Orophea sichaikhanii (Annonaceae), a new species from southern Thailand, with a key to the species of Orophea in Thailand and notes on some species

Background and aims – Recent botanical expeditions in Ranong Province, southern Thailand yielded unidentifiable collections of Orophea subgenus Sphaerocarpon (Annonaceae). To elucidate the taxonomic status of these collections, detailed morphological examinations and comparisons with morphologically similar species are made.Material and methods – This study followed standard practices of herbarium taxonomy. Specimens of Orophea spp. in BKF, CMUB, L, and QBG herbaria were studied. Digitised type specimens deposited in BM, E, G, K, and L herbaria were accessed. A stereo microscope was used for morphological observations and measurements. Key results – A new species Orophea sichaikhanii is described and illustrated. The Peninsular Malaysian O. hastata and O. kingiana are the species most similar to the new species. Orophea sichaikhanii is different from O. hastata in several traits: indumentum on ovaries and young twigs; length of pedicels, inner petals, and inner petal claw; and inner petal colour and tip. The new species differs from O. kingiana by having dissimilar colour and tip of inner petals; lower number of stamens and carpels per flower; and glabrous ovaries. Additionally, a key to the species of Orophea in Thailand and notes on certain species are provided.