## 2 Main Factors Affecting Spectral Reflection Characteristics of Leaves
### Other Influencing Factors  
Leaf age influences reflectance. Changes in the optical properties of leaves happen primarily during the young and mature senescence stages of plants. Annual plants or deciduous leaves maintain constant optical properties most of the time, which is a temporal function of chloroplast content [32]. In the near-infrared band, the reflectance changes only when the leaves wilt and the internal structure changes. In the mid-infrared band, leaf reflectivity increases as the leaves die. Notably, when the leaves are yellow, the reduction in water content is relatively delayed. Fertillizer (nutrient) depletion affects reflectance. Lack of nutrients mainly affects the chlorophyll content and the cross-sectional structure of leaves, so reflectance is obviously related to the nutrients of crops. Chlorosis and nitrogen deficiency are common. Chlorosis mainly affects leaf reflectance in the visible spectrum, and nitrogen deficiency changes the entire reflectance spectrum. The visible light reflectance increases (due to the reduced chlorophyll content), and the near-mid-infrared reflectance decreases (due to the decreased number of cell layers) [33]. Diseases affect the reflectance of leaves. When leaves are infected with plant diseases, their structures or biochemical compositions change, which will cause corresponding variations in the spectral characteristics of leaves [34]. Generally, four situations will occur. First, the pigment contents (yellow pigment) of leaves will change, which only affect the optical properties of leaves in the visible spectrum. Second, leaf necrosis will occur, and the reflectance of the necrotic part is comparable to that of aging leaves. Third, the leaves will produce additional pigments, which will increase or decrease the reflectivity of leaves in different parts of the spectrum. Fourth, because the transpiration rate of leaves only changes the radiation temperature of leaves, rather than the optical properties of leaves, diseases of leaves can be detected in the infrared band. Based on the influencing factors of the spectral reflection characteristics of leaves and the law of refraction, [PERSON] pointed out that the significant symmetry of the solar spectrum transmission and the reflection curve of leaves are caused by the scattering of incident radiation by the cell stacking structure [29]. [PERSON] et al. conducted experimental studies on the transmission and reflection characteristics of leaves and analyzed the impact of scattering on visible light absorption [35]. It was found the scattering in leaves increased the optical path of light in leaves and accelerated visible light absorption. Notably, [PERSON] et al. pointed out the rough structure of the leaf surface can turn parallel light into diffuse light, promoting light absorption within the leaf [36]. Thus, when studying the transflective characteristics of leaves, it is necessary to consider the effects of surface roughness and internal scattering on the transflective characteristics at the same time. To study the relationship between leaf structure or components and visible light absorption, researchers built leaf transmission and reflection models to quantitatively analyze the reflection, absorption, and transmission of solar light in leaves. The classic PROSPECT model describes solar radiation in leaves and is widely used in the field of remote sensing (RS) [37; 38; 39; 40]. The core idea of this model is comparing the cell stack structure to a flat plate structure in form. The absorption in flat plates simulates the absorption in leaves, and the reflection between flat plates simulates the scattering of incident radiation by the cell stack structure. The important input parameters of the PROSPECT model include the absorption coefficient and content of each substance, structural parameters, and surface incident angle. The structural parameter is the number of flat plate structure layers and is used to describe the scattering intensity within the blade.