Measuring the Light Penetration and Distribution within Leaf Tissues

The depth of light penetration into leaves and spectral distribution within leaf tissues were measured using a fiber optic microprobe system (Fig. 18.1) (Qi et al., 2003a), which was modified based on Vogelmann and Bjorn (1984) and Vogelmann et al. (1991). The microprobes were fabricated in Dr. T. C. Vogelmann's lab, using 150 |im diameter (OD) multimode step-index fibers made of fused silica (Polymicro Technologies, Phoenix, AZ, USA). The fibers were heated and drawn to a tip diameter of around 10 |im. The tapered regions of the probes were then coated with evaporated chromium and truncated with a diamond knife. Thus, light entry was confined to the tip of the probes that had near-perfect Gaussian acceptance angles (50% acceptance half width) of 27° to 34°.To operate the microprobe, the tip was threaded through a needle eye and firmly glued onto it (Fig. 18.2(a)); the other end of the probe was connected to a spectroradiometer (Model 754, Optronic Laboratory, Florida, USA) (Fig. 18.2(b)). The needle carrying the probe tip was firmly mounted to a Stepper Mike (Model 18515, Oriel, Stratford, CT, USA) on an XYZ translator (Fig. 18.2(c)). The leaf sample was mounted between two aligned Plexiglas slides perforated with a 5 mm diameter opening in the center of both slides to allow the light to illuminate and the fiber to penetrate. The leaf sample was placed between the slides, with the abaxial side facing the fiber probe and the adaxial side facing the radiation beam from a 75-W Xenon-arc lamp (Hanovia 901C-1), which generated 4 W/m2/nm UV-B flux at 310 nm, or 0.23 W/m2 erythemal UV-B at the leaf surface level. This amount of UV-B was comparable to the ambient level of the UV-B in Baton Rouge, LA, USA, measured by the USDA UVMRP. The erythemal UV-B radiation at the Baton Rouge Station peaked in June at 0.3 W/m2 UV-B with the maximum daily sum of 8.2 kJ/m2.

The measurements of light penetration were made at zero orientation at which the largest portion of light flux moves through the leaf. Photons captured by the microprobe were measured with the calibrated spectroradiometer. The light penetration was measured at four wavelengths including UV-B at 310 nm, UV-A at 360 nm, blue at 430 nm, and red at 680 nm. The spectroradiometer was preset for the wavelength selected before each measurement. The microprobe was auto-advanced at 4 |rm/s through a vein-free region from the abaxial side toward the irradiated surface. Light transmitted to the probe was calculated as the relative amount of light, expressed as the ratio of light measured by the probe inside the

Figure 18.1 Diagram of the fiber optical microprobe system for measurement of light penetration, modified based on Vogelmann and Bjorn (1984). C = Computer, FOM = Fiber Optic Microprobe, S = Leaf Sample, N = Needle, SR = Spectroradiometer, SM = Stepper Motor, XYZ-T = XYZ Translator, SMC = Stepping Motor Controller, PGM = Plexiglas Mount, XAL&LH = Xenon-arc Lamp and Lamp House, CS = Current Source, hv = high voltage (Qi et al., 2003a)

Figure 18.1 Diagram of the fiber optical microprobe system for measurement of light penetration, modified based on Vogelmann and Bjorn (1984). C = Computer, FOM = Fiber Optic Microprobe, S = Leaf Sample, N = Needle, SR = Spectroradiometer, SM = Stepper Motor, XYZ-T = XYZ Translator, SMC = Stepping Motor Controller, PGM = Plexiglas Mount, XAL&LH = Xenon-arc Lamp and Lamp House, CS = Current Source, hv = high voltage (Qi et al., 2003a)

Figure 18.2 The fiber optic microprobe tip is threaded through the eye of a needle and firmly glued on to it (a), and the other end of the microprobe is connected to the OL754 Spectroradiometer (b). A leaf sample is mounted between two aligned Plexiglas slides perforated with a 5mm diameter opening in the center of each slide to allow the light to illuminate from the leaf adaxial surface and the microprobe to penetrate from the leaf abaxial surface (c)

Figure 18.2 The fiber optic microprobe tip is threaded through the eye of a needle and firmly glued on to it (a), and the other end of the microprobe is connected to the OL754 Spectroradiometer (b). A leaf sample is mounted between two aligned Plexiglas slides perforated with a 5mm diameter opening in the center of each slide to allow the light to illuminate from the leaf adaxial surface and the microprobe to penetrate from the leaf abaxial surface (c)

tissue to light measured by the probe without the tissue (probe normal to incident light, the control). Fifteen mature leaves per species were measured for each of the four wavelengths selected.

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