Final answer:
Near infrared photos reflect off healthy vegetation due to chlorophyll, which absorbs more in the red than in the near-IR spectrum. Plants utilize the phytochrome system to detect light quality, enabling them to grow toward light and away from shade to compete for light in their environment.
Step-by-step explanation:
When near infrared photos intercept healthy vegetation, the near infrared light is largely reflected by the leaves due to their chlorophyll content. Chlorophyll absorbs strongly in the red region of the visible spectrum, aiding in photosynthesis, but not as much in the near-infrared (IR) region. In addition to red light, plants also use light in the violet-blue region of the spectrum for structural development, which is absorbed by different sensory photoreceptors. The phytochrome system in plants responds to the ratio of red to far-red light, allowing plants to sense shaded environments and optimize their growth direction towards light sources.
However, when in the shade, plants are exposed to more far-red than red light, which signals to the phytochrome system in the shaded leaves to convert to the Pr (inactive) form, slowing their growth. Conversely, in areas receiving more red light, growth is stimulated by converting phytochrome to the active Pfr form. This critical adaptation reflects an evolutionary advantage allowing plants to grow away from shade and compete effectively for light in dense plant communities.