Physiological and Biochemical Adaptations to Repeated Drought-Rehydration Cycles in Swartz: Implications for Growth and Stress Resilience.

Swartz is a rapidly growing plant known for its lightweight wood; it is widely utilized for timber production and ecological restoration. We investigated the effects of different numbers of drought-rehydration cycles on seedlings cultivated at the Xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences. The experiment comprised three treatments: normal watering (CK, 80-85% field capacity), one drought-rehydration cycle (D1, one rewatering), and three drought-rehydration cycles (D2, three rewaterings). We characterized the effects of these treatments on seedling growth, biomass allocation, non-structural carbohydrates (NSCs), malondialdehyde (MDA), catalase (CAT) activity, peroxidase (POD) activity, superoxide dismutase (SOD) activity, proline content, and soluble protein content. The number of drought-rehydration cycles had a significant effect on the growth characteristics and physiological and biochemical properties of leaves. As the number of drought-rehydration cycles increased, the height increased significantly (by 17.17% under D2). The leaf biomass ratio, soluble sugar content, and starch content decreased (15.05%, 15.79%, and 46.92% reductions under the D2 treatment); the stem biomass ratio and root biomass ratio increased; CAT activity increased and then decreased (it was highest at 343.67 mg·g·min under D1); and the POD and SOD activities, the MDA content, the soluble protein content, and the soluble sugar/starch ratio increased significantly (395.42%, 461.82%, 74.72%, 191.07%, and 59.79% higher under D2). The plasticity of growth was much greater than that of physiological and biochemical traits. In summary, seedlings adapted to multiple drought-rehydration cycles by increasing the accumulation of soluble proteins (likely associated with osmotic protection), activating enzymes (POD and SOD), promoting the conversion of NSCs (increasing stored carbon consumption), and allocating more biomass to plant height growth than to diameter expansion. Under climate change scenarios with intensified drought frequency, elucidating the drought resistance mechanisms of is critical to silvicultural practices in tropical plantation.