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The father of allelopathy

Professor Dr Hans Molisch (1856-1937) was, appropriately, the son of a farmer. Both his parents had a love of gardening. Molisch lived in the German city of Brunn where several scientific institutes were located. Gregor Mendel was foundation professor of natural sciences at Brunn University and visited the Molisch family garden in 1865 (Narwal and Jain, 1994). Thus, were the seeds sown for his subsequent, distinguished career as a botanist which was under-pinned by his understanding of chemistry, in which field he also published. These two disciplines continue to underpin contemporary research into allelopathy.

Studies at the University of Vienna were followed by a distinguished academic career in which Molisch taught and carried out research in Czechoslovakia, Austria, Japan, and India. He traveled also in Indonesia, China and North America (Narwal and Jain,1994).

Molisch is, perhaps, best remembered for two books: Microchemistry of Plants (1912) and The Influence of One Plant on Another: Allelopathy (1937). This, his last major contribution to the literature, blended his disciplinary interests and is credited with introducing the term ‘allelopathy’, thus earning him the sobriquet ‘the father of allelopathy’. Molisch also espoused the now generally accepted view of allelopathy as describing both positive and negative effects of exposure to allelochemicals (Rizvi et al, 1992).

Pioneers of allelopathy

There were, of course, pioneers in the world of allelopathy, albeit unfamiliar by that name, pre-Molisch. Bioactive compounds from plants have been harvested and deployed for a variety of purposes, not least in health and medicine, since before the dawn of agriculture more than 10 000 years ago. The fact that many of the most potent pharmaceuticals and poisons known to man are of natural, plant origin is often over-looked by the more ardent proponents of all things ‘organic’!

While it may not quite have been a case of:
“Eye of newt and toe of frog,
Wool of bat and tongue of dog,
Of the ravin’d salt-sea shark,
Root of hemlock# digg’d i’ the dark,
Double, double, toil and trouble:
Fire burn and cauldron bubble.”
William Shakespeare ‘Macbeth’ 1606
(#Tsuga sp.)

Tose witches or wizards who used natural compounds to effect cures were often subjected to the ducking-stool, or worse, for their pains. Early agronomists may have fared better as they offered their proto-allelopathic remedies but advisory parchments of the period have not, unfortunately, survived.

Putnam (1985) notes that among the earliest records of the detrimental impact of what is now recognized as allelopathy in agriculture are the writings of the Greek philosophers Democritus (c.460-370BC) and Theophrastus (c.372-286BC). The former traveled widely and was the most learned thinker of his day. Fragments of his mathematical, physical, ethical and musical works survive, including his ‘atomic system’. The latter was a student of Aristotle and was responsible for preserving much of his teacher’s works. Theophrastus himself wrote two books on plants.

Willis (2000), reviewing aspects of the history of allelopathy notes that the Romans, too, recognized allelopathic phenomena. Circa 36BC, the author Varro, who wrote more than 600 books, observed the harmful effect that walnut tress could have on adjacent species. Early naturalists continued to make observations of ‘allelopathic’ phenomena throughout the ensuing centuries but Willis (1996) dates the first phase of allelopathy, proper, as the period 1785-1845 “the age of de Candolle”.

By the nineteenth century serious attempts were being made to provide a scientific rationale for observations of allelopathic phenomena. One of the best known identities of this period was Augustin Pyrame de Candolle. As early as 1805 he wrote on the topic of root excretions and applied these studies in the context of crop rotation. The technology of the day, however, did not permit him to identify the bioactive compounds which he perceived to be present. As Willis (2002) writes: “To many, de Candolle is the real father of allelopathy….he developed a perspicacious theory of plant interaction, relevant to agriculture and natural ecosystems, based on chemical substances assumedly released from plants”. Perhaps de Candolle should be recognized as the grand-father of allelopathy?

In any event it was appropriate that the acknowledged father of allelopathy, Hans Molisch, should feature in the first article of the inaugural edition of Allelopathy Journal, published in 1994. Greeting the arrival of the journal, Professor Al Putnam noted that a multidisciplinary approach, as exhibited by Molisch, was essential to successful studies of allelopathy. “Rarely” he wrote “does one individual possess all of the skills necessary to solve all of these problems” (Putnam, 1994). Collaboration between scientists would bring together the necessary mix of skills and would be “appropriate for publication in this Journal where it can reach a multidisciplinary international audience”.

