Silicon- and Sargassum muticum Extract-Based Seed Priming Promotes Water Deficit Tolerance and Protects Photosystems in Sesamum indicum L.

Water deficit strongly constrains sesame growth and productivity, highlighting the need for sustainable strategies to maintain plant performance under limited water availability. Although silicon (Si) and seaweed extracts have been reported to improve plant responses to abiotic stress, the effects of their application as seed priming agents, particularly in combination, on subsequent PSI and PSII performance in sesame under water deficit remain insufficiently characterized. This study aimed to evaluate whether seed priming with silicon treatment (Si; nominal 4 mM SiO2), Sargassum muticum extract (SWE; 10%), or their combination improves sesame tolerance to water deficit, with particular emphasis on growth, water status, gas exchange, photosynthetic pigments, and PSI and PSII performance. The experiments were carried out in greenhouses under well-watered and water-deficit conditions (80% and 40% field capacity (FC), respectively). We assessed plant growth, leaf relative water content (LRWC), photosynthetic pigments, gas exchange, and PSI and PSII activities. Water deficit strongly reduced shoot and root growth and significantly decreased chlorophyll content, photosynthetic efficiency, and biomass. Seed priming considerably reduced these negative impacts. Si priming and SWE priming alone significantly enhanced shoot length (by 62% and 72%, respectively) relative to unprimed water-deficit-stressed plants. Under water deficit, SWE priming produced the strongest chlorophyll response, increasing chlorophyll a, chlorophyll b, and total chlorophyll by 325.2%, 262.6%, and 240.3%, respectively, whereas Si priming increased carotenoid content by 414.6%. Relative to unprimed water-deficit-stressed plants, the combined Si + SWE priming treatment increased whole plant dry biomass by 119% net photosynthetic rate by 533.9%, and transpiration rate by 522.2%, while also improving several PSI and PSII photochemical efficiencies, increasing regulated energy dissipation, and reducing unregulated energy loss under water deficit. In contrast, SWE priming alone produced the highest chlorophyll a, chlorophyll b, and total chlorophyll contents, whereas Si priming enhanced carotenoid accumulation. Furthermore, the combined Si + SWE treatment improved PSI and PSII photochemical efficiency, increased regulated energy dissipation, and reduced unregulated energy loss under water deficit. Overall, seed priming with Si and SWE represents an effective, economical, and sustainable strategy for improving sesame tolerance to water deficit.