Leaf Optical Properties

The absorption and reflection of irradiance within the plant canopy is determined by the optical properties of the leaves, in addition to their geometric orientation. Seagrass leaves, like those of all photoautotrophs, contain optically active pigments. Chlorophylls (Chls) a and b are the most abundant pigments and the only ones responsible for photosynthetic light harvesting in seagrasses. Although the pigments are principally responsible for determining the absorptance (A) and reflectance (R) of the leaf, the optical properties of intact leaves result from the complex way in which the pigments are arranged within chloroplasts and cells. This "package effect" reduces the light harvesting efficiency of the chlorophyll within the leaf and flattens the absorption spectrum of the intact leaf with respect to that of the optical properties of the freely dissolved pigments (Duysens, 1956; Larkum et al., Chapter 14).

Fig. 3. Photomicrograph of a cross section of a turtlegrass leaf showing two layers of chloroplast-dense epidermis, a chloroplast-free mesophyll and air-filled lacunae running parallel to the central axis of the leaf. Image provided by F. Dobbs.

Seagrass leaf anatomy is relatively constant across species with respect to leaf thickness, chloroplast distribution, lacuna volume and even the developmental sequence leading to mesophyll differentiation (Tomlinson, 1980). Unlike terrestrial leaves, chloroplasts are restricted to the epidermis and there is no spongy mesophyll (Fig. 3). This arrangement presumably facilitates gas exchange between the leaf surface and the surrounding water. The restriction of chloroplasts to thin epidermal layers in seagrass leaves may also be advantageous in light limited environments, as has been shown for shade-adapted terrestrial leaves (Lee and Graham, 1986). This restriction, however, enhances the package effect such that large differences in leaf chlorophyll content are required to produce even minor differences between the absorption spectra of sun and shade-adapted seagrass leaves (Cum-mings and Zimmerman, 2003). Absorption and reflectance spectra of seagrass leaves are qualitatively typical of vascular plant and green algae pigment systems dominated by Chls a and b (Fig. 4). In general, leaves that contain higher concentrations of chlorophyll pigments have higher absorption coef ficients and lower reflectances, but the differences are not linearly proportional to pigment concentration because of the package effect. For example, a 9% increase in the PAR-averaged absorptance of low light-grown eelgrass (Zostera marina L.) leaves relative to turtlegrass (Thalassia testudinum Banks ex Konig) leaves from a high light, tropical environment required a five-fold increase in Chl a + b content (Cummings and Zimmerman, 2003). Despite the structural restrictions on chlorophyll distribution that produce strong package effects, clean seagrass leaves (even those with relatively little chlorophyll) can absorb at least 75% of the incident light, even in green biased light environments. This is similar to the absorptances of higher plant leaves and thalli of macrophytic algae (Givnish, 1987; Smith and Alberte, 1994). Scattering caused by refractive index changes at the lacuna/tissue boundaries may promote light absorption by increasing the effective optical pathlength within the leaf, but this has not been investigated in detail.

Measuring the optical properties of intact leaves requires the use of a spectrophotometer fitted with

Fig. 4. Optical properties of eelgrass and turtlegrass leaves. (A) Absorption spectra plotted as absorption coefficients (left axis, per m of leaf thickness) and as optical density (right axis, per unit leaf thickness). (B) Reflectance spectra. Note that absorption is lower, and reflection is higher from the less pigmented turtlegrass leaf. From Zimmerman (2003). Copyright (2003) by the American Society of Limnology and Oceanography, Inc.

Fig. 4. Optical properties of eelgrass and turtlegrass leaves. (A) Absorption spectra plotted as absorption coefficients (left axis, per m of leaf thickness) and as optical density (right axis, per unit leaf thickness). (B) Reflectance spectra. Note that absorption is lower, and reflection is higher from the less pigmented turtlegrass leaf. From Zimmerman (2003). Copyright (2003) by the American Society of Limnology and Oceanography, Inc.

an integrating sphere that permits accurate measurement of the scattered radiant flux emanating from turbid samples (Fig. 5). The spectrophotomet-rically measured optical density [D(À)], however, results from both the absorption and reflectance, or backscattering of light emanating from the leaf surface. Consequently, the raw optical density must be transformed into leaf absorptance (AL) and corrected for leaf reflectance (RL) in order to calculate the light absorbed by the canopy:

The absorptance then must be transformed into an absorption coefficient [aL(À)], which represents the probability of photon survival, for use in the radiative transfer calculation described below:

Although AL(A) represents a dimensionless ratio [see Eq. (2)], aL(X) assumes units of inverse meters (m-1) by virtue of its normalization to leaf thickness (*l).

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