Li-Hsuan Ho

• Drought is a significant environmental factor that can impair plant growth and development, leading to reduced crop productivity or even plant death. Maintaining sugar distribution from source to sink is crucial for increasing crop production under water limitation conditions. Numerous studies have suggested that nutrition fertilization, especially potassium (K), can enhance plant growth and yield production. To investigate the mechanism of K in sugar long-distance transportation under drought stress, we established a soil-grow system and a hydroponic-grow system with varying amounts of potassium supplementation and analyzed the biochemical and molecular responses in Arabidopsis and potato plants under drought stress conditions. Our findings showed that excess potassium fertilization limited sucrose metabolism, leading to lower drought tolerance in Arabidopsis in both grow systems. However, higher potassium supplementation altered sugar relocation and potassium movement, resulting in an increase in starch yield production in both potato plants with different sink strength capacities. We also proposed that a low amount of sodium increases Arabidopsis drought tolerance under low potassium conditions since a low amount of sodium can improve the control of osmotic potential, leading to more water being retained in plant cells. Silicon (Si) has received considerable attention recently for its potential in mitigating drought stress, although the effects vary among different plant species. To investigate the mechanism of Si in drought stress tolerance, we applied monosilicic acid in hydroponic media and then applied PEG8000 to simulate drought stress. Our findings revealed that Si-dependent drought mitigation occurred more in the shoot than in the root of Arabidopsis, and we observed silicon accumulation in the shoot of Arabidopsis. In Si-treated plants, more glucose was accumulated in the vacuole, leading to better osmotic potential control under drought stress. RNA sequencing analysis showed that Si altered the activity of sugar transporters and the sugar metabolism process, and increased photosynthesis. However, Si-dependent regulation in sugar transporter showed different responses in potato. Understanding the mechanism of Si in potato requires further studies. Overall, our dissertation provides important information for clarifying the mechanism of Si in drought stress, which forms the basis for further investigation.