363
14.8.1 Mesophyll Index of Refraction
The refractive indices of cell walls in the visible wavelengths have been empirically
estimated to be 1.4 (Knipling 1970), 1.425 (Gausman 1974), and 1.415 in living
cells and 1.53 in dried cell walls (Woolley 1975). It is very difficult to characterize
the refractive index for pigments, cellulose, cell walls, proteins, etc. given our
inability to measure them in situ. One reason the refractive index is known for so
few plant biochemicals is that in their functional state, these cell constituents are
bound to membranes in complexes with proteins and other molecules, so when they
are isolated, their 3-D structure has been lost along with their bond structures and
other molecular interactions. The function of proteins, pigments, enzymes, (including ribulase-1,5-bisphosphate carboxylase, RUBP-Case, RuBisCO), and amino
acids (all of which contain N) depends on their structure, and empirical methods
have not worked well to obtain their absorption coefficients in vivo, so they cannot
be accurately predicted in radiative transfer models. It has long been known that N
is generally allocated proportional to optimal photosynthesis (Field and Mooney
1986). More than half of all leaf N is allocated to photosynthetic proteins (Makino
and Osmond 1991; Hikosaka and Terashima 1996). Because N forms many types of
bonds, it has been necessary to estimate the total concentration of foliar N from
training data in empirical models, based on either photosynthesis models or statistical models like partial least squares and other self-learning methods (see Serbin and
Townsend, Chap. 3). In recent years, statistical methods have become the preferred
approach to estimate leaf N concentration. These can be accurately applied as long
as the new data have the same statistical structure and ranges as the original test
data. The total concentration of foliar N is often predicted directly from empirical
models using methods like partial least squares regression (Smith et al. 2002;
Ollinger and Smith 2005; Singh et al. 2015) or, more recently, by self-learning
methods, such as wavelets, used by Cheng et al. (2014) to estimate leaf mass area
(dry weight · area
−1
). These methods can easily be overfitted, so care is needed to
produce a realistic result (Féret et al. 2011). Nonetheless, these models can be accurate for the vegetation types and concentrations they are trained against, and they are
being used to estimate functional properties and biological diversity (Asner et al.
2014a, b; Asner and Martin 2016; Féret et al. 2014a, b).
14.8.2 Molecular Absorption Processes
For a wavelength of light to be absorbed, the amount of energy in a photon must
equal the specific energy difference between the resting (ground) state of the electron (S 0 ) and its excited state in an allowable unoccupied higher energy level
(Fig. 14.7). It is the separation of water into an H+ and an OH– ion that provides the
electron that is transferred through the electron transport chain in photosynthesis.
The magnesium ion in the tetrapyrrolic ring of the chlorophyll molecule helps initially stabilize the charged state long enough to transfer it to a phaeophytin in the
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
14.8.1 Mesophyll Index of Refraction
The refractive indices of cell walls in the visible wavelengths have been empirically
estimated to be 1.4 (Knipling 1970), 1.425 (Gausman 1974), and 1.415 in living
cells and 1.53 in dried cell walls (Woolley 1975). It is very difficult to characterize
the refractive index for pigments, cellulose, cell walls, proteins, etc. given our
inability to measure them in situ. One reason the refractive index is known for so
few plant biochemicals is that in their functional state, these cell constituents are
bound to membranes in complexes with proteins and other molecules, so when they
are isolated, their 3-D structure has been lost along with their bond structures and
other molecular interactions. The function of proteins, pigments, enzymes, (including ribulase-1,5-bisphosphate carboxylase, RUBP-Case, RuBisCO), and amino
acids (all of which contain N) depends on their structure, and empirical methods
have not worked well to obtain their absorption coefficients in vivo, so they cannot
be accurately predicted in radiative transfer models. It has long been known that N
is generally allocated proportional to optimal photosynthesis (Field and Mooney
1986). More than half of all leaf N is allocated to photosynthetic proteins (Makino
and Osmond 1991; Hikosaka and Terashima 1996). Because N forms many types of
bonds, it has been necessary to estimate the total concentration of foliar N from
training data in empirical models, based on either photosynthesis models or statistical models like partial least squares and other self-learning methods (see Serbin and
Townsend, Chap. 3). In recent years, statistical methods have become the preferred
approach to estimate leaf N concentration. These can be accurately applied as long
as the new data have the same statistical structure and ranges as the original test
data. The total concentration of foliar N is often predicted directly from empirical
models using methods like partial least squares regression (Smith et al. 2002;
Ollinger and Smith 2005; Singh et al. 2015) or, more recently, by self-learning
methods, such as wavelets, used by Cheng et al. (2014) to estimate leaf mass area
(dry weight · area
−1
). These methods can easily be overfitted, so care is needed to
produce a realistic result (Féret et al. 2011). Nonetheless, these models can be accurate for the vegetation types and concentrations they are trained against, and they are
being used to estimate functional properties and biological diversity (Asner et al.
2014a, b; Asner and Martin 2016; Féret et al. 2014a, b).
14.8.2 Molecular Absorption Processes
For a wavelength of light to be absorbed, the amount of energy in a photon must
equal the specific energy difference between the resting (ground) state of the electron (S 0 ) and its excited state in an allowable unoccupied higher energy level
(Fig. 14.7). It is the separation of water into an H+ and an OH– ion that provides the
electron that is transferred through the electron transport chain in photosynthesis.
The magnesium ion in the tetrapyrrolic ring of the chlorophyll molecule helps initially stabilize the charged state long enough to transfer it to a phaeophytin in the
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…