This is a post written by Darwin Hickman on the history of allelopathy. I learnt a lot, and we hope you enjoy!
I’ve been a joint PhD student with the University of Nottingham (including the AR_Lab) and Rothamsted Research for four years now, working on interactions between crops and weeds and how we can tip the balance in favour of the crop. Throughout this time, I’ve worn many hats (both literally and figuratively), at turns leaning into the fields of agronomy, weed biology, plant physiology, chemical ecology, soil microbiology and even chemistry (a scary prospect when I started to be sure). Rarely do projects of this nature take such a holistic approach given constraints in time, funding or expertise so I’ve had more opportunity to really delve into this weird and underappreciated area of science. One of the things that I really engaged with, but struggled to find an outlet for, was the way that our understanding of these interactions has developed over time, and how difficult progress has been, and that’s why I’m writing this, because it’s quite the story.
I’ve spent a substantial portion of my PhD explaining its core concept: Allelopathy. In honesty, I wasn’t aware of it until I came upon the studentship which I now work on (even as a fledgling weed ecologist), and I think that speaks to its nature as an inherently intangible phenomenon. It says a lot that G. Bruce Williamson in 1990 described the challenge of proving allelopathy as a ‘neck riddle’, the impossibility of which a condemned prisoner would stake his life. A lot of definitions of allelopathy are out there, the first being the succinct summary offered by Hans Molisch in 1937 when he coined the term: “The influence of one plant on another”. More specifically, the most-commonly understood mechanism of allelopathy is that one plant species releases defensive compounds (‘allelochemicals’) to actively inhibit other (usually plant) species which may compete with it, usually from the roots. As such, it’s incredibly difficult to prove, as it is very closely associated with resource competition (which differs in that the antagonistic plant deprives its competitor of something it needs, rather than giving it something it doesn’t want). J. L. Harper, highly quotable visionary in plant ecology, would describe it as “logically impossible to prove that it doesn't happen and perhaps nearly impossible to prove absolutely that it does”. In spite of such difficulty, though, and the slight obscurity of allelopathy, it has a bizarre and fascinating history from long before the days of Harper or Molisch.
It started when Theophrastus, ancient Greek philosopher, student of Aristotle, and ‘Father of Botany’, observed in the time of Alexander the Great that chickpea yields declined over successive seasons, the cause of which was unknown at the time. The Roman Pliny the Elder, writing around three centuries later, would note that ‘venoms’, ‘juices’, and ‘heavy shade’ from one plant would affect surrounding plants, in some cases following their removal from soil. To quote the man: “The oak and the olive are parted by such inveterate hatred that if one be planted in the hole from which the other has been dug out, they die, the oak indeed dying if planted near the walnut… on going away they leave their venom behind when the plant is torn up from the root.” Obviously, these suggestions are primitive, but they are strikingly accurate considering the beliefs of the public in this period, that plants were in fact a basic type of animal which slept with their heads in the ground. Such perceptiveness in interpreting natural phenomena is especially salutary when one realises that Pliny’s response to the famously destructive volcanic eruption at Pompeii was to have a bath rather than run for his life.
These observations from antiquity, then, gave the impression that something beyond resource competition was occurring between plants, even if it was a largely unknown quantity without a name to describe it. Unfortunately, the Middle Ages would be unkind to allelopathy, perhaps unsurprising for a period whose ideas of natural history involved the manticore and the cockatrice (although in fairness, Pliny had written about his fair share of bizarre mythological beasties). One accepted myth from this time was that of the ‘Vegetable Lamb of Tartary’, a bizarre beast which resembled a sheep attached to a plant stem. The idea was that this poor creature would ravenously eat all plants around it, then starve to death and wilt away when it could not find further food, conveniently to the point that all that was left to indicate its presence was a ring of desolation. In actual fact, this story was inspired by observations of wool-like tree cotton, and the bizarre and ovine form of the woolly barometz fern rhizome, as well as some misidentified allelopathic effects.
Allelopathy would remain at least partially in the dark following the Scientific Revolution in the West. Erasmus Darwin in his Loves of Plants, a collection of poems and scientific notes concerning botany, makes great reference to the Upas tree of Java, the ‘Hydra-tree of Death’, as he calls it. This tree was supposedly not just allelopathic to other plants but toxic to all life for around 20 miles, at least according to the testimony of a Dutch surgeon named Foersch: “The Bohun-Upas… is surrounded on all sides by a circle of high hills and mountains; and the country round it… is entirely barren. Not a tree, nor a shrub, nor even the least plant or grass is to be seen.” In a peculiar (if highly unethical) slice of early Science, Darwin Sr. Sr. would detail in his notes on the Upas that he obtained some seeds (the source of which was not explained), fed them to a dog, and observed that it convulsed and died. What Foersch did not mention, however, was that the Upas he saw was close to the smoking vent of a volcano (who knew volcanoes had so much to do with this subject?), the toxic gases of which were the true cause of the reported devastation. The source of Erasmus’ killer seeds is lost to history. In the modern day, an Upas tree sits in the middle of the fourth largest city in Malaysia, home to over half a million people, which is named after the species: Ipoh. Incidentally, my grandfather was based in Ipoh for military service in the 1950s and showed no signs of being poisoned by an accursed tree.
