Morphology, Phytochemistry and Growth Regulation in Barleria L. (Acanthaceae): A Critical Narrative Review

Abstract

Background and Significance: Barleria L. is among the largest genera of Acanthaceae, comprising roughly three hundred palaeotropical species that carry substantial medicinal, ornamental and ecological value. Three research communities have engaged with the genus largely in isolation: systematists concerned with delimitation and structural diagnostics, natural-product chemists concerned with iridoid and phenylethanoid glycosides, and a small horticultural and biotechnological community concerned with propagation and growth manipulation. Purpose and Scope: This review critically evaluates the evidence linking morphological and anatomical traits, phytochemical composition, and growth regulation approaches in Barleria, and assesses whether the three domains can currently be integrated in a defensible way. Approach: Open scholarly indexes and citation networks were searched from January 1970 to 4 July 2026 using topic-specific Boolean concepts covering taxonomy, micromorphology, secondary metabolism and growth manipulation. Sources were appraised for design adequacy, analytical transparency, taxonomic verifiability and evidential contribution rather than citation count. Principal Findings: Molecular phylogenetics has repeatedly overturned morphology-based infrageneric groupings, and reconstructions of character evolution indicate pervasive homoplasy in the traits historically used for classification. Micromorphological and anatomical work is descriptively rich but taxonomically narrow, resting on few species. Phytochemical coverage is dominated by four species, and quantitative comparisons across studies are undermined by heterogeneous extraction, inconsistent reference standards and frequently unverified voucher identity. Growth regulation evidence is the weakest domain: a small number of single-site tissue culture, seed dormancy and growth retardant studies exist, few have been replicated, and almost none reports both morphological and chemical outcomes. Unresolved Questions: Whether secretory trichome complements predict metabolite yield, whether chemotypes correspond to phylogenetic lineages, and whether growth regulator interventions alter metabolite profiles predictably all remain open. Implications: Integration will require phylogenetically structured, voucher-verified, multi-site studies that measure structure, chemistry and growth response within the same experimental material rather than in separate literatures.