The phenomenon that the growing organs of plants are bent by unidirectional light is called phototropism. For higher plants, phototropism mainly refers to the positive phototropism of aboveground stems and leaves of plants. In the past, it was thought that roots had no phototropism, but in recent years, Arabidopsis thaliana was used as the research material, and it was found that roots had negative phototropism.

Phototropism of plants is most sensitive to shoots, coleoptiles and seedlings growing in the dark. The stem tips of plants such as sunflower and cotton can also rotate with the sun. The etiolated seedlings of Gramineae plants such as oats, wheat and corn, and hypocotyls of peas and sunflowers are often used as phototropism research materials.

Phototropism is an ecological response of plants, such as the phototropism of stems and leaves, which can make leaves in the best position to absorb light energy for photosynthesis.

Extended data:

Sunflower phototropism

The auxin of sunflower is mainly formed at the shoot tip and transported to the base. The distribution of auxin is influenced by light: the concentration of auxin is low on the light side and high on the backlight side.

In this way, the growth area on the illumination side grows slowly, and the growth area on the backlight side grows faster, so that the stem bends phototropically. The growing area of sunflower stem contains high concentration of lutein.

Contrary to auxin, this substance inhibits cell growth.