Elsewhere, however, the beginnings of modern allelopathy were taking shape. As early as 1727, ‘Dutch Hippocrates’ Herman Boerhaave was suggesting that chemical compounds were emitted from plant roots. This hypothesis was mostly proven by fellow Dutchman and part-time military physician Sebald Brugmans with the collection of droplets (admittedly of unknown composition) from root tips of darnel ryegrass. These piecemeal advancements set the table for Augustin de Candolle, colleague of Lamarck and inspiration for Charles Darwin, in the early 1800s. He, and a student by the name of Macaire-Princep, grew plants in various solutions which reacted with exudate compounds to confirm their presence. From this, de Candolle rationalised and advocated the use of crop rotation as an agronomic approach to avoid ‘soil sickness’, the mysterious decline of crop yield with repeated sowings.
So, thanks to de Candolle and his predecessors, it was apparent that something was happening, but the next step required was the identification of some offending compounds. Juglone, or to use its full name, 5-hydroxy-1,4-naphthalenedione, would be isolated from black walnut in the latter half of the 19th Century, explaining the ‘heavy shade’ that Pliny had alluded to, as well as toxicity of these plant parts to other species both inside and outside of the plant kingdom. By the early 1900s, the USDA Bureau of Soils, specifically Oswald Schreiner and Edmund Shorey, would isolate multiple chemical compounds from agricultural soils, such as arginine and histidine, known to be of plant origin. Across the Atlantic, gentleman scientist Percival Spencer de Umfreville Pickering (Spencer to his friends), and the 11th Duke of Bedford, were venturing into pioneering physiological work in their fruit orchards in Woburn. Part of this pursuit may have been the former’s attempts to remain engaged in plant chemistry in spite of ill health, having lost his eye in a lab explosion in 1890. Pickering would bring these experiments into his own glasshouses, noting that washings from soil where mustard plants had grown were inhibitory to a variety of fruit plants in 1917. He used this as evidence that allelopathy (although still unnamed at this point) was a widespread phenomenon. Of course, this needed a name, which was where Molisch came in in 1937.
Pickering’s allelopathy experiment from 1917: The large pots contain fruit tree saplings, the small pots in front contain mustard plants. On the left, mustard root exudates are freely washed into the larger pot, severely inhibiting the growth of the fruit sapling. In the middle, these exudates cannot enter the growth medium of the sapling, which has developed much more. On the right, the negative control with no mustard and a large, healthy sapling. (Pickering, 1917).
At this point then, allelopathy was indisputably ‘a thing’, but attempts to observe and explore it in the field would be scant until the 1950s. Enter, Cornelius Muller. His domain was the deserts of Colorado and Southern California, theoretically a simplified ecosystem from which to untangle these many interlinked interactions between shrubs. Even here, he would state (correctly) in 1953 that “The natural habitat… is far too intricate a system of influences and factors, physical and biological, to hope that there may be found a single factor controlling the complicated life of a perennial species”. In spite of this defeatist tone, Muller would eventually detail with comprehension the allelopathy of purple sage 13 years later, describing both in-field radial inhibition of grasses and similar effects against cucumber in the lab. Another 12 years thereafter, he and a co-worker, Stephen Gliessman, would determine the allelopathic dominance of bracken with a similar degree of diligence. Meanwhile, Elroy Rice and his colleagues delved into allelopathy in agriculture in the 1960s to the 1980s, noting broad-scale allelopathy in Johnsongrass, sunflower, broomsedge, and a variety of other plant species. With this background, Rice produced a book, simply titled ‘Allelopathy’ in 1975 which summarised work to that date, and remains influential in the field, essentially setting the stage for modern allelopathy studies.
Strides forward have been made by some in the following years. Broad-scale allelopathy has been reported by rice, leading to the promotion of breeding programmes to optimise weed suppression. Sorghum allelopathy has been tracked from field to molecule- specifically the potent benzoquinone allelochemical sorgoleone. Phenolic acids have been extensively studied and their various bioactivities hotly debated.
But there is still a great deal of progress to be made. As noted, Williamson in the 1990s basically claimed the definitive isolation of allelopathy to be impossible. He, like numerous influential workers, set frameworks for a best possible verification of allelopathy occurring, so difficult that modern works rarely satisfy them entirely. John T. Romeo in 2000 noted of allelopathy works that “Greater than 40% of the papers submitted… are rejected… a disproportionately higher percentage than in other subdisciplines of chemical ecology”, due in large part to this difficulty. In particular he singled out the ‘grind and find’ approach, producing whole-plant extracts with little ecological relevance but great bioactive potential, and the ‘thrill of the kill’ approach which prioritises the elucidation of a toxic effect regardless of dose or its practicality. No doubt considering Muller and Rice, Romeo argued that “the pressures of survival in science today focus overwhelmingly on frequent publication and, therefore, usually preclude the kinds of exhaustive work necessary to make a convincing case for allelopathy, at least the components of the process need to be done with precision and rigour”. A number of reviews have been presented in the following years which philosophise on allelopathy and how to most effectively investigate the subject, but there are still issues with respectability. Cruelly, a 2003 piece in Science entitled ‘Making Allelopathy Respectable’ was marred when the work it spotlighted was affected by a series of retractions and alterations resulting from a lack of reproducibility. Even today, then, it remains the case that, as Romeo says, “Within chemical ecology, allelopathy remains the poor stepchild, often lacking in legitimacy and respectability”. With the looming twin spectres of widespread resistance to novel herbicides and increasingly stringent regulation of these formulations, the poor reputation of allelopathy needs to change for the benefit of modern ecology. Through integrity and diligence, I believe it will.
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