PLANT BIOPHYSICS - Y. I. Posudin - 2004

I. PHYSICAL PROPERTIES OF PLANT CELLS AND TISSUES

5. OPTICAL PROPERTIES OF PLANTS

5.2. SPECTRAL PROPERTIES OF THE LEAF

The cuticle covering the epidermis is responsible for the specular reflection of optical radiation when it hits the leaf surface at a non-zero angle of incidence. Increasing the angle of incidence leads to a higher intensity of specularly reflected radiation. Thus, the total reflected radiation consists of diffuse radiation—which depends on the light wavelength, internal leaf Structure, pigment composition, and Water content—and specular radiation, which is produced by the cuticle and is partially polarized. Leaves reflect only 6-10% of the optical radiation in the visible spectrum.

Optical radiation transmittance by a leaf ranges from 3% to 40% of the incident radiation. Soft, pliable, and thin leaves exhibit higher transmittance compared to rigid and coarse leaves.

The absorption, reflection, and transmission spectra of a plant leaf are shown in Fig. 5.3. These spectra are characterized by three specific regions.

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Fig. 5.3. Absorption, reflection, and transmission spectra of a plant leaf.

The visible region (400-700 nm) is marked by weak reflection (maximum 15%) and very low optical radiation transmittance. Leaf absorption accounts for 60-80% of the incident radiation. In higher plants, the primary pigments responsible for absorbing optical radiation in the visible spectrum are chlorophylls (with absorption maxima between 450 nm and 660 nm), as well as carotenoids, xanthophylls, and anthocyanins. Absorption is also influenced by the Internal Structure of the leaf: the spongy mesophyll absorbs optical radiation more intensively than the palisade Cells. The screening effect produced by pigments and the scattering of optical radiation inside the leaf give rise to internal light gradients. All plant pigments become nearly transparent at wavelengths exceeding 700 nm. Water absorption exhibits a minimum at 300-580 nm.

The near-infrared region (700 nm-1.3 µm) is characterized by a low leaf absorption rate of about 10%, causing optical radiation to be either reflected or transmitted by the leaf. The reflectance can reach up to 50% due to internal structural elements such as Cellulose. In this spectral region, water has absorption maxima at 0.9 µm and 1.1 µm.

The mid-infrared region (1.3-2.5 µm) is distinguished by intense water absorption bands within the plant tissue at 1.45 µm, 1.95 µm, 2.74 µm, and 6.3 µm.

The spectral properties of a leaf are influenced by its internal structure, Morphology, and physiology, surface characteristics (such as roughness and pubescence), age, water content, mineral deficiency, and pests. This very dependence of spectral parameters on environmental and physiological factors forms The basis of spectral monitoring Methods for plant canopies.

Review Questions

1. State the main theories explaining the mechanisms of optical radiation propagation through a leaf.

2. What factors determine the spectral properties of a leaf in the visible region of the spectrum?

3. What factors determine the spectral properties of a leaf in the near-infrared region?

4. What factors determine the spectral properties of a leaf in the mid-infrared region?