Phototropism

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Phototropism

Phototropism is the orientation of an organism in response to asymmetric illumination. Phototropism is commonly observed in the stems of higher plants, which bend toward a light source as they grow. Phototropism can be positive (bending toward a light source) or negative (bending away from a light source), depending on the organism and nature of the illumination. Phototropism and other tropisms are different from nastic movements, which are also common in plants. A tropism is the orientation of an organism in response to an external stimulus in which the stimulus determines the orientation of the movement. A nastic movement is a growth movement in which the stimulus does not determine the orientation of the movement.

History of phototropism research

Plant physiologists have investigated phototropism for over 100 years. The best-known early research on phototropism was by Charles Darwin, who reported his experiments in a book published in 1880, The Power of Movement in Plants. Although Darwