11.2.5Root Evolution Under a UV-B E

11.2.5Root Evolution Under a UV-B EnvironmentAs above-mentioned, plant UV-B receptors have recently been discovered and arebeing studied for their elusive functions in roots. Why does the root system havesuch a sensitive mechanism for the perception of light? In general, roots arethought to be always in the dark underground environment, protected fromsunshine. It is obviously important that roots “have to” be in the soil in order to fixthe plant body properly and take up water and certain nutrition. In fact, this couldbe the reason why plant roots evolved abilities to recognize the light source andaccomplish escape tropism from the illuminated condition (Figure 11.2). Although it has been demonstrated using physical models that UV-B can only penetrate intothe ground less than 100 mm due to the absorbance of soil [37,38], light-penetratingcracks in the ground or removal of soil may occur at anytime in nature. Forinstance, roots may be easily exposed to light when large animals trample thesurface of the ground, and through climate change, earthquakes, heavy rain, and soon. In addition, it is also important to speed up the root growth rate of the radicleimmediately after germination of plant seeds scattered on the ground (Figure 11.3).