Photosynthetic efficiency and stress responses in medicinal Bulbine natalensis Baker under simulated warming and drought futures for Southern Africa

Climate warming in Southern Africa is predicted to range between +1.1 °C and +4.5°C by 2050, with an increased drought frequency and severity threatening regional biodiversity. Understanding how wild medicinal plants respond to these stresses is crucial to predict their vulnerability and future species range shifts. We simulated passive daytime warming for a year using open-top chambers (OTCs): OTC30 (+ 1.3°C-2.1°C) and OTC50 (+ 2.2°C-3.1°C). Bulbine natalensis Baker, a valued succulent herb, was subjected to a 3 × 3 factorial experiment combining these warming scenarios with three soil moisture levels: Low (50% field capacity), Moderate (75% FC), and High (100% FC). While OTC30 warming had insignificant physiological effects, the extreme OTC50 reduced Photosystem II (PSII) quantum yields (F V /F M ) by 5% and performance index (PI ABS ) by 14%, while increasing cross-sectional energy dissipation (DI O /C S ) by 53%. PSII was more sensitive to warming than to drought, because succulent leaf water buffering insulated the photosystem. However, severe drought (50% FC) significantly suppressed shoot, root, and total dry biomass. Under warming, plants exhibited significant allometric shifts, allocating more carbon to shoots (+0.12 g g −1 ) at the expense of root yields. Overall, B. natalensis exhibits high resilience, showing 100% survival across all treatments. However, the warming-induced allometric shift away from belowground roots - the primary organ harvested for medicine - poses a major ecological and economic threat to future corm and root yields. Further research is recommended to define critical temperature thresholds (T crit ) and optimal photosynthetic temperatures (T opt ).