Such a broad interest in the chemical interactions of plants and their application in natural and managed systems became apparent in what Willis (1997a) characterizes as the second phase of allelopathy history, between 1900 and 1920. Spencer U Pickering (1858-1920) was a pioneer of allelopathy active during this period (Willis, 1994a). A physical chemist by training, Pickering is known for his work on the effects of grass cover on fruit trees; the effect of crops on other crops, and the effect of heat on soils (Willis, 1994a). His work was greatly facilitated by the patronage of the Duke of Bedford on whose Woburn estate, in southern England, Pickering worked for a quarter of a century. Interestingly, the noble duke appears not to have hesitated to exercise aristocratic privilege, appearing as senior author in many of the Pickering papers (for example, Bedford and Pickering, 1914)!

Profiles of a total of fourteen pioneers of allelopathy have appeared in issues of Allelopathy Journal, to date. These include the distinguished American taxonomist and ecologist Cornelius Muller, who investigated allelopathy in the context of dominance and suppression in natural communities (Willis, 1995); Oswald Schreiner of Germany, the first person to isolate and identify phytotoxins associated with unproductive soils (Willis, 1996), and Gerhard Grummer, who “was the first to consolidate allelopathy as a phenomenon in plant ecology” (Willis, 1997b).

‘Allelopaths’ (a term very loosely translated from the Greek and meaning ‘those who suffer together’) were active, also, in the southern hemisphere. Fernando Almeida (1924-1993), for example, a Portugese immigrant to Brazil, was a pioneer in the study of allelopathy resulting from mulch biomass decomposition in reduced cultivation systems (Passini and Rodrigues, 1999), bringing allelopathy studies firmly into the context of contemporary management in farming systems.

Hans Molisch’ death occurred shortly before the outbreak of the Second World War. The sixtieth anniversary of the end of that conflict has been celebrated this year. But, although the world war concluded in 1945, its legacy saw many communities of scientists working in comparative isolation for more than four decades, until the conclusion of the so-called ‘Cold War’.

Andrei Grodzinsky (1926-1988), the father of allelopathy in the Soviet Union, was at his most scientifically active during this period and, accordingly, is less well known than is warranted by his contributions. Unlike Molisch, he was not able to travel widely but, nevertheless, made a major contribution to the science of allelopathy through his observations of bioactives (allelochemicals) released by plants and their stimulatory and/or inhibitory effects. He published in the western literature, for example, a paper on the phytosanitary effects of cruciferous crops in crop rotation appearing posthumously (Grodzinsky, 1992). Like his contemporaries in North America, he espoused a wide role for allelopathy as a factor in ecology (Golvko, Roschina and Narwal, 1995). Indeed, Willis (1994b) writes that “Grodzinksi has long tried to present a much less restrained view of allelopathy and has also bravely alluded that plants may even be able to react to energy fields of other plants”. Grodzinky’s role as ‘father-figure’ influenced the work of other Soviet allelopathy scientists, who are reported as contributing more than 5000 scientific papers and 20 monographs on allelopathy between 1931-1990 (Roshchina and Narwal,1998).

The principal interest of German F Naumov (1927-1997), a contemporary of Grodzinsky in the Soviet Union, was the allelochemicals produced by germinating seeds of field crops (Proskurnin et al, 2003). Naumov recognized that these compounds may affect the germination and early growth of other plant species but that they may also influence crop growth and development via interactions with micro-organisms. Thus, he studied the fungicidal activity of allelochemicals and also investigated the influence of these bioactive compounds on symbiotic nitrogen fixation.

The contributions of Czech scientists during this period, too, ranged widely across the identification of allelochemicals; their modes of action, and their ecological significance (Vicherkova,1997). These workers were intrigued by the interplay of allelopathy (addition of chemicals to the environment) and competition (depletion of environmental resources) in the overall ‘interference’ of one plant with another. They also ranged over topics such as autopathy and investigated the effects of volatile terpenes and essential oils on plant physiological processes.

How much of this vast body of literature let alone that published in China, the subject of a special edition of Allelopathy Journal in 2005, has ever been properly explored by scientists in the western hemisphere is an open question. What is clear is that, even in the twenty-first century, language barriers, sometimes coupled with insularity, continue to present barriers to information flow.

