Highway construction in the karst areas of Guangxi has caused water- and soil-deficient rocky desertification slopes. Selecting plant species with high biomass and good drought tolerance is key for rapid vegetation recovery. This study measured two plant species’ light and CO2 response curves during extreme drought. At light saturation point (Isat), the net photosynthetic rates (Pn) of Pueraria lobata (P. lobata) and Bougainvillea spectabilis (B. spectabilis) were low (4.14 and 2.38 μmol·m-2·s-1, respectively). Under drought conditions, they demonstrated distinct adaptive strategies. As irradiance increased, the water use efficiency (WUE), transpiration rate (Tr), and stomatal conductance (Gs) of P. lobata increased faster than those of B. spectabilis. As intercellular CO2 concentration (Ci) increased, the measured maximum Pn of P. lobata and B. spectabilis were 26.6 and 14.1 μmol·m-2·s-1, respectively. The Tr and Gs of P. lobata first decreased rapidly, then remained stable, while WUE increased linearly. However, Tr and Gs of B. spectabilis changed less, and WUE increased less than that of P. lobata. This shows that P. lobata can better adapt to water deficits and atmospheric CO2 increases than B. spectabilis, with faster biomass accumulation. Measuring photosynthetic response characteristics is useful for quickly screening suitable plants for rocky desertified slope recovery.