Plants And Light Phototropism

Sunflowers loom large in Fort Collins gardens

The Maximillian sunflower (Helianthus maximillanii) is a perennial sunflower that is native to North America. It is a reliable bloomer in the Colorado area. It easily withstands the intense heat and produces buckets of 3-inch yellow flowers that are loved by butterflies and birds. It reaches heights of 3 to 10 feet and is drought-tolerant once established. The sunchoke or Jerusalem artichoke (Helianthus tuberosus) is another example of a perennial sunflower native to North America. It produces bright yellow blooms above ground while growing an edible tuber below ground. Jerusalem artichokes grow easily in Colorado and the gardener may be tempted to leave them undisturbed. However, the plant needs to be dug and replanted every few years or the quality of the tuber will degrade. Sunflowers have a rough, hairy stems with broad, coarse leaves and circular heads of flowers. The head is not a single flower but is made of thousands of small flowers or florets that are crowded together and join in the center. The outer petal-bearing florets can be any color from yellow to red to orange to brown, while the florets inside the head mature into seeds. Some sunflower varieties have an inability to self-pollinate and require insects to move the pollen between plants, a job that bees happily perform. Many gardeners believe that the sunflower will track the sun, an action known as heliotropism. However, only young sunflowers have leaves and buds that change from east to west during the day. Once the plant is mature, the flower will typically face east and will not move. The plant is a bit of a gymnast while growing and demonstrates phototropism, which occurs when the petioles (leaf stalk) bend and twist toward the light during the day then unbend and untwist at night. Sunflowers need at least six hours of full sun a day and should be spaced 12 inches apart. Large sunflowers should be spread sporadically throughout the garden to avoid crowding. Avoid high nitrogen soil that encourages plant growth but fewer blooms. Sunflowers can be planted when day and night temperatures are above 50 degrees with optimum temperatures for growth being 70 to 78 degrees.

Sunflowers are not considered highly drought tolerant although the plant does have an extensive branched taproot, which may penetrate up to nearly 7 feet and can aid the plant during water stress. A critical time for water stress is the period 20 days before and 20 days after flowering.

Do all plants have phototropism? | TheFreeResource.com

Most plants are phototropic, which means that they respond to a light stimulus and grow toward the light. The reason for this is that plants contain a hormone called auxin which reacts to light. If a plant, for example, has one side exposed to light while the other side is in the shade, the auxins will expand in the shade and cause the plant to grow toward the light. There is thus more growth in the shade.

There are two reasons why the side of plants that is in the shade grows faster than the side that is in the sun . In grass and smaller plants, the growth hormone becomes more concentrated on the shady side and this causes this side of the plant to grow more quickly. However, in other plants, such as sunflowers and radishes, there is a natural chemical that slows down the growth of the plant’s sunny side. In both cases, the plant turns towards the light and the part of the plant that is in the shade grows faster.

Phototropism is a very strong force; most plants need light in order to be able to grow through the process of photosynthesis. Plants will do everything they can to obtain that light, even if it means they have to grow almost parallel to the ground in order to take advantage of the light. Phototropism is known to take place throughout the life of a plant. But it is most pronounced and can be manipulated more efficiently when the plant is still young.

There are two kinds of phototropism- positive and negative. Positive phototropism is the way a plant grows towards the light. It enables the plant to position its leaves so that it will be able to receive the energy it needs to grow and reproduce. On the other hand, negative phototropism takes place when the plant’s roots grow away from the light. This is also an important process, because it allows the plant to find the nutrients, soil and water it needs and to anchor itself in the ground.

Phototropism was first discovered in 1809. The Swiss botanist, Augustin de Candolle, verified that it was light that caused plants to grow, rather than air, as was previously believed. 70 years later, Charles Darwin and his son, Francis, also studied plants and plant growth. The Darwins found that a stimulus and a response can occur in different parts of the plant. This led to the subsequent discovery of plant hormones called auxins. Auxins cause the cells of plants to elongate during the process of phototropism and the plants to bend towards the light. A Dutch botanist named Frits Went made this discovery and named the growth hormone auxin, from the Greek word which means “to increase”. It was only in the 1940s that the chemical properties of the auxin hormone were identified as indole-3-acetic acid.

Plant tropisms

In most cases, red light does not induce phototropism in flowering plants. Red and far- red light are sensed by the phytochromes (Figure 4.1)—photoreceptors ...

Light sensing in plants

At high intensity of blue light phototropism is attenuated. ... In higher plants , however, little is known concerning the interactions of gravitropism and ...

Photomorphogenesis in Plants and Bacteria, Function and Signal Transduction Mechanisms

As is evident during phototropism, light perception and signalling are critical for polar auxin transport throughout the stem of a plant. ...

Light and plant development

10.5.3 Root phototropism Like its shoots, Arabidopsis roots exhibit blue-light- mediated phototropism, ...

Photoreceptors and light signalling

The latter are still by far the best-characterized plant light-sensors [reviewed in ... This differential growth reaction, commonly known as phototropism, ...