Aims Drought stress is one of the most important factors limiting the growth of plants in the harsh karst habitats of southwestern China, especially at the seedling establishment stage. Our objective was to elucidate the adaptive strategies against drought stress of native plants with different growth forms.
Methods We investigated leaf water potential, photosynthetic capacity, pigments, osmotic solutes, antioxidant enzymes, and growth of the seedlings of four plant species under four drought intensities. The species were Pyracantha fortuneana (evergreen shrub), Rosa cymosa (deciduous shrub), Cinnamomum bodinieri (evergreen tree), and Platycarya longipes (deciduous tree).
Important findings As drought stress intensified, leaf water potential, photosynthetic capacity, chlorophylls content, biomass increase, leaf mass ratio (LMR), leaf area ratio (LAR) and specific leaf area (SLA) gradually decreased, while thermal dissipation (NPQ), ratio of carotenoids to chlorophylls, malondialdehyde content and root mass ratio (RMR) increased in the four species. Intrinsic water use efficiency (An/gs), content of osmotic solutes and activity of antioxidant enzymes in C. bodinieri and Platycarya longipes increased under mild and/or moderate drought and decreased under severe drought, while An/gs, proline content and superoxide dismutase activity gradually increased in Pyracantha fortuneana and R. cymosa as drought intensified. Under severe drought, Pyracantha fortuneana and R. cymosa showed smaller decreases of predawn water potential and chlorophylls content and maintained higher photosynthetic capacity and larger percent biomass increase than C. bodinieri and Platycarya longipes, due to lower SLA and LAR, higher An/gs and NPQ and higher capacities of osmotic adjustment and antioxidant protection. Under moderate drought, C. bodinieri exhibited small decrease of predawn water potential and higher LMR and LAR than the deciduous species, and maintained unusually high proline accumulation and high activities of antioxidant enzymes. However, the low maximum quantum efficiency of PSII photochemistry and net assimilation rate, the sharp decreases of water potential, LMR, LAR and biomass and the low capacities of osmotic adjustment and antioxidant protection under severe drought indicated C. bodinieri’s weak tolerance. Under
well-watered condition, Platycarya longipes with high RMR showed higher LAR, chlorophylls content and biomass increase than the two shrubs. However, in response to drought stress, Platycarya longipes revealed sharp reduction of biomass due to large drought-induced decreases of gas exchange, LAR, LMR and chlorophylls content. It minimized water loss by both stomatal closure and reducing transpiration leaf area through shedding leaves and also reduced light harvesting by pigments breakdown. These results suggested that Pyracantha fortuneana, R. cymosa and C. bodinieri adopted tolerance strategies against drought stress with C. bodinieri having lower tolerance to severe drought, whereas Platycarya longipes was more sensitive to drought stress and employed avoidance strategies.