The thermodynamic availability of water plays a pivotal role in fundamental processes that are crucial to materials and plant sciences. It governs key chemical interactions in materials and drives essential processes in plants, such as photosynthesis, transpiration, and nutrient transport. However, despite its importance, there is a lack of techniques for non-invasive monitoring and visualization of the thermodynamic availability of water or water potential. This dissertation introduces the design and application of optical techniques rooted in reflectance and fluorescence. These techniques convert the thermodynamic state of water into an optical signature, thereby paving the way for better understanding and engineering of both synthetic and natural systems. At first, we demonstrate the use of optical reflectance to study the interplay between aqueous solutions and nanoporous materials, revealing optical signatures corresponding to the critical process of water adsorption, desorption, and crystallization in nanopores; these processes play essential roles in applications such as aquifer contamination, carbon dioxide sequestration, synthesis of materials and water purification. We then present the design, synthesis, and characterization of AquaDust, a novel nano reporter of the thermodynamic state of water (water potential). We demonstrate AquaDust as a sensor for minimally disruptive measurements of water potential in leaves of intact plants at high spatial and temporal resolution. This creates opportunities for improving our understanding of the mechanisms coupling variations in water potential to biological and physical processes. We then use AquaDust to provide a detailed characterization of the water potential in the otherwise inaccessible living tissues of leaves spanning the last 100 microns traveled by water in the plant during transpiration, from the xylem to the stomata. Our measurements reveal a strong loss of hydraulic conductance in these tissues with decreasing soil water availability but not with evaporative demand, suggesting an active role played by the living tissues in the leaves in regulating transpiration from plants. To conclude, we unveil a dual-method regulation system for water loss in plants. This system involves a two-fold response: the stomatal response, which adjusts according to both soil moisture levels and evaporative demand, and the regulation of mesophyll conductance to water, which is primarily linked to the availability of water in the soil. Our research upends longstanding beliefs about the exclusive role of stomata in water regulation, highlighting the integral part played by mesophyll tissues in minimizing water loss while continuing to absorb carbon. These insights pave the way for devising strategies to boost plant resilience in the face of increasing drought conditions and a changing climate.