Photosynthetic efficiency and stress indices in medicinal Bulbine natalensis Baker respond to warming and drought futures for Southern Africa

Climate warming in Southern Africa is predicted to range between 1.1 ˚ and 4.5˚ by 2050, with an increased frequency, duration and severity of droughts, which affect plant physiology. We used two types of open-top warming chambers, which passively warmed by 1.3–2.1˚ (OTC30) and by 2.2–3.1˚ (OTC50) above ambient temperatures, to simulate daytime warming for a year. Bulbine natalensis Baker, a valued medicinal plant in Southern Africa, was grown at low, moderate and high soil moisture levels under Ambient, OTC30 or OTC50 to test the impacts of drought and warming on the chlorophyll concentrations and efficiency of the photosystem (PSII) across seasons. While warming with OTC30 had insignificant effects, OTC50 led to 5% lower quantum yields of PSII (Fv/Fm), 14% lower performance index (PI (abs)), and a 53% rise in energy dissipation (Dio/CS) of PSII. Although plants displayed more stress and less chlorophyll concentrations during warmer Spring and Summer than cooler Autumn and Winter, the seasonal stress did not materially reduce PSII efficiency. Chlorophyll concentrations peaked with higher soil moisture during the cooler, but not the warmer seasons. Overall, B. natalensis was more temperature- than drought-sensitive, possibly due to its high leaf-water storage capacity. Given that all OTC30- and OTC50-warmed plants survived, thermal thresholds of B. natalensis were not evident. The 3.1˚ passive warming (OTC50), which matched the 2050s median for Southern Africa, caused significant plant stress but not mortality. Therefore, studying optimal temperatures for maximum photosynthesis (Topt) and critical temperature thresholds (Tcrit) for PSII function in B. natalensis is recommended.