Complex taxonomy in Opuntioideae: is floral morphometry essential to identify Opuntia species?

Correct species identification is critical for studies on biodiversity, ecology, and conservation. Determining Opuntia s.s. species is difficult because they have similar traits and are phenotypically plastic. Taxonomic keys are based on vegetative traits rather than reproductive ones such as flowers, because they are assumed to be too similar. We analyzed morphometric characteristics of flowers and cladodes over 6 years to determine which of these is most useful for differentiating Opuntia species from the Chihuahuan Desert. For each species (Opuntia robusta H.L. Wendl. ex Pfeiff., O. cantabrigiensis Lynch, O. tomentosa Salm-Dyck, and O. streptacantha Lem.), we tagged 20 hermaphroditic and 40 dioecious plants (totaling 100) from 2014 to 2020 to complete the sample size of flowers and cladodes. Seventeen morphometric characters were measured for new cladodes and 15 for flowers, and discriminant analysis was applied to determine which traits enabled species delimitation. Six of the 17 cladode characteristics combined explained 89% of the variation, while 9 floral characteristics combined explained 94% of the variation. Floral morphometrics proved to be very useful to accurately differentiate species and should be included, in addition to cladodes, in future taxonomic studies. Here, we provide the first taxonomic key that includes floral traits to identify Opuntia and a new description of each studied species.