The prophet Elroy Rice (1917-2001) – and his disciples

The thirtieth anniversary of the publication of the first edition of Professor Elroy Rice’s book titled, simply, “Allelopathy” fell in 2004. Surely, no work on the discipline has been more influential or more widely cited in the literature. A flavour of this work is given in an autobiographical contribution to the ‘Pioneers in Allelopathy’ series (Rice, 1996).

Significantly, much of what Rice wrote and described 30 years ago has been neither surpassed nor superceded in the literature. The potential scope of allelopathy in natural and managed ecosystems; the means of liberation of allelochemicals from plants; the families of compounds themselves, and their likely modes of action are all included (Rice, 1974). It could be argued that all that has been accomplished since is the refinement that accompanies the application of ever more sophisticated technology…..

Such an appraisal would be unfair to the undoubted flowering of allelopathy which occurred during the1970s, through the 1980s and into the 1990s. During this period allelopaths were active around the World. They included, in Canada, Neil Towers; in Europe, Michael Wink; in Japan, Junya Mizutani; in Korea, Bong-Seop Kil; in Mexico, Ana Luisa Anaya; in Taiwan, Chang-Hung Chou and CC Young; in India, Syamasundar Joshi – who published one of the rare examples of allelopathy being deployed to good effect in the field by living plants, in this case the effect of Cassia sericea Sw. on Parthenium hysterophorus L.- and in the USA, HH Cheng, Stephen Duke; Frank Einhellig; Stephen Gliessman; Gerald Leather; Al Putnam; CD Tang, Doug Worsham, and George Waller, of whom, more later. These are examples, only, of the ‘multidisciplinary international audience’ for allelopathy and no disrespect is intended to the many workers who do not receive specific mention. As noted, above, there was much activity behind the ‘Iron Curtain’ a great deal of which remains inadequately explored.

Gerald Leather and Bong-Seop Kil were guests in my laboratory at the University of New England (New South Wales) which, along with Rick Willis’ laboratory in Melbourne, were the main centres for allelopathy in Australia at that time.

The topics covered by this large group of very productive workers illustrates the extent to which allelopathy must be viewed in breadth. Allelopathy was, for example, recognized as but one of the many stresses which impact plants in their environment. In some instances allelopathy may emerge as a dominant, identifiable influence. In others it is merely part of the background. An allelopathic effect which is stark in one season may be invisible in the next.

Allelopathy was recognized in fresh water and marine systems, demonstrating the capacity for allelochemicals to retain their bioactivity even when greatly diluted. Similarly, the fate of allelochemicals in the rhizosphere and, more generally, in soil was being explored. Emergent, contemporary interest in sustainability of natural and managed systems focused workers on the role which allelopathy might play, for example, in reducing reliance on synthetic pesticides. The application of mass spectrometry and high pressure liquid chromatography greatly facilitated the identification and quantification of allelochemicals.

Perhaps most important of all was the recognition that those compounds which are to allelopaths, ‘allelochemicals’, have a broad role in phytochemical ecology (Towers et al, 1989), and that they have a long history which has witnessed their employment by man for a variety of purposes. This has never been put better than by Whittaker (1970) who wrote, specifically of the alkaloids: “Man’s use of alkaloids for flavour, mild stimulation, medicinal effect, or pleasurable self-destruction should not obscure a common theme: they are probably, although not necessarily in all cases, repellents and toxins, evolutionary expressions of quiet antagonism of a plant to its enemies”. This antagonism may be perceived as being part of the array of defences which plants use to offset their sedentary habit (Lovett, 1985) and/or as a means of conveying messages: “Allelochemicals may, therefore, be considered as part of a network of communication in which disparate organisms give similar responses to similar compounds or families of compounds. Plants producing biologically active compounds at relatively high concentrations may be perceived as utilizing chemical defences” (Lovett et al,1989).

Taking advantage of an author’s prerogative to be a little nationalistic, it is worth recording that Australia well illustrates the rich diversity of allelopathy (Lovett, 1986). As in other countries, allelopathy has often been observed but not recognized, for example, in early studies of phytotoxicity occurring from the decomposition of stubble in the field (Lovett, 1987). Nevertheless, allelopathy in Australia has been identified with plants native and introduced; in communities from rangelands to eucalypt forests; in pasture and crop systems, and in situations where bacterial involvement may be a pre-requisite for its expression (Lovett, 1987). Later, it has been recognized that chemical compounds which may act as plant messengers between plants and widely disparate organisms have potential as management tools in agricultural and other production systems (Lovett, 1989).

