When crowns sprout in deep shade, or under black polyethylene sheets, they have the distinctive look of potato sprouts. The stems are elongated and cream colored; the leaves are small and almost colorless. These are all phototrophic responses, that is growth responses controlled by light quantity (amount) and quality (wavelengths). Plants need light to produce the green pigment, chlorophyll. Thus without light the sprouts are almost colorless. If these white sprouts are exposed to light they will soon turn green.
The elongation of the stem (etiolation) in the absence of light is an adaptation for seed germination. As the stem emerges from a soil covered seed it must grow towards the light. This growth is regulated by a hormone, auxin, produced in the shoot tip. Spindly young shoots with small leaves pass more easily through the soil. When the plant reaches the light, chemicals known as phytochromes react with light to reverse this etiolation process. Leaves begin to grow, stem elongation slows, and the thickness of the stem increases. Red and far red wavelengths of light are the most important in reversing the etiolation process.
Dr. Gillian Newberry