The Thale Cress (Arabidopsis thaliana) is regulated by blue to UV light (plantphys.net)

Phycomyces, a fungus, also exhibit phototropism

Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. Arabidopsis thaliana ( A-ra-bi-dóp-sis tha-li-á-na; thale cress, mouse-ear cress or Arabidopsis) is a small Phycomyces is a genus of Fungi in the Zygomycota family They are known for their strong Phototropism response In other words, it is the growth and response to a light stimuli. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. A tropism (from Greek tropos, to turn is a biological Phenomenon, indicating growth or turning movement of a biological Organism, usually a plant Growth towards a light source is a positive phototropism, while growth away from light is called negative phototropism (or Skototropism). Most plant shoots exhibit positive phototropism, while roots usually exhibit negative phototropism, although gravitropism may play a larger role in root behavior and growth. Gravitropism ''geotropism'' is a turning or growth movement by a Plant or Fungus in response to Gravity. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them.

Phototropism in plants such as Arabidopsis thaliana is regulated by blue light receptors called phototropins. Arabidopsis thaliana ( A-ra-bi-dóp-sis tha-li-á-na; thale cress, mouse-ear cress or Arabidopsis) is a small Phototropins are Photoreceptor proteins (specifically Flavoproteins that mediate Phototropism responses in higher Plants. Other photosensitive receptors in plants include phytochromes that sense red light and cryptochromes that sense blue light. Phytochrome is a Photoreceptor, a Pigment that Plants use to detect light Cryptochrome is a name used for the Blue Light Photoreceptors of Plants and Animals The word cryptochrome derives from Different organs of the plant may exhibit different phototropic reactions to different wavelengths of light. Stem tips exhibit positive phototropic reactions to blue light, while root tips exhibit negative phototropic reactions to blue light. Both root tips and most stem tips exhibit positive phototropism to red light.

Phototropism is enabled by auxins. Auxins are a class of Plant growth substance (often called Phytohormone or Plant hormone) Auxins are plant hormones that have many functions. In this respect, auxins are responsible for expelling H+ ions (creating proton pumps) which decreases pH in the cells on the dark side of the plant. This acidification of the cell wall region activates enzymes known as expansins which break bonds in the cell wall structure, making the cell walls less rigid. In addition, the acidic environment causes disruption of hydrogen bonds in the cellulose that makes up the cell wall. The decrease in cell wall strength causes cells to swell, exerting the mechanical pressure that drives phototropic movement. Phototropism relates to photosynthesis.

Other light responses

• Etiolation is the response of a plant when light is nearly (or completely) absent. Etiolation occurs when plants are grown in either partial or complete absence of light and is characterized by long weak stems smaller sparser leaves due to longer internodes and a
• Heliotropism is the diurnal motion of plant parts (flowers or leaves) in response to the direction of the sun. Heliotropism is the Diurnal motion of plant parts (flowers or leaves in response to the direction of the sun A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the It is not a phototropism since it does not involve growth.
• Photonasty involves the movement of plant parts that does not involve growth but is triggered by light. The plant movement is not determined by the direction of light so it is not a phototropism. Photonasty in prayer plant (Maranta leuconeura) involves the downward movement of leaves when they receive light in the morning.
• Phototaxis is movement of an entire organism in which the direction of movement is determined by the direction of light. Phototaxis is a kind of Taxis that occurs when a whole organism moves in response to the stimulus Light. It occurs in some motile microbes such as Euglena and algae. Euglena are a common group of Unicellular Protists, of the class Euglenoidea of the Phylum Euglenophyta. It is not a phototropism because growth is not required.
• Photo-orientation occurs within a plant cell when chloroplasts change their positions depending upon light intensity. Chloroplasts are Organelles found in Plant cells and eukaryotic Algae that conduct Photosynthesis. This was discovered in 1987 by Chelsea Polevy and Kelsey Joyce when experimenting in their laboratory. When the light intensity is high, chloroplasts move to the edge of the cell to reduce photobleaching (destruction of chlorophyll). Chlorophyll is a green Pigment found in most Plants Algae and Cyanobacteria. ^[1] In low light, chloroplasts tend to spread out within the protoplasm to maximize their capture of light energy. Photo-orientation is also not a phototropism.

References

See also

• Scotobiology

External links

• Time lapse films, Plants-In-Motion