Healthy leaves exhibit unique spectral reflectance properties, especially in the near infrared (NIR) region. Understanding which plant parameter drives this reflectance is important in fields like remote sensing and plant physiology.

Leaf Reflectance in the Near Infrared (NIR)

The reflectance of a healthy leaf is typically low in the visible light spectrum (around 400-700 nm) due to the absorption of light by photosynthetic pigments like chlorophyll and carotenoids. However, in the near infrared region (around 700-1300 nm), reflectance dramatically increases, reaching values often between 40% and 60%.

Why Healthy Leaves Have High NIR Reflectance

Unlike visible light, near infrared light is not strongly absorbed by photosynthetic pigments. While water content does absorb NIR light at specific wavelengths (e.g., around 970 nm, 1200 nm), the overall high reflectance across the broader NIR region is primarily due to the internal structure of the leaf.

Internal Leaf Structure and Scattering

The internal structure of a leaf consists mainly of the palisade mesophyll and the spongy mesophyll tissues. The spongy mesophyll, located typically below the palisade layer, contains a significant number of irregular cells and intercellular air spaces. When near infrared light penetrates the leaf, it is scattered multiple times as it encounters the interfaces between the cell walls (higher refractive index) and the air spaces (lower refractive index) within the spongy mesophyll.

This multiple scattering event effectively bounces a large portion of the near infrared light back out of the leaf, resulting in high reflectance.

Comparing Factors

• Concentration of chlorophyll: Primarily affects visible light absorption. Has minimal effect on NIR reflectance.
• Concentration of carotenes and xanthophylls: Primarily affect visible light absorption. Have minimal effect on NIR reflectance.
• Water content in the leaf: Absorbs NIR light at specific wavelengths, thus *reducing* reflectance at those points. While significant, it is the structural scattering from air spaces that is the *major* driver of the overall high reflectance level across the NIR plateau in healthy leaves.
• Arrangement of spongy and palisade mesophyll tissue of the leaf: The presence and arrangement of intercellular air spaces within the spongy mesophyll cause significant scattering of NIR light, leading to high reflectance. This structural effect is considered the dominant factor for the high reflectance observed in healthy leaves in the NIR region.

Therefore, the internal scattering caused by the arrangement of cells and air spaces in the spongy mesophyll is the primary reason for the high reflectance of healthy leaves in the near infrared region.