Climate change has caused extreme weather conditions, and resulted in a large water stress in agriculture. Monitoring plant water stress is crucial for both the study of on plant drought responses and the improvement of the agricultural water use efficiency. However, current commercially available water stress sensors either lack of accuracy and resolution, or are too complicated to use. In this study, we developed a micro-tensiometer (µTM), which measures plant water stress in real time by monitoring the stem water potential (Ψ_stem) and the soil water potential (Ψ_soil) - the two most important plant water stress indicators - with high accuracy, high resolution, minimum sample destruction, and optimum local geometrical integration with the sample. The μTM translates the water energy state into electronic signal by implementing traditional tensiometry in a microelectromechanical system (MEMS) with the nanoporous silicon membrane (PoSi) technique. This design significantly increased the measurement range from >-0.1 MPa to >-10 MPa. With the MEMS approach, the sensing area was reduced by two orders of magnitude (from >10 cm^2 to 0.25 cm^2). In situ embedding strategies were developed for the µTM through testing on apple trees. In an in-plant experiment (GH2), the µTM (~ -2.5 MPa) showed up to 1.5 MPa difference from the traditional Scholander pressure chamber (~ -1.0 MPa). This result led to the hypothesis that a vapor gap existed between the µTM and the tissue, and could result in a 7.77 MPa error per degree Celsius of temperature difference (ΔT) between the sample and sensor at 25 ºC. Different strategies were tried to reduce the vapor gap in the fourth experiment (GH4). The µTM with the best contact showed a linear correlation (R^2=0.93) with the Scholander. Other discoveries from the GH4, and their related hypotheses were discussed as well.