusions and future directions Multiple steps in the mechanism of parthenium weed invasion may operate simultaneously. Strong morphological features and a unique reproductive biology enables parthenium weed to adapt to a wide range of environmental conditions. The superior morpho-physiological traits make parthenium weed a good competitor in its introduced range mainly because of the absence of its natural enemies. An efficient photosynthesis mechanism, tolerance to numerous abiotic stresses, and unique soil interactions are parts of the physiological component of parthenium weed invasion. Parthenium weed plant produces a variety of allelochemicals, and releases them in multiple ways, that helps the weed to suppress neighboring plants. So, allelopathy may be another component of the parthenium weed invasion mechanism. In addition to these morphological and physiological aspects, greater genetic diversity and improvement in growth and reproductive capacity under a changing climate are also thought to be the important phenomena contributing towards its invasion success. In future, research on parthenium weed management may be oriented to explore its invasive biology. Seed ecology of parthenium weed is worth considerable while mapping the broad-spectrum invasion mechanism. The comparative analysis of the germination response for

different biotypes and populations may improve the understanding on the role of germination biology parthenium weed invasion. The dimensions of competition and losses in different crops due to parthenium weed infestation needs to be explored. Further research is required to evaluate the interaction between parthenium weed and its natural enemies in the introduced region. On physiological fronts, the research on response of parthenium weed to abiotic stresses may reveal the actual mechanism behind its successful establishment in disturbed landscapes. The study of positive or negative soil feedbacks for parthenium weed in its native and introduced ranges may also contribute in solving the myth of its invasion mechanism. Furthermore, the possible expression of novel allelochemicals and their potential role in invasion is yet to be evaluated. The role of genetic diversity and mutations in invasion across the different biogeographic conditions is another domain seeking attention of researchers. The mechanisms contributing towards genetic variations under different environmental conditions may also be investigated. Climate change has a direct relation to parthenium weed invasion. Multi-disciplinary research efforts should be made to study the impact of changing climatic conditions on different features of the invasion mechanism. Predictive modeling, associated with geographical surveys and socio-economic studies, may help to explain more precisely parthenium weed invasion patterns. More importantly, the research should be conducted with equal emphasis on all the perspectives so that a clear picture of parthenium weed invasion biology may be developed in the near future. Author contribution statement AAB conceived the idea of the review and prepared the initial outline. SWA, BSC and MF provided feedback to improve the outline. AAB composed the manuscript while SWA, BSC, AS and MF edited and improved the manuscript on technical and

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linguistic grounds. BSC and MF helped to prepare the tables while SWA aided figure preparation. MF and AS helped with the literature review in the South Asian region. AS prepared the maps. Acknowledgments Ali Ahsan Bajwa is thankful to Australian Government and The University of Queensland, Australia for the provision of an International Postgraduate Research Scholarship (IPRS) and UQ Centennial (UQCent) Scholarship, respectively. Authors are thankful to their colleagues: Bharat Babu Shrestha (Nepal), Karma Chophyll (Bhutan), Buddhi Marambe (Sri Lanka), Rezaul Karim (Bangladesh), and Gul Hassan (Pakistan) who helped in provision or confirmation of parthenium weed distribution data in South Asia to develop maps.

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