What Defines a Plant Chemotype? Conceptual Boundaries, Chemical Criteria, and Methodological Challenges

Plant chemical diversity constitutes an essential dimension of biodiversity, arising from interactions among genetic background, biosynthetic regulation, the environment, ontogeny, ecology, and evolutionary history. Although the term chemotype is widely used for distinguishable chemical patterns within a taxon, its meaning remains inconsistent across plant biology, phytochemistry, pharmacognosy, chemical ecology, and natural-product standardization. This conceptual review examines the historical development, conceptual boundaries, chemical criteria, biological significance, and methodological requirements of plant chemotypes. We define a chemotype as a distinguishable, recurrent, relatively stable, analytically reproducible, and biologically interpretable intraspecific chemical pattern expressed by individuals, populations, or lineages of the same taxon. Assignment should be based primarily on chemical discontinuity, diagnostic markers, recurrence, relative stability, and analytical reproducibility; ecological, functional, biosynthetic, and genetic information should be treated as supporting evidence when available. The framework applies to volatile and non-volatile systems, including essential oils, cannabinoids, alkaloids, phenolics, and glucosinolates, while recognizing that platform coverage, annotation confidence, and taxon-specific biology can alter the inferred groups. We distinguish chemotypes from chemical phenotypes, chemical ecotypes, chemical geotypes, chronotypes, chemovars, and chemophenetic groups and propose three evidence levels: putative, supported, and confirmed chemotypes.