I am a plant ecologist and have contributed towards understanding various mechanisms through which allelochemicals provide competitive or invasive success to naturalized and/or nonnative weeds. My research on allelochemicals forms the basis of understanding the competitive dominance of weeds and explains the role of species’ life forms, climatic and edaphic factors in the regulation of allelochemical activity. This contribution has helped to determine species interaction at various levels of ecological organization.

Our group has recently discovered two novel mechanisms that explain invasiveness of non-native species: (i) increased competitive ability in their introduced ranges, and (ii) enhanced accumulation of native soil pathogens. Our researches on Chromolaena odorata, one of the world’s most destructive tropical invasive weeds, revealed that its rhizosphere soils accumulate high concentrations of a generalist soil pathogen, thus creating a negative feedback for native plant species which are more sensitive to the pathogens than the invader. These results are novel and unusual in the sense they suggest that the impacts of an invader on other species are due to the exacerbation of biotic interactions that are naturally part of the native ecosystem (Journal of Ecology 2008 96, 58-67).

My international collaboration with researchers in China and Mexico on Ageratina adenophora, a native of Mexico, and a noxious invasive plant in India and China showed that the species has evolved a mechanism of increased N allocation to photosynthesis (growth) and reduced allocation to cell walls, resulting in poorer structural defenses (Proceedings of National Academy of Sciences, USA [PNAS], 2009, 106: 1853-1856). These studies provide first evidence of mechanism behind the commonly observed and genetically controlled enhanced plant growth/ vigor in invasives when introduced to non native ranges. More recently, we provided a substantial evidence for the novel weapons hypothesis and the role of volatile organic compounds in the invasion success of Ageratina adenophora (Ecology, in press).

The research conducted by me and by my group over the years have provided a rigorous test of the allelochemical hypothesis in the invasive success of a non-native species in its naturalized range (Trends in Plant Science 2006, 11, 574-580). We recently described three main pathways for invasion success of species: (i) plant-soil feedbacks in the invaded range; (ii) manipulation of local soil biota by exotic plants, and/or (iii) role of local soil communities in influencing allelopathic potential of novel chemicals (Trends in Ecology & Evolution, 2010 25, 512-519).

While working at the Max Planck Institute for Chemical Ecology, Germany on DFG fellowship, I reported that genetically modified silenced plants can be used in the production of a putative allelochemical, which in turn help in determining whether the emitted chemical functions allelopathically (Planta, 2009). Since some allelochemicals are currently been targeted to produce transgenic allelopathic crops, this study assumes significance in establishing their allelopathic potential before these transgenic crops are commercialised.

My earlier work has extended the research frontier on allelopathic mechanisms and methods in laboratory and field settings, on the role of allelochemicals in ecological systems, on allelopathic interactions in weed-crop and weed-weed settings, and helped to explain the rapid spread of specific exotic alien species. These studies highlight the importance of basic studies of soil ecology in understanding the role of allelochemicals in plant productivity. I have successfully demonstrated the influence of habitat, climate and site on the release and accumulation of phenolics and thus on the phytotoxic potential of Pluchea (Weed Science, 1996, 44:393-396; American Journal of Botany, 1998, 85:64-69; Canadian Journal of Botany, 1996, 74:1445-1450). My research revealed that allelochemicals such as hesperidin, taxifolin 3-arabinoside, phloroglucinol, chlorogenic acid, quercitrin, and formononetin 7-0-glucoside released from roots and foliage of Pluchea lanceolata, a pernicious perennial weed, inhibit the growth of associated crop species by interfering with their physiological and biochemical pathways (Plant and Soil, 1990, 122:298-302; American Journal of Botany 1992, 79: 977-981; American Journal of Botany 1994, 81:799-804; Journal of Chemical Ecology 1991, 17: 1585-1591; Journal of Chemical Ecology 1992, 18: 713-718; Physiologia Plantarum, 1994, 92:571-576). My studies showed that phenolics exuded by the deep root and rhizome systems could enter the ground water (Journal of Chemical Ecology, 1996, 22:1123-1131) and we demonstrated that groundwater from wells in the fields infested with Pluchea with dense subterranean parts, when used for irrigating mustard crop, caused marked inhibition of crop growth. Phenolics released by Pluchea also influence the accumulation and availability of soil nutrients for several crops plants (American Journal of Botany, 1998, 85:64-69). These studies revealed mechanisms for weed competition and explained the role of species, climatic, and edaphic factors in the regulation of allelochemical activity in ecological systems.

For a long time, agronomists had believed that allelopathy is prevalent among perennial weeds. My work on annual weeds such as Polypogon monspeliensis and Stellaria media has shown that allelochemicals play an important role in the growth and establishment of polycarpic annuals. A polycarpic annual weed such as Stellaria media with several life cycles in a single crop season can build up allelochemicals to toxic levels. However, monocarpic annual weeds do not exhibit such a strategy (Plant and Soil, 1995, 173:251-256; Canadian Journal of Botany, 1999, 76:1317-1321).

