Characterization and Water Content Estimation Method of Living Plant Leaves Using Terahertz Waves

An increasing global aridification due to climate change has made the health monitoring of vegetation indispensable to maintaining the food supply chain. Cost-effective and smart irrigation systems are required not only to ensure the efficient distribution of water, but also to track the moisture of plant leaves, which is an important marker of the overall health of the plant. This paper presents a novel electromagnetic method to monitor the water content (WC) and characterisation in plant leaves using the absorption spectra of water molecules in the terahertz (THz) frequency for four consecutive days. We extracted the material properties of leaves of eight types of pot herbs from the scattering parameters, measured using a material characterisation kit in the frequency range of 0.75 to 1.1 THz. From the computed permittivity, it is deduced that the leaf specimens increasingly become transparent to the THz waves as they dry out with the passage of days. Moreover, the loss in weight and thickness of leaves were observed due to the natural evaporation of leaf moisture cells and change occurred in the morphology of fresh and water-stressed leaves. It is also illustrated that loss observed in WC on day 1 was in the range of 5% to 22%, and increased from 83.12% to 99.33% on day 4. Furthermore, we observed an exponential decaying trend in the peaks of the real part of the permittivity from day 1 to 4, which was reminiscent of the trend observed in the weight of all leaves. Thus, results in paper demonstrated that timely detection of water stress in leaves can help to take proactive action in relation to plants health monitoring, and for precision agriculture applications, which is of high importance to improve the overall productivity.

Keywords: vegetation health monitoring; leaf water content; terahertz; sensing; plants health

Over the past decade, terahertz (THz) technology has seen an increased amount of interest in the scientific community chiefly due to its non-ionising and less pervasive radiation properties [1]. There has been significant progress in tapping the so-called THz gap 0.3 THz to 3 THz of the electromagnetic spectrum. The THz technology has found extensive use in applications such as the imaging of concealed items [2], material characterisation [1], diagnostic applications including treatment of skin and dental care [3,4], effective and quality control of food [5], and telecommunication [1,6,7]. Furthermore, a distinguishing feature of the THz waves is that the water molecules exhibit a strong absorption spectrum in the pertinent frequency range, leading to novel bio-sensing applications.

Despite these substantial contributions, the utility of the THz technology in the environmental control/monitoring systems has not been explored in depth, especially for vegetation monitoring [8,9]. Unlike the microwave-based remote sensing techniques, the THz technology can provide detailed insight into the health of a plant specimen in terms of the water content (WC) in the leaves [10]. Plant leaves comprise of a composite biological structure of tissues, distinct bio-molecules like cellulose and synthesis compounds including proteins, carbohydrates and many other molecular weight compounds, as illustrated in Figure 1. On an individual basis, they vastly differ in terms of material properties such as relative permittivity [11]. Furthermore, water is not only an essential component but an important nutrient to the process of photosynthesis, and transpiration in the overall process of growth [12]. Due to the high sensitivity and strong penetration feature of THz, it has a strong potential to disseminate through plants leaves at cellular level as shown in Figure 1 and can yield significant information of WC in leaves. Hence, it is significant to highlight the frequency dependence of the permittivity of leaves. Designing a smart and plant-specific irrigation system that monitors the leaf WC in a non-invasive manner is, therefore, critical in the current circumstances governed by global climate change that demand water conservation. Over the years, significant contributions have been made [9,10,11,12,13], that address estimating the leaf’s WC. There are techniques that offer high reliability, yet they are unsuitable for long-term studies of the same plant leaves because of validity of measurements cannot be guaranteed [11,14,15,16,17].

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In this paper, a novel, non-invasive technique for characterising the water content (WC), and in turn the health of plant leaves was proposed using THz waves. The electromagnetic properties of eight types of leaves were determined for four consecutive days through the measured scattering parameters. The weight and thickness of the leaves were also recorded at the same time. We observed that the leaves became increasingly transparent to the terahertz (THz) waves through the course of four days experiment, as seen by the peaks in the real part of permittivity. Similar decaying trends were observed in the peak values of the real part of the extracted relative permittivity as the decreasing weight due to loss of WC. The significance of this paper lies in the simple, cost-effective technique and other advantages such as: (a) This paper proposes a unique technique to characterise and estimate WC of eight various leaves in terms of electromagnetic parameters at THz frequency range from 0.75 to 1.1 THz. (b) The electromagnetic parameters are measured in simple, fast, and non-invasive manner using a THz material characterisation kit. Moreover, The structural integrity and configuration of leaves were also considered by employing two polytetrafluoroethylene (PTFE) caps which were fitted internally to the waveguide. (c) This paper establishes a notable correlation between electromagnetic parameters with WC in leaves i.e., change in the WC of leaves is reflected in the electromagnetic parameters at certain frequencies. In the age of a climate change driven water conservation, the proposed scheme can be used to design efficient irrigation systems on-site without any need to remove the leaves from plants.

Conceptualization, A.Z., H.T.A. and Q.H.A.; software, A.Z. and H.T.A.; resources, D.R.S.C. and K.A.Q.; writing–original draft preparation, A.Z. and H.T.A.; writing–review and editing, Q.H.A., M.A.I. and A.A.; supervision, Q.H.A. and M.A.I.; project administration, Q.H.A.

This research was funded under EPSRC DTA studentship which is awarded to Adnan Zahid for his PhD.

The authors declare no conflict of interest.