Justicia adhatoda reveals two morphotypes with possible functional significance

Justicia adhatoda L. (Acanthaceae), an Old-World species of Justicia, is found in almost all geographical regions of India. Indian botanists have persistently used two accepted synonyms of J. adhatoda, namely, Adhatoda vasica and Adhatoda zeylanica, treating them as names of separate species, but without considering or making any reference to variation of forms in the species. Here, different aspects of variation-phenotypic, genotypic, and distributional-in Indian populations of J. adhatoda were studied to determine whether the two names might have been used to designate distinguishable forms of the species. We conducted field studies in different regions of India, laboratory studies of diverse phenotypic traits in experimental plots (anatomical, biochemical, reproductive, and morphometric), and a preliminary study of genetic variation using homologous cytochrome P450 gene fragments. We assessed herbarium samples from across India and the taxonomic literature for pointers indicating the presence of distinguishable forms. Population-level phenotypic and genetic variation pointed to the presence of two distinct morphotypes of the species, which separately tend to occur in dry and wet regions. Each form retains its original phenotype, either when the two forms are transplanted and cultivated together, or when found growing in regions (presumed introduced) outside its normal distributional range. Morphological studies and metabolic profiling (leaf and seed fatty acids, wax load and wax composition in leaf) suggest functional adaptation of the two forms, one to drier and the other to wetter regions. We could distinguish these forms in herbarium specimens dating back to 1821, but neither herbarium specimens nor the taxonomic literature reveal any reference to two forms. We propose that the forms be recognized as two distinct morphotypes of Justicia adhatoda.