My personal experience of allelopathy, greatly assisted by my students and colleagues in Australia and overseas, has been varied and never less than fascinating. It began in 1977 when Professors John Harper and Geoff Sagar at the University of Wales, Bangor, offered me facilities for sabbatical leave and suggested that a little exploration of some curious phenomena might be of interest. I began by following some of the pioneer work of Grummer and Beyer (1960), finding that the toxicity of the Brassica weed species Camelina sativa (L.) Crantz depended on the presence of free-living bacteria in the Camelina phyllosphere. Returning to Australia, this initial work stimulated studies with a number of weed species, notably Salvia reflexa Hornem. and Datura stramonium L. Like other allelopaths, my students and I were much addicted to bioassays but, at least, confined ourselves to aqueous leaching of plant parts. With Datura we were able to demonstrate allelopathy under controlled conditions and to recover the tropane alkaloids scopolamine and hyoscyamine, which are released by this weed, from field soils. In experiments with field-collected soil, inhibition of radicle elongation in sunflower was observed (Levitt and Lovett, 1984). Twenty years on this remains a rare example of allelopathic potential in the laboratory being strongly related to the field.

Simultaneous with the field work we were also concerned to determine the primary effects of allelochemicals, which are commonly expressed as impaired germination or reduced radicle length. Evidence was found that the alkaloids of Datura interfere with cell division, damage mitochondria and interfere with energy metabolism (Lovett and Ryuntyu, 1988).

Although recognised as being attenuated, as a consequence of selection for other attributes, crop plants also exhibit allelopathy. Studies of allelopathy with Brassica crop residues (Mason-Sedun et al 1986) showed that not only Brassica species but also cultivars differed in their expression of allelopathy. From this work, my students and I progressed to work with barley, Hordeum vulgare L. The release of the alkaloids gramine and hordenine from growing barley plants was quantified. These allelochemicals affected the growth of a test plant, Sinapis alba L., but it was also found that hordenine affected the growth of the common armyworm, Mythimna convecta, an insect pest of barley and the fungal pathogen Drechslera teres (Lovett, 1991).

This broad compass of allelopathy was, of course, being echoed in other laboratories in other parts of the world. What has happened, then, in the decade or more since I vacated the allelopathy bench?

Towards an allelopathic future?

Contemporary with Elroy Rice for much of his career, Professor George Waller ranks among the pioneers of allelopathy (Waller, 1998) in bringing the rigour of the chemist to his studies of agricultural and biochemical topics. He was founding editor of Mass Spectrometry Reviews (Vols 3-18) and is an authority on two classes of compound widely associated with allelopathy, the alkaloids and the terpenoids.

In September 1994, with Professor Shamser S Narwal, George Waller led the establishment of the International Allelopathy Society. The Society exists to promote understanding of allelopathy, referring to any process involving compounds produced by plants, microorganisms, fungi and viral secondary metabolites that influence the growth and development of agricultural, forestry, biological and ecological systems (excluding animals). Writing a decade later , Waller (2004) avers that, in order to sustain production while preserving resources for the future “The world’s need for research and development in allelopathy in agriculture, forestry, and ecology is of extreme urgency”. To meet this need will require allelopathy to contribute as part of the natural and managed systems of which it is a part. Are we meeting this challenge?

Certainly, a new generation of allelopaths is active. But, pondering on this retrospective, it would be easy to argue that the promise of allelopathy, pre- and post-Molisch, has remained exactly that. For example, papers describing apparent allelopathic phenomena continue to abound in the literature. The dictionary definition of this term is thought-provoking: ‘Phenomenon: a fact or occurrence that appears or is perceived, especially one of which the cause is in question’ (Oxford, 1996). Interesting as the reported phenomena may be allelopathy runs the risk of becoming an evolutionary dead end unless its practitioners can translate phenomena into performance: ‘Performance: achievement under test conditions’(Oxford, 1996).

An analysis of papers published in Allelopathy Journal is instructive. Up to April 2005 (Volume 15 No.2) some 294 papers, including short communications, had appeared. Of these, 125 (42.5%) describe phenomena, typically, ‘the allelopathic potential of species x against species y’. There is some cause for optimism in that up to the end of the year 2000 the percentage was 47%, falling to 38% in the ensuing years.

