In what form is the light energy transferred to the root cells?

Light energy is used by the plants to prepare food. This process is called photosynthesis. Photosynthesis takes place in the green leaves that contain chlorophyll pigment. During the process of photosynthesis, light energy, carbon dioxide and water is used for the production of glucose and oxygen. Glucose acts as food for the plants that is stored in the form of starch for later use. The food is transferred to all the parts of the plant body, including the roots with the help of phloem tissues.
Thus, light energy is transferred to the root cells in the form of chemical energy (glucose).

Spherical shaped cell might increase the surface area and decrease the volume.