Leaf dielectric properties influence microwave scattering from a vegetation canopy. The dielectric properties of leaves are primarily a function of leaf water content. Understanding the effect of water stress on leaf dielectric properties will give insight in how plant dynamics change as a result of water stress, and how radar can be used for early water stress detection over agricultural canopies. This paper presents in-vivo measurements of leaf dielectric properties. Different relationships between leaf water content and leaf dielectric properties were found tomato leaves at various heights. The dielectric properties of live stressed and unstressed tomato plants were measured during a controlled, two-week experiment. A clear difference was found between the leaf dielectric properties of stressed and unstressed leaves, which can be attributed to increase in water stress. This results of this study show changes in plant dynamics due to water stress lead to a difference in leaf dielectric properties between stressed and unstressed plants.