Terahertz Spectroscopy in Assessing Temperature-Shock Effects on Citrus

Abstract

Rapid assessment of physiological status is a precondition for addressing biological stress in trees so that they may recover. Environmental stress can cause water deficit in plants, while terahertz (THz) spectroscopy is sensitive to changes in aqueous solutions within organisms. This has given the THz sensor a competitive edge for evaluating plant phenotypes, especially under similar environmental stress, if there are existing differences in the corresponding THz information. In this study, we utilized THz technology in association with traditional weighing methods to explore physiological changes in citrus leaves under different temperature, duration, and stress treatment conditions. It was found that the higher the temperature and the longer the exposure duration, the more severe the reduction in the relative absorption coefficient. There was a positive correlation between the trends and the increase in the ion permeability of cells. In addition, based on the effective medium theory, THz spectral information can be transformed into information on free water and bound water in the leaves. Under different treatment conditions, water content shows different trends and degrees of change on the time scale, and accuracy was verified by traditional weighing methods. These findings revealed that characteristics of THz information can serve as a simple and clear indicator for judging a plant's physiological status.

Keywords: THz spectroscopy; high temperature; plant; sensor; water deficit.

Figure 1

Schematic diagram of the experimental process.

Figure 2

THz-TDS system (a), THz spectral measurement in working mode (b), and sample holder with leaf sample (c).

Figure 3

Relative absorption coefficient of leaves after high−temperature stress at 30 °C (a), 35 °C (b), 40 °C (c) for 1, 2, 4, 6 and 16 h, and (d) linear fitting curves. The original reference zero is shown as red dashed line.

Figure 4

Volume fraction of water, free water, and bound water and the ratio of bound water and free water in leaves after high-temperature stress at 30 °C, 35 °C, and 40 °C for 1, 2, 4, 6, and 16 h based on THz spectroscopy.

Figure 5

Correlation analysis of relative electrolyte leakage rate and relative absorption coefficient.

Figure 6

Correlation analysis of leaf WC (a), FWC (b), BWC (c), and BWC/FWC (d) content measured using THz spectroscopy and gravimetric methods.

Figure 7

Linear fitting curves of relative absorption coefficients of leaves under five consecutive intermittent high−temperature cycles.

Figure 8

Relative change in volume fraction of water, free water, bound water, and the ratio of bound water and free water in leaves under 5 consecutive intermittent high−temperature cycles at 30 °C, 35 °C, and 40 °C using THz spectroscopy.