Measurement of light gradients and spectral regime in plant tissue with a fiber optic probe

TLDR

The study presents a fiber‑optic method to measure light gradients and spectral regimes inside plant tissue. A 200‑µm fiber probe with a truncated 20–70‑µm tip and 17–20° acceptance angle was inserted into Crassula falcata leaves via a micromanipulator, directing captured light to a spectroradiometer to assess spectral quality and quantity at various depths. Collimated light is almost completely scattered within the first millimeter, leaving only green and far‑red transmission; the resulting light gradient shows 550‑nm light 1.2× incident just below the surface, dropping to 0.5× at 1.4 mm, while 750‑nm light rises to 2.9× within 0.5 mm before falling to 0.5× at 4.5 mm, highlighting the substantial role of scattering in shaping internal light gradients.

Abstract

A method is described in which light gradients and spectral regime can be measured within plant tissue using fiber optics. A fiber optic probe was made by modifying a single optical fiber (200 μm diameter) so that it had a light harvesting end that was a truncated tip 20–70 μm in diameter. The probe was a directional sensor with a half‐band acceptance angle of 17–20°. Light measurements were made as the fiber optic probe was driven through plant tissue by a motorized micromanipulator, and the light that entered the fiber tip was piped to a spectroradiometer. By irradiating green leaf tissue of the succulent Crassula falcata L. with collimated light and inserting the probe from different directions, it was possible to measure light quality and quantity at different depths. Collimated light was scattered completely by the initial 1.0 mm of leaf tissue, which also greatly attenuated all light except the green and far‐red. Light scatter contributed significantly to light quantity and had a pronounced spectral structure. Immediately beneath the irradiated surface the amount of light at 550 nm was 1.2 times that of the incident light. The light gradient declined rapidly to 0.5 times incident light at 1.4 mm depth. In contrast, the amount of light at 750 nm increased during the initial 0.5 mm to 2.9 times incident light and then declined linearly to 0.5 times incident light at the dark side of the leaf (4.5 mm). The implications of the magnitude of the contribution of light scatter to the light gradient is also discussed.