As Regional Editor for Allelopathy Journal I am privileged to read some of the latest and most stimulating contributions to the discipline. But, sadly, I am still invited to review papers in which the plant victims have been dried, ground and extracted with solvents for anything up to two weeks, or more; the resulting solutions have been applied to Petri dishes containing the hapless target seeds, and claims of discovery of allelochemicals and apparent allelopathic activity have duly been made. Frequently, I am told that roots appear to be more sensitive than shoots, and that low concentrations of allelochemicals stimulate while higher ones inhibit. The most sophisticated chemistry is sometimes applied but the compounds identified are depressingly familiar.

The scope and fascination of allelopathy are so broad that it is all too easy to fall into this trap of reinventing the wheel. But the new generation of allelopaths must be encouraged to pay full attention to the work of the pioneers and, rather than mimicking their achievements, strive to move the discipline forward. In an equivalent context the German philosopher GWF Hegel (1770-1831) observed: “What experience and history teach is this – that nations and governments have never learned anything from history, or acted upon any lessons they might have drawn from it” (Hegel, 1830).

So many enthusiastic authors, the present writer among them, have identified the potential of allelopathy to play an important role in the management of plant-based systems, including agriculture, horticulture and forestry. Yet, with relatively few exceptions, allelopathy has not made a recognised impact as a management tool in common use.

Lest I appear unduly censorious there are, among the 294 papers in Allelopathy Journal, some excellent examples of ‘performance’. In my analysis I looked, especially, for the word ‘management’ in the title of a paper as this implies that allelopathy is actually being applied to deliver useful outcomes. Four papers, 1.4% of total contributions, meet this criterion. All are from Asian countries.

In chronological order, the first example is from India and describes the application of oilseed cakes containing extracts of neem (Azadirachta indica A.Juss), castor (Ricinus cimmunis L.), mustard (Brassica campestris L.) and rocket salad (Eruca sativa Mill.) to soil under controlled conditions. The amended cakes reduced the multiplication of two species of applied nematode, the effects being similar to those achieved with commercial nematicides (Anver and Alam, 2000). The findings are relevant to crop protection in several leguminous crops which are important in the sub-continent.

Eiji Tsuzuki commenced allelopathy research in Japan in the 1970s and is recognized as a pioneer of allelopathy (Xuan, 2004). With a strong background in agronomy, Professor Tsuzuki’s contribution lies in the exploitation of allelochemicals in crops to reduce dependence on synthetic pesticides. This has long been one of the objectives of allelopathy researchers. Thus, as the second example of ‘management’ papers, Tsuzuki and Dong (2003) applied pellets of several species of buckwheat (Fagopyrum spp.) containing a number of commonly identified allelochemicals (for example, ferulic and caffeic acids) to several plant species under controlled conditions, where inhibitory activity was observed. Most importantly, the work was translated to the field where buckwheat pellets were shown to inhibit weeds in rice fields without adverse effects on the crop.

The third example is that of Cheema and Irshad (2004) who, working in Pakistan, investigated the effect of ‘SWE’, water extract of Sorghum bicolor L., on barnyard grass (Echinochloa crus-galli (L.) Beauv) in rice (Oryza sativa L.) SWE significantly reduced mass, but not plant density, of barnyard grass in the field, relative to control. This reduction was reflected by a positive effect on yield components of rice.

Finally, Ni and Zhang (2005) reviewed the use of allelopathy for weed management in China, citing mulching with wheat straw as an effective technique for use in the field. They also found that while rice accessions showed differences in allelopathic potential these were not expressed in the field. Nevertheless the variation present may provide the basis for further selection for enhanced allelopathic activity in this important crop. This work echoes the promise of selection for allelopathic activity in crops foreshadowed by the work of Putnam and Duke (1974) with cucumber (Cucumis sativus L.).

Of course, the 294 papers published in Allelopathy Journal are part only of the recent literature. Nevertheless, they reflect a broader trend. From January 1996 to April 2005, Allelopathy Journal published almost 3500 abstracts of papers on the discipline. A spot check of abstracts published in issues 3(2) [July 1996] and 14(2) [October 2004] showed that titles containing the word ‘potential’ accounted for 10.5% of the former but only 8.7% of the latter, while papers containing the word ‘management’ in the title were equal at about 1%. While small in number, it is encouraging that management papers are emanating from those countries where Professor Waller and his colleagues in the International Allelopathy Society have identified the greatest need.

