M.Koch Water status measurements of plants using THz spectroscopy

Water status measurements of plants using THz spectroscopy

Martin Koch1 1Philipps-Universität Marburg, Physics Department, 35032 Marburg, Germany

Abstract—THz spectroscopy is well suited to study the water status of living plants. In this talk we will give an overview on the existing work and approaches.

I. INTRODUCTION

Terahertz (THz) waves are strongly absorbed by liquid water. Therefore, THz spectroscopy is well suited to study the water status of living plants. This fact has been known for nearly 25 years, since the pioneering work of Hu and Nuss who recorded an image of a freshly cut leaf using THz time domain spectroscopy [1] and Mittleman who visualized the water uptake of a living plant [2]. A quantitative determination of the water status in living plants is desired by plant breeders as well as by plant physiologists. Here, we review the work carried out over the years highlighting a few approaches.

In the years after the first demonstrations mentioned above not much happened, except that it was shown that THz spectroscopy is fast enough to follow fast processes in plants, like e. g. the rapid leaf movements of mimosa pudica after mechanical stimulus (see Fig. 1) [3].

In 2009 Jördens and coworkes presented an electromagnetic model for plant leaves consisting of an effective medium theory which describes their permittivity at terahertz frequencies [4]. This model was later implemented in an algorithm [5].

In 2011 a photomixing based CW system was used for the first time to monitor the water status of plants [6].

The first to observe effects of dark-light cycles and abscisic acid on the water dynamics using a THz TDS system were Castro-Camus and coworkes who studied Arabidopsis thaliana [7].

Water status measurements on living plants are typically carried out over the course of days or weeks. It is not an efficient use of equipment, if a THz is occupied for such a long time, just to measure a single plant. To overcome this limitation Born and coworkers developed a THz TDS system which allows for the continuous monitoring of up to 20 plants simultaneously. By monitoring the relative change in transmission, they were able to narrow down the permanent wilting point of silver fir seedlings [8].

In 2019 Gente et al. presented a first battery operated THz quasi-time-domain spectroscopy system for continuous outdoor in vivo leaf-water monitoring [9].

Over the years also other THz sources have been employed for plant water status measurements. While Pagano et al. used terahertz quantum cascade lasers for transmission measurements [10], other groups used microwave network analysers [11,12].

Finally, an alternative approach by Gente et al. should be mentioned. They measured the scattering of 35 GHz waves from entire plants to determine the plant water status [13].

Transmission (%)

102 101 100 99 98 97 96 95 94 93

Stimulus

-10 0 10 20 30 40 50 60 70 80 90 100 Time (seconds)

Fig. 1. THz transmission through tertiary pulvini after mechanical stimulus of mimosa pudica. From the drop in THz transmission one can conclude that there must be a net increase of turgor in the tertiary pulvini; from [3].

REFERENCES

[1] B.B. Hu, and M.C. Nuss, “Imaging with terahertz waves”, Opt. Lett. 20, 1716 (1995) [2] D.M. Mittleman, R.H. Jacobsen, M.C. Nuss, “T-ray imaging”, IEEE Journal of Selected Topics in Quantum Electronics 2, 679 (1996) [3] M. Koch et al., “THz imaging of biological samples”, in “Terahertz Sources and Systems”, Ed. R.E. Miles et al, p. 241-258, Kluwer Academic Press, (2001). [4] C. Jördens et al., “Evaluation of the Leaf Water Status by means of the Permittivity at Terahertz Frequencies”, J. Biol. Phys, 35, 255 (2009) [5] R. Gente et al, “Determination of Leaf Water Content from Terahertz Time-Domain Spectroscopic Data”, J. Infrared Milli Terahz Waves 34, 316 (2013) [6] B. Breitenstein et al., “Introducing Terahertz Technology into Plant Biology: A Novel Method to Monitor Changes in Leaf Water Status”, Journal of Applied Botany and Food Quality 84, 158 (2011) [7] E. Castro-Camus et al., “Leaf water dynamics of Arabidopsis thaliana monitored in-vivo using terahertz time-domain spectroscopy”, Scientific Reports 3, 2910 (2013) [8] N. Born et al, “Monitoring plant drought stress response using terahertz time-domain spectroscopy”, Plant physiology 164, 1571 (2014) [9] R. Gente et al., “Outdoor Measurements of Leaf Water Content Using THz Quasi Time-Domain Spectroscopy”, J. Infrared Milli Terahz Waves, 39, 943 (2018) [10] M Pagano et al, “THz Water Transmittance and Leaf Surface Area: An Effective Nondestructive Method for Determining Leaf Water Content”, Sensors 19 (22), 4838 (2019) [11] V. Torres et al, “Monitoring Water Status of Grapevine by Means of THz Waves”, J. Infrared Millim. Terahertz Waves 37, 507 (2016). [12] A. Zahid et al, Characterization and Water Content Estimation Method of Living Plant Leaves Using Terahertz Waves”, Appl. Sci. 9, 2781 (2019) [13] R. Gente et al, “Contactless water status measurements on plants at 35 GHz”, J. Infrared Milli Terahz Waves, 36, 312 (2015)