14.5.1 Reflection

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A considerable fraction of solar radiation incident on leaves is reflected (Figure 14.18), and that fraction varies enormously with wavelength. Reflectance shows a modest peak around green wavelengths, causing plants illuminated with white light to appear green to our eyes. Significantly, the spectral sensitivity of our eyes also peaks at those wavelengths corresponding to green. Reflectance is especially low for blue and red wave-lengths (preferentially absorbed for photosynthesis; Figures 1.8, 1.9).

Leaf reflectivity increases abruptly in the near infrared, to a maximum between about 0.75 and 1.0 µm. This feature enables leaves to forestall energy gain and thus heat build up due to absorption of wavelengths that contribute nothing to photosynthetic carbon gain (but recall photomorphogenic effects of red and far-red radiation either side of 0.7 µm; Section 8.4).

Confining reflectance measurements to visible wavelengths provides an estimate of ‘albedo’ or ‘whiteishness’ of a surface, and plant communities are known to vary in albedo according to habitat and adaptive response. Expressed as a percentage of incident visible radiation, some desert plants show values up to 35% due to thick cuticle or dense pubescence, but most crop communities are around 25%. Pine forests are even lower due to canopy structure and needle geometry, while large water bodies are lowest of all at around 5%. Albedo thus varies according to vegetation type as well as growing conditions, and serves as a basis for remote sensing, enabling satellite detection of changes in crop condition or patterns of land use.