Plant specialized metabolism changes throughout development, but developmental context remains inconsistently incorporated into phytochemical interpretation. This integrative narrative review synthesizes evidence on chemical ontogeny in plants, defined here as the qualitative and quantitative reorganization of specialized metabolism along plant and organ development. We distinguish ontogenetic variation from phenology, seasonality, organ maturation, developmental plasticity, and environmentally induced chemical plasticity, and examine the genetic, hormonal, anatomical, ecological, and analytical processes that generate stage-dependent chemical profiles. Across seeds, seedlings, vegetative organs, reproductive structures, fruits, and senescent tissues, the evidence shows recurrent changes in metabolite abundance, composition, spatial allocation, and bioactivity. These trajectories can alter ecological interactions, marker stability, harvest recommendations, and the reproducibility of botanical raw materials. Analytical strategies should therefore be selected according to the biological question and available resources, ranging from validated targeted or single-omics designs to spatial, single-cell, and multi-omics integration when mechanistically justified. Chemical ontogeny is proposed not as evidence that developmental metabolism is newly discovered, but as an operational framework for stage-aware sampling, reporting, interpretation, quality control, and bioprospecting.

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.