Terahertz spectroscopy for quantification of free water and bound water in leaf

Abstract The water in a plant is known to exist as free water and bound water, and the proportional change between the two forms is an important indicator of the physiological state of the plant. Terahertz (THz) spectroscopic techniques are effective in measuring the water content and describe the hydration state, due to the coincidence of THz photonic characteristics and the hydrogen bond in both their time scale and energy level range. In the same vein, the ability of the THz spectroscopy in discrimination of free water content (FWC) and bound water content (BWC) in a plant should be a natural extension, but has thus far not been realized. Here, a THz nondestructive method for quantitatively detecting the FWC and BWC in leaves based on the effective medium theory is proposed and demonstrated. Good consistency is obtained between the THz-based method and the traditional method by experimental verification using the leaves of four different species as samples, with R2 about 0.9, employing the particle swarm optimization algorithm for the spectral analysis at 0.7 and 0.9 THz, exhibiting drastically improved efficiency for quantitative detection. Further, THz-based monitoring has shown certain unique advantages to be used to estimate the activity of plant branches. This simpler yet highly effective methodology opens up the possibility for a broad application range of the FWC/BWC index in physiological experiments and agricultural managements.