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This chapter describes the optical properties of leaves at the epidermis and how, to a large extent, the anatomical and morphological structure of the epidermis moderates, controls, and influences the optical properties of the leaf mesophyll and its functioning. We start with the properties of waxes, hairs, and the three-dimensional surface structures and explain many optical phenomena related to scattering of light away from the leaf such as iridescence and specular reflectance, and how surface roughness interacts with water. We discuss how light is focused into the leaf by the epidermal cells, illustrating how this is critical to leaf functions like exchanges of energy and gases.
The purpose of this introductory chapter is to provide a general survey to readers from various backgrounds about how we have thought about leaf properties related to their interactions with light. For example, questions such as “is it colored because of how light contacts with the surface or because some colors of light are absorbed by particular materials?” These questions aroused curiosity about how the nature of interactions with light influence leaf properties, such as observations of leaf color differences on the upper and lower foliar surfaces or why leaves change color in the fall. Investigations from Aristotle up to the 19th century focused on the causes of leaf color and its variation and how these relate to how leaves function. Finally, we introduce some of the earliest studies on the physical mechanisms for the color patterns observed.
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.
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.
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).
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.
A model is a simplified mathematical representation of a phenomenon that simulates its functioning. Science has long used, explicitly or implicitly, approaches that associate models with pure theory and experimentation. These three lines of research are inseparable. We expect a theory to explain the experimental results and predict new results; from an experiment we generally expect it to verify the validity of existing theories and to collect new data. With the exception of a few cases where experimentation is not possible, the confrontation of a model with experimental data is needed to validate it. Sometimes, however, this comparison is misleading.
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.