heliotropizmus

Heliotropizmus is a phenomenon observed in some plants where their leaves and flowers track the movement of the sun across the sky. This movement is a form of phototropism, a growth response of a plant to a light stimulus. The term itself is derived from the Greek words "helios" meaning sun and "tropos" meaning turn.

The most well-known example of heliotropism is the sunflower (Helianthus annuus). Young sunflower plants exhibit heliotropism, with their flower heads facing east in the morning and slowly turning to follow the sun westward throughout the day. By night, they typically return to face east, preparing for the next sunrise. This behavior helps maximize sunlight exposure for photosynthesis, particularly in the young, developing plant. As the sunflower matures and its stem stiffens, the flower head often becomes fixed, usually facing east.

Other plants can also display heliotropism, though perhaps less dramatically than sunflowers. Certain legumes, some species of beans, and even some aquatic plants have been observed to exhibit this sun-tracking behavior. The mechanism behind heliotropism involves specialized cells in the plant stem or petiole that respond to light. These cells, often called pulvini, contain hormones that regulate cell elongation. When light hits one side of the pulvinus more intensely than the other, differential growth occurs, causing the plant part to bend towards the light. This adaptation is crucial for plants in environments where sunlight can be a limiting factor for survival and growth.