The historical bases of the concept of allelopathy

Conclusion In the light of contemporary allelopathic research, the intuitively based statements of the early botanists stand up surprisingly well. The walnut tree is now understood to affect the growth of neighboring plants via juglone leached from the leaves, roots, and fruits.¹¹⁸ The replant or soil sickness problem of peach orchards has been related to the toxigenic breakdown of amygdalin, a constituent of peach roots.¹¹⁹ The declining yield of many crop species grown under continuous monoculture has been linked to the accumulation of allelopathic substances in the soil, especially through the mediation of microorganisms.¹²⁰ Numerous plants cited by de Candolle as being injurious, such as Erigeron,¹²¹ thistle (Cirsium),¹²² flax (Linum),¹²³ and various crucifers (such as Brassica nigra),¹²⁴ have been found to posses marked allelopathic activity. Over fifty years before the discovery of rhizobia, de Candolle considered the excretory material of legumes to be beneficial to cereals.¹²⁵Modern reviews of allelopathy commonly credit de Candolle with an insight that was not equaled by the technology of his era.¹²⁶ In fairness to his detractors, his toxin theory of plant interactions was largely the by-product of an outdated and misconstrued notion of plant nutrition. His critics and most earlier botanists had similarly erred in seeking a single factor responsible for plant growth, much as had the alchemists sought the legendary philosopher's stone. Taking all this into account and considering the forceful personality of Liebig, one can readily appreciate how, 130 years ago, Liebig's theories preempted and stifled those of de Candolle.Today, with modern techniques of plant physiology and soil biochemistry, allelopathy has been shown to be a real but subtle factor in the dynamics of natural and agricultural plant communities. It is unfortunate that the single-mindedness characteristic of previous centuries still persists. The dichotomy between allelopathy and competition is exacerbated by the inherited bias toward the nutritional model of plant interaction fostered by Liebig, and is accentuated in the fact that in modern nutritional studies it is still basically unnecessary to consider plant-plant chemical interactions and their concomitant effects, whereas in allelopathic investigations the converse is regarded as axiomatic.In summary, de Candolle should not be seen as a prophet crying in the wilderness, as Fisher would have it.¹²⁷ The bases of de Candolle's concept of allelopathy were the dubious experiments of Macaire and his own obsolete theory of plant nutrition. Despite this, modern experimental work indicates that allelopathy is important in many plant interactions. De Candolle seems to have been right, at least in part—but for the wrong reasons.