Weed species are known to prevent regeneration of forest ecosystems. In Canadian boreal forests, Kalmia angustifolia-infested sites had poor conifer regeneration. I investigated the role of Kalmia and Ledum groenlandicum in the regeneration failure of black spruce (Picea mariana) stands in Canadian forests. I observed that Kalmia releases a group of phenolic molecules in the rhizosphere of black spruce and thereby prevents nutrient availability to its seedlings. The strategy followed by Kalmia creates nutrient-poor microsites that lead to black spruce regeneration failure (Acta Oecologia 1999, 20: 87-92; Canadian Journal of Forest Research, 1997, 26: 1899-1904; Plant Ecology 1997, 133: 29-36).

Residues of a number of plant species including the biomass of certain weeds are used for soil amendment in crop fields. Although these weeds do contain chemicals that can inhibit plant growth, the exact mechanism is not known. My research on allelochemical behavior of hairy vetch (Vicia villosa), mugwort (Artemisia vulgaris) and Verbesina encelioides has demonstrated that: (i) growth inhibition of a crop species due to the presence of hairy vetch cannot be attributed to its allelochemicals (Crop Protection, 2001, 20:261-265), (ii) allelopathic chemicals from Verbesina roots are the main cause of its rapid spread to semi-arid regions of northeastern India (Canadian Journal of Botany, 1999 77: 1419-1424), and (iii) chemical changes in amended soil including higher amounts of phenolics are the main factors for growth reduction in areas with red clover (Trifolium repens) (Weed Technology 1999, 13:176-182).

One of the basic questions is to ascertain whether allelopathic effects are due to a single chemical or combined activity of several chemicals. I have probed into this aspect and noted that allelopathy results largely from the interaction and a concerted action of several compounds. (Physiologia Plantarum 2002, 114: 422-428). My research has reemphasized that due to the ecosystem complexities, ecological patterns cannot be explained by a single cause (Plant and Soil 2003 256: 1-11). I have described in detail the conceptual framework of ecological functioning of allelochemicals in my studies (Planta 2003, 217: 529-539).

Our studies on Phalaris minor, an alien weed of wheat fields of northwestern India, has brought to light some serious environmental and economic consequences. Continuous and excessive use of the herbicide (isoproturon) for elimination of this weed has resulted in selection of resistant biotypes of Phalaris minor and as result negatively impacted wheat production. We found that a nontoxic herbicide, isoxaflutole, to be a better alternate herbicide (Plant and Soil, 2004, 258: 161-168). Our work suggested that there is a need to assess and quantify the impact of alien species at several levels of ecological complexity in order to gain influence in their management and control. Farmers often incorporate rice straw (burned or unburned) prior to sowing wheat. My research has established that incorporation of unburned rice straw causes significant damage to seedling growth of next season crop (Canadian Journal of Botany 2004, 82: 161-168). Incorporation of burned rice straw adversely influences the phytotoxicity of isoxaflutole to the weed, Phalaris (Plant and Soil, 2005, 277: 25-30). My recent studies provide initial evidence that rice straw restricts P. minor growth by altering soil chemical and microbiological properties (Biology & Fertility of Soils, 2007 43: 557-563). I have shown that photosystem II herbicides such as propanil could have deleterious effects on phototrophic (cyanobacterial) communities, which are an integral part of rice ecosystem (Paddy & Water Environment, 2010, 8: 277-282).

My laboratory studies have led to vital research insights in the broad field of Ecology and allelopathy and invasion specifically. I have examined experimental complexities in evaluating the allelopathic activities in laboratory bioassays using Chenopodium murale as an experimental system (Soil Biology & Biochemistry 2006, 38: 256-262). One of my important contributions is the understanding of experimental complexities in evaluating phytotoxicity of chemicals with different molecular targets (Environmental & Experimental Botany, 2005, 53: 97-104). Researches led by me with my coworkers have provided cellular evidence of allelochemical interference to mustard seedling growth (Plant Physiology & Biochemistry, 2005, 43: 77-81). We suggested that there is vast potential of both increasing the specificity of herbicides by using mixtures and lowering the total dose for weed control, while at the same time delaying the development of herbicide resistance by using mixtures with different molecular targets (Pest Science Management, 2006, 62: 1092-1097).

I have extensively contributed to the field of allelopathy with special empahsis on taking ecosystem level approach to study allelopathy (Botanical Review 1994, 60: 182-196; 1996, 62: 186-202; 1997, 63: 221-230; 2006, 72: 153-178; Advances in Agronomy 1999, 67: 141-231; Perspectives in Plant Ecology, Evolution and Systematics 2001, 4: 3-12; Agronomy Journal 2001, 93: 79-84; Forest Ecology & Management 2002; 160: 75-84; Critical Reviews in Plant Sciences 2003, 22: 221-238; Plant and Soil 2003, 256: 1-11; Planta 2003, 217: 529-539; Plant and Soil 274: 227-236).