The ‘management’ test is extremely crude. However, relaxing the criterion from ‘management’ to papers which had a clear component of field work showed that such papers accounted for only about 10% in 1996 and that the proportion had probably decreased by 2004!

The impossible dream?

It seems clear that allelopaths, in general, often supported by increasingly sophisticated chemistry, molecular biology and mathematical modeling, are not engaged in translating even well established and documented manifestations of allelopathy to the field. Why is this the case?

One reason is that, for the scientist, bioassays are easy to carry out and complex chemical tests are becoming more readily available. Work in controlled environments yields quicker results than does field work, and the flow of publications is concomitantly greater.

A less cynical view is that allelopathy is one component, only, of a complex system. To deploy it requires a multidisciplinary effort, as exemplified by the ‘pioneers of allelopathy’. In other words, allelopathy is but one thread in the web of life and it can be extremely difficult to disentangle it and to establish its importance. Fitter (2003) points to this complexity in his paper ‘Making allelopathy respectable’, suggesting that there are several cases where successful invasion by weeds is due, at least in part, to phytochemical activity. One example cited is that of the success of the European Vulpia spp. in Australia, An et al (2005). However, it is cause for reflection that Fitter chose this title 66 years after Molisch’ published his treatise on allelopathy.

A third reason is that even where allelopathy has been shown to have a role in, say, weed management, for the farmer to use it as part of a management system is much more difficult than the simple alternative of employing a commercial herbicide.

This point leads to another cherished but largely unrealized allelopathic objective. Allelochemicals have been repeatedly heralded as the basis for new families of herbicides. Sadly, their impact has been very limited and commercial interest in allelochemicals has languished. The compounds involved are well known and probably not patentable. While, in some instances, they have been elucidated the modes of action of allelochemicals have not been sufficiently novel as to excite investment. The work of Tsuzuki and Dong (2003) does, at least, indicate that allelochemicals can hold their own with synthetic pesticides in some circumstances and the diminishing number of affordable, effective pesticides gives cause for some optimism as to the future of allelopathy as an element in crop protection. This may be particularly the case in those countries where economic considerations preclude the routine use of conventional chemical approaches. Developing economies in Asia and in eastern Europe fit this category and they are proving to be a rich source of papers on allelopathy.

What may offer a renewed vision for allelopathy is that, in these early years of the twenty-first century, the global consumer is exerting increasing influence on production systems. To employ the jargon, ‘market pull’ is being exerted. Consumers are demanding fresh, healthy and ‘natural’ foods which are definitely not transgenic (GM) in origin. Conventional plant beeding to develop the inherent self defence of crop plants, reducing the need for pesticide and herbicide application, may yet deliver benefits in this context (see, for example, Dilday et al, 1998; Wu et al, 2002).

The market for alternative pharmaceuticals, too, is reaching new heights. ‘Bioactives’ are recognised and acceptable, not least because the technology to quantify, evaluate, dispense and deploy them with confidence now exists. The wide publicity which the carotenoid, lycopene, found principally in tomatoes, has received as an antioxidant which reduces the risk of prostate, lung, stomach and a range of other canacers (Giovannucci, 1999) is an excellent example. Thus, the climate is right to promote allelopathy and allelochemicals in their own right, that is, not as substitutes for or as competitors with conventional technologies.

Consistent with this approach, Zhuoquan et al (2005) refer to recent researches on biogenic volatile organic compounds (BVOCs) produced by plants and their relations with other plants, insects, pathogens, nematodes and other microbes. The volatiles concerned appear to be known allelochemicals but the paper stresses that their role in non ‘plant-plant’ relations warrants a distinction between a broader ‘chemoecology’ and allelopathy.

It is this distinction which may be the basis for a new allelopathy dream.

Acknowledgements

The assistance of Dr An Min, a member of Professor Jim Pratley’s very active allelopathy group at Charles Sturt University, and Professor Shamsher S Narwal, Chief Editor of ‘Allelopathy Journal’, in providing background material for this paper is gratefully acknowledged.

Through the generosity of my former students my contribution, albeit limited, has been recognised in a number of allelopathy publications up to 2003, ten years after leaving the laboratory. I am deeply appreciative of this courtesy.

*Emeritus Professor in the University of New England, NSW. Formerly: Managing Director, Grains Research and Development Corporation, Australia; Professor of Agronomy, University of New England; Professor of Agricultural Science, University of Tasmania; Visiting Professor, University of Helsinki.

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