# Characterization and Water Content Estimation Method of Living Plant Leaves Using Terahertz Waves
## 1 Introduction  
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 detailedinsight 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]. Thermogravimetric analysis has been used for the quantification of WC in plants leaves. However, due to its destructive nature and problems owing to its harmfulness to specimens have markedly reduced the usefulness, and thus is not suitable for estimation of WC in plants leaves. On the other hand, some non-destructive methods have previously been used to determine the water status in plant leaves, which include thermal and hyper-spectral imaging [18; 19], infrared [20] and magnetic resonance imaging (MRI) [21]. However, these techniques are limited by the resolution and thus, cannot provide any cellular-scale information about the plants [19]. In addition, these technique do not consider any environmental influence due to its low photon energy [14; 19]. Lately, there has been a growing trend in the field of plant physiology and characterisation of liquid to use THz time-domain spectroscopy (TDS) [11; 22], which is considered a non-invasive technique, and has been deployed in the field of plant physiology to detect anomalies proactively. Moreover, it has an enormous potential to measure the leaf water status under certain conditions, such as drought stress [11; 13; 15; 23; 24] and dehydration kinetics [19]. In addition, THz-TDS technique can also investigate the structural behaviour and complex traits of leaves under any environment [14; 19]. As compared to others, THz-TDS technique has proven to be more effective and reliable.