Heliotropism

Heliotropism refers to a plant’s ability to track the sun’s movement.

For many centuries, it was believed that a plant’s tendency to follow the sun as it crossed the sky was a passive action caused by water loss on the side of the plant exposed to sunlight. Now, we know that there is far more to it than that.

Sunflower

Growing toward sunlight

Instead of passively shrinking to one side as the sun’s harsh rays boil away a plant’s bodily fluids, we now know that plants actively grow toward (or away from) sunlight. [When a plant grows away from sunlight, it is called skototropism.]

Experiments conducted in the 1800’s demonstrated that plants will respond to any type of light: street lights, grow lights, or sunlight. When plants are attracted to this light, it is called phototropism. Phototropism is a function of the hypocotyl, or individual cells found in the same region. Hypocotyls are the embryonic stem found below the seed leaves (cotyledons) and directly above the root. You can easily see examples of phototropism when seedlings first emerge and they don’t get enough sunlight - they become leggy and lean toward whatever light they can. This is phototropism.

In heliotropism, not any old light source will do. It is only radiation from the sun that causes the reaction. And the mechanical causes of these two types of movements are very different.

Mechanics of plant movements

When plants move in response to the position of an external stimulus, it is called a tropic [TRO-pic] movement. If a plant’s movement is independent of the stimuli’s position, it is called a nastic movement. In phototropism, plant hormones (auxins), found in the meristem tissue of leaf and stem tips, photoreceptors, and multiple signaling pathways are used to direct a plant to grow more rapidly toward sunlight. In heliotropism, a structure called the pulvinus is used to direct movement.

The power of pulvinus

The pulvinus is an amazing, fluid-controlled joint found at the base of a plant leaf stem (petiole) or just below a flower.

Pulvinus of 'Oxalis rosea', from: Charles Darwin (1880): The Power of Movement in Plants. Public Domain

The pulvinus causes movement by altering fluid pressure in the surrounding plant tissue. These changes in fluid pressure start when sucrose is moved from the phloem into the apoplast. The apoplast is the conjoined spaces between plant cells. As sugar is pumped into the apoplast, potassium ions are pushed out, followed by water molecules. This changes the pressure within the affected cells, causing movement. This is called turgor-mediated heliotropism. But not all heliotropic flowers have a pulvinus. Those that do not are still able to move by permanently expanding individual cells. This is called growth-mediated heliotropism. Pulvini are also used in response to nyctinastic and thigmonastic movements.

Heliotropic flowers

Heliotropic flowers face the sun from dawn to dusk. Slowly tracking the sun’s path across the sky, these flowers are believed to use heliotropism as a way to improve pollination, fertilization, and seed development. Heliotropic flowers often have five times as many beneficial insects present, due to the added warmth. [Many tropical flowers exhibit a modified form of heliotropism in which flowers maintain an indirect tracking of the the sun. This is believed to reduce the chance of potential overheating.] Beans, alfalfa, sunflowers, and many other species turn their blooms to follow the sun’s path across the sky each day. But sunflowers only use heliotropism in their early development, in the bud stage. Once a sunflower head emerges, it may track the sun for a short time, as an expression of phototropism, until the flower head reaches full size. The majority of sunflowers found in the northern hemisphere nearly always end up facing east.

Leaf heliotropism

Like floral heliotropism, leaf heliotropism is the method by which plants focus their leaves perpendicularly to the sun’s morning rays (diaheliotropism), or parallel to midday sun (paraheliotropism). Diaheliotropism allows leaves to capture the maximum amount of energy from the sun, while paraheliotropism protects plants from overheating and dehydrating.

How do your plants move during the day?