Introduction
Rafflesia R.Br. ex (Thomson 1820) is a genus of parasitic plants that famously includes the world’s largest flowers (Nais 2001). There are 42 accepted species of Rafflesia in the Malesian Floristic Region (Adam et al. 2022; POWO 2023), and in the last two decades, the Philippines has been established to be a center of diversity for the genus (Barcelona et al. 2009; Pelser et al. 2019; Malabrigo et al. 2023). Rafflesia occurs on five major islands of the Philippines: Luzon, Mindanao, Samar, Negros, and Panay. Among these islands, Luzon, which has the largest land area in the country (109,965 km²), has the highest diversity of species (Barcelona et al. 2006, 2008, 2009; Madulid et al. 2007, 2008; Malabrigo 2010; Galindon et al. 2016; Valenzuela et al. 2017). Rafflesia are endoparasites that lack leaves, stems and roots, spending most of their life cycle embedded within the tissues of their hosts – vines in the genus Tetrastigma, family Vitaceae (Thorogood et al. 2021). Parasitic plants, and endoparasites in particular, have long been subject to taxonomic confusion and debate, owing to a dearth of stable morphological characters (the absence of leaves for example). Recent discoveries in the genus Rafflesia in the Philippines, such as R. consueloae Galindon, Ong & Fernando and R. camarinensis F.B.Valenz., Jaucian-Adan, Agoo & Madulid, highlight the need for further exploration in the region. Meanwhile, known species have also been the subject of considerable taxonomic confusion (Malabrigo et al . 2023). A firm and objective grasp of taxonomy is needed to inform effective conservation practice, for we cannot adequately conserve what we do not know to exist.
The new species we describe was brought to the attention of the first author in 2019 by staff of the Department of Environment and Natural Resources. The plant was photographed in 2013 by members of the Balatoc, Kalinga Tribal Inc. (BKTI) during a resources inventory. Careful examination reveals that this taxon is morphologically distinct from closely related and co-occurring species in the region, and requires recognition at the specific rank. The plant we name Rafflesia balatociana is the 9th species of Rafflesia to be described from Luzon Island and the 16th species in the Philippines.