Study on the Relationship among the Machilus Spectral Behavior and Light Intensity and Physiological Activity
2.2 Configuration of Irradiance Measurement
For purifying the circumstances of reflected radiance measurements of vegetation, the remote cosine receptor and the LI-1800 is connected by a fiber probe and is settle to beneath the Machilus leaf to avoid the noises influence from canopy structure and background. Figure 2 depicts such configuration of the irradiance measurements. Because the measurements are made under plant canopies, it is therefore that what the machine recorded is irradiance and hence could reflect the leaf reflected irradiance amount. This study also adopted an accurate cosine correction for each measurement to avoid a significant error occurrence, that is dividing the cosine receptor scanned irradiance data (watts.m
-2.nm
-1) file by the instrument internal COSC file (LI-COR, 1989). Both photosynthetic photon flux density (PPDF) and illuminance (lux) is also calculated in each scan of leaf reflected irradiance. The value of PPDF and illuminance is then used for representing the photosynthetic rate of leaf and the light intensity from the hemisphere.
Figure 2. Configuration of leaf reflected irradiance measurement (adopted form LI-COR, 1989)
3. Result
3.1 Relationship between Photosynthetic Rate and Light Intensity
Relationship between leaf photosynthetic rate (Pn rate) and incident light intensity (Lux) was examined by regression method. The Pn rate could be more precisely estimated by light intensity in a parabolic regression equation in view of the determination coefficient (R
2) of the regression line showed in figure 3a (left plot). More robust relations of those two variables could be achieved by standardization method. Figure 3b (right plot) depicts such stronger relationship of both standardized Pn rate and light intensity. R2 of the linear and the 2 orders polynomial equation is 0.8025 and 0.9225, which is large than the equations build from the original variables about 12 percent and 7 percent. Most important logically meaning of the standardized Pn rate and standardized light intensity relationship is that the polynomial regression line describes leaf Pn rate is increasing positively with the light intensity until the light intensity approaching a critical value where only little marginal effect of photosynthesis will happen. This is evident from the solid curvature line of figure 3b and is agree with the cases of Kramer and Kozlowski (1979).
Figure 3. Relationships between the photosynthetic rate (Pn) of Machilus leaf and hemisphere light intensity (Lux). Left plot (a) and right plot (b) shows the fitted regression lines derived from the original variables, Pn and Lux, and their standardized variables.
3.2 Spectral Behavior of Machilus Leaf
The reflected irradiance (watts.m
-2.nm
-1) of Machilus leaf
starting from 300 nm and ending at 1100 nm is
recorded by a wavelength (
l ) interval of 2 nm and is depicted in figure 4. Patterns of the reflected irradiance (RI) curves (peak-and-valley) around the wavelength of the five samples are almost identical to the ones of a healthy green leaf or vegetation (Lillesand and Kiefer, 1994) and also conform to the spectral irradiance pattern of reflected radiation (LI-COR, 1982).
Figure 4. Reflected spectral irradiance curves of Machilus green leaves.
