369
conditions such as sun vs. shade, cold temperature, and others (Thayer and Björkman
1990, Demmig-Adams and Adams 1992, Hannoufa and Houssain 2012). For example, some species with shade-grown leaves may have high concentrations of
α-carotene, and some with sun-grown leaves have only trace concentrations (Thayer
and Björkman 1990). Comparing among species with shade-grown leaves, concentrations of lutein or neoxanthin can vary as much as factors of two (Thayer and
Björkman 1990). Such differences in concentrations of specific carotenoids and/or
their pool sizes provide potential for use in identifying biodiversity patterns if remote
sensing instruments and analytics have power to resolve some of these differences.
14.8.6 Xanthophyll Pigments
Oxygen-containing carotenoids contribute to regulation of photosynthetic functioning. For example, a reversible bond changes between violaxanthin and zeaxanthin
as light environments change from low light to high light conditions, causing a
small increase in reflectance around 530 nm that protects the photosynthetic reaction center from the additional light. Gamon et al. (1992) provided the first experimental evidence that this signal could be measured with spectrometers. The
photochemical reflectance index (PRI) by Gamon et al. (1992) has had extensive
use and is assumed to follow short-term (minutes to hours) xanthophyll cycle
changes; over longer periods, the PRI more likely represents changes in chlorophyll/carotenoid ratios (Gamon et al. 2015).
14.8.6.1 Apparent Concentration vs. Actual Concentration
In some cases, the apparent concentration of a chemical is overestimated or underestimated due to the probability of light interacting with weakly or strongly absorbing chemicals in nonhomogeneous media. An in vivo phenomenon termed the
detour effect increases the probability of absorption for nonhomogeneous distributions of weakly absorbing molecules (Fukshansky et al. 1993; Terashima et al.
2009). This effect lengthens the optical path length within the leaf and increases
photon scattering, enhancing the potential for a weakly absorbing molecule to interact with the photon (Terashima et al. 2009). A different phenomenon termed the
sieve effect has, generally speaking, an opposite effect. The sieve effect decreases
the expected absorption by concentrating strongly absorbing molecules (pigments,
water, and other compounds) in a small area of the cell volume (e.g., in organelles).
Consequently, the molecules have more limited opportunities to interact with a photon and be absorbed. It effectively reduces the path length, and its effect is most
noticeable at wavelengths where light is strongly absorbed [e.g., chlorophyll pigments in the blue and red wavelengths (Evans et al. 2004) or water in the SWIR
(Baranoski and Eng 2007)]. Terashima et al. (2009) show that for weakly absorbed
light such as in green wavelengths, the loss of absorption due to the sieve effect is
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
conditions such as sun vs. shade, cold temperature, and others (Thayer and Björkman
1990, Demmig-Adams and Adams 1992, Hannoufa and Houssain 2012). For example, some species with shade-grown leaves may have high concentrations of
α-carotene, and some with sun-grown leaves have only trace concentrations (Thayer
and Björkman 1990). Comparing among species with shade-grown leaves, concentrations of lutein or neoxanthin can vary as much as factors of two (Thayer and
Björkman 1990). Such differences in concentrations of specific carotenoids and/or
their pool sizes provide potential for use in identifying biodiversity patterns if remote
sensing instruments and analytics have power to resolve some of these differences.
14.8.6 Xanthophyll Pigments
Oxygen-containing carotenoids contribute to regulation of photosynthetic functioning. For example, a reversible bond changes between violaxanthin and zeaxanthin
as light environments change from low light to high light conditions, causing a
small increase in reflectance around 530 nm that protects the photosynthetic reaction center from the additional light. Gamon et al. (1992) provided the first experimental evidence that this signal could be measured with spectrometers. The
photochemical reflectance index (PRI) by Gamon et al. (1992) has had extensive
use and is assumed to follow short-term (minutes to hours) xanthophyll cycle
changes; over longer periods, the PRI more likely represents changes in chlorophyll/carotenoid ratios (Gamon et al. 2015).
14.8.6.1 Apparent Concentration vs. Actual Concentration
In some cases, the apparent concentration of a chemical is overestimated or underestimated due to the probability of light interacting with weakly or strongly absorbing chemicals in nonhomogeneous media. An in vivo phenomenon termed the
detour effect increases the probability of absorption for nonhomogeneous distributions of weakly absorbing molecules (Fukshansky et al. 1993; Terashima et al.
2009). This effect lengthens the optical path length within the leaf and increases
photon scattering, enhancing the potential for a weakly absorbing molecule to interact with the photon (Terashima et al. 2009). A different phenomenon termed the
sieve effect has, generally speaking, an opposite effect. The sieve effect decreases
the expected absorption by concentrating strongly absorbing molecules (pigments,
water, and other compounds) in a small area of the cell volume (e.g., in organelles).
Consequently, the molecules have more limited opportunities to interact with a photon and be absorbed. It effectively reduces the path length, and its effect is most
noticeable at wavelengths where light is strongly absorbed [e.g., chlorophyll pigments in the blue and red wavelengths (Evans et al. 2004) or water in the SWIR
(Baranoski and Eng 2007)]. Terashima et al. (2009) show that for weakly absorbed
light such as in green wavelengths, the loss of absorption due to the sieve effect is
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…