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Changes in leaf internal structure may affect leaf optical properties to varying degrees in different parts of the solar spectrum, but they are most evident in the near infrared (NIR) where absorption by pigments is minimal. The mesophyll anatomy of leaves of terrestrial plants is highly variable. To illustrate the effect of such variability on leaf optical properties, Gausman et al. (1971c) selected 11 species displaying a wide range of internal structures (compact, dorsiventral, isolateral, and succulent) and thicknesses.
Leaf directional-hemispherical or bidirectional reflectance and transmittance spectra can directly feed canopy reflectance models as input parameters, but the measurement of these properties is not an end in itself. A large number of spectral analysis methods have been proposed to detect plant biochemistry, ranging from simple band ratios to inversion of radiative transfer methods of varying complexity. The estimation of leaf biophysical parameters is often developed in parallel with the estimation of canopy characteristics, using the same methods as detailed below.
The term “stress” was defined by Jackson (1986) as any disturbance that adversely influences plant growth. Various types of stress can be caused by abiotic (water deficit, nutrient deficiency, salinity, heavy metal, herbicide, air pollution, etc.) and biotic (bacteria, fungi, viruses, insects, etc.) factors. They may induce changes in leaf anatomy, chemistry, and physiology, which will result in changes in leaf optical properties (e.g., Carter, 1993).
As seen in Chapter 8, radiative transfer models have advanced our understanding of light interception by plant leaves throughout the history of remote sensing. They describe absorption and scattering, the two main physical processes involved during the interaction of light with matter. The open-source prospect leaf optical properties model (Jacquemoud and Baret, 1990; Lu et al., 2013) has been the most extensively used radiative transfer model over the past 25 years.
Applications of leaf spectroscopy have many different end uses. Leaf level information is crucial to quantify the state of physiological processes, for example the energy budget and transpiration. It is used to monitor photosynthetic rates and respiration rates. It provides a basis to scale environmental processes from the molecule to the planet. Leaf spectroscopy is also used in remote sensing studies to calibrate processes and provide ground truth data for interpretation, and in agriculture to indirectly calibrate foliar nutrients like nitrogen concentration.
Great progress has been made over the last two decades in the simulation of photon transport within vegetation canopies using radiosity or ray tracing models. At the leaf scale, similarly, it is possible to track a single photon from cell to cell and to derive the optical properties of the entire blade by following the paths of hundreds, thousands, or even millions of photons (see Section 8.2.5). Ray tracing techniques require a detailed description of leaf geometrical properties, as well as knowledge of the mechanisms involved in the scattering and absorption of light at different levels of organization from organelle to leaf and at different wavelengths.
A fundamental understanding of leaf optics has widespread applications ranging from botany, environmental sciences, physics, and astronomy, to applications in art, movies, and videogames. Improved understanding of how plant leaves interact with light permits a more complete understanding and quantifying of ecological processes and functions, not only on Earth, but also for exploration of other planets. Through primary production, plants provide the energy and nutrient resources that supports all living systems on Earth. In addition, the leaf energy budget, largely dictated by the amount of energy absorbed, impacts carbon sequestration, respiration, and transpiration rates, thus providing feedback mechanisms between biogeochemical cycles and the climate system.
This chapter aims to describe the basic anatomy of common groups of higher plants. including the tissues of leaves, their main cell types, and the biochemical constituents that characterize their functional properties. It should provide enough detail on the main construction of plant leaves and how major groups of plants are distinguished based on anatomy, morphology, cell type distribution, and biochemistry. The three-dimensional structure and arrangement of the organelles, cells, and tissues in the leaf are critical to understanding the photon transport in leaf tissue and how these traits relate to the physiological processes of photosynthesis, respiration, and transpiration.
This chapter provides the basis for the absorption and internal scattering properties of leaves derived from theoretical spectroscopy of various chemical components. The absorption of electromagnetic radiation by leaf constituents occurs in different regions of the spectrum. Molecular electronic transitions take place in the ultraviolet (UV) and visible spectrum. This occurs when electrons in a molecule are excited from one energy level to a higher energy level. Transitions between two levels can occur upon the absorption of a photon.