Temporal and Spatial Variability of Water Status in Plant Leaves by Terahertz Imaging

Water and its distribution and transport dynamics in green plant leaves are vital to the growth of plants. Owing to the high sensitivity of terahertz (THz) wave to water, THz spectroscopy has great advantages in analyzing the water status of plant leaves. This paper presents a new approach to estimate the water status of plant leaves by the THz time-domain spectroscopy (THz-TDS) technique. Spatial distribution of THz transmission amplitudes located in vein xylem and mesophyll of all three kinds of leaves including wintersweet, ginkgo, and bamboo is detected by THz-TDS measurements. Based on the transmission amplitude, reconstructed THz images show that the water loss in the basal leaf region is more than that in the distal region during the natural drying process for all three plants. A good agreement is reached between the THz imaging method and the direct water weight measurement. To illustrate the accuracy and the sensitivity of the THz technique, the temporal and spatial variations of the water content in the damaged ginkgo leaf with a wound by cutting are also investigated for comparison. The water flow from the basal region to the distal region of the leaf is inferred according to the variation of THz transmission amplitude with the leaf region in different dehydration periods, which is consistent with the string-of-lakes model prediction. This paper shows the feasibility of using THz technology to monitor the temporal and spatial variability of the water status in plant leaves.