Physiological, Photosynthetic and Stomatal Ultrastructural Responses of Quercus acutissima Seedlings to Drought Stress and Rewatering

The physiological mechanisms of drought-stress response in Quercus acutissima were explored with the aim to develop potentially valuable drought-resistant species that are adapted to arid regions and barren mountains. Potting experiments of Q. acutissima that simulated drought-stress conditions, and morphological, physiological, photosynthetic, and ultrastructural changes were investigated at different stages of drought stress, including after rehydration and recovery. During drought stress and rewatering, the leaves exhibited yellowing and abscission, followed by the sprouting of new leaf buds. The relative water content (RWC) changed under the drought-rewatering treatment, with a decreasing and then increasing trend, while the relative electrical conductivity (REC) had a more gradual increasing and then decreasing trend. The activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and the proline (Pro) and soluble sugar (SS) contents increased and then decreased. The change in soluble protein (SP) content showed an N-shaped trend of increasing, decreasing, and then increasing again. The malondialdehyde (MDA) content decreased and then slowly increased. From the drought to recovery phase of the experiment, the net photosynthesis (Pn), stomatal conductance (Gs), and transpiration rate (Tr) decreased gradually at first, and then the Pn increased significantly, while the Gs and Tr increased slowly. During this period, the internal CO2 concentration (Ci) did not decrease significantly until the last stage of the drought treatment, and then it increased slowly thereafter. The open stomata count in the Q. acutissima leaves was reduced significantly as drought stress increased, but after rewatering, the stomata recovered rapidly, with their opening size increasing. The number of leaf epidermal trichomes gradually declined to a low count in response to drought stress, but it rapidly recovered and increased within a short period of time after rehydration. Q. acutissima was found to have a strong drought tolerance and recovery ability after exposure to drought stress, and it may be an effective pioneer species for reforestation in barren lands.