Growth response of Thunbergia grandiflora (Roxb. ex Rottler) Roxb. (Skyflower) on shading and anatomical characteristics under various soil series.
Thunbergia grandiflora Roxb. (Acanthaceae) or "skyflower" is an exotic species that was reported to be aggressively spreading out of control in the tropical and subtropical ecosystems. This study assessed the invasive characteristics of the species by analyzing its growth characteristics on varying shading intensity and soil series, as well as anatomical characteristics in varying soil series. Growth characteristics include the height and root collar diameter (RCD), while anatomical parameters include the thickness of all dermal, ground, and vascular tissues. Based on the results of the shading experiment, T. grandiflora grew significantly in terms of height and RCD under full light conditions (1784.75 lx). The species can survive intense light condition but is suppressed under partial to fully shaded treatments. In the soil series experiment, the highest growth and RCD were observed in the Luisiana soil series with strongly acidic soil (4.1 pH). The leaf and stem anatomical structures of T. grandiflora have the characteristics typical of invasive species that can adapt to certain harsh conditions. This includes the presence of dense non-glandular trichomes, cystolith, multi-layered collenchyma cells in the hypodermis, sclerenchymatous tissues, wider xylem pores, and closely pack parenchyma cells in the stem. Therefore, anatomical structures of this species suggest its ability as an invasive species. Consequently, the invasive characteristics can be suppressed by controlling the shade and soil conditions during the wildling stage of the species. The species can be bioinvasive and the extent may depend on the prevailing conditions in the site. However, further researches on the species' root physiological and anatomical responses to other biotic and abiotic factors in the environment are recommended. Also, knowing the invasiveness of T. grandiflora will help us identify possible strategies to limit the spread of the species, including early detection of the occurrence of species, avoiding disturbance in the area, and identifying species as biocontrol.