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Fig. 2 | Plant Methods

Fig. 2

From: Non-invasive absolute measurement of leaf water content using terahertz quantum cascade lasers

Fig. 2

a Measured optical depth \(\tau\) as function of the leaf water mass \(M_W\). The experimental data (black squares) are linearly fitted (red straight line) as \(\tau = C_1 + C_2 \cdot M_W\). The linear fit has a coefficient of determination \(R^2 = 0.31\); the coefficients are \(C_1 = 3.2 \pm 0.1\) and \(C_2 = 2.0 \pm 0.1 \, \text {g}^{-1}\). b Measured optical depth versus \(M_{W} A^{-1}\). The coefficient of determination is greatly improved with respect to the previous model, by the simple measurement of the leaf projective area A. In this case the linear model produces a best fit with \(R^2 = 0.74\). There are two statistically significant parameters: the intercept \(C_1 = 1.0 \pm 0.1\) may be ascribed to residual scattering and absorption from the leaf dry mass and vapor, whereas the slope \(K = 0.33 \pm 0.03 \, \text {cm}^2 \, \text {mg}^{-1}\) is linked to the effective absorption coefficient of water; according to Eqs. (5) and (6) \(\alpha _{\text {eff}} = 330 \pm 30 \, \text {cm}^{-1}\). If the intercept is set to 0 as proposed in Eq. (6), the absorption coefficient changes to \(\alpha _{\text {eff}} = 444 \pm 2 \, \text {cm}^{-1}\), and the model fit accuracy is reduced (see the green line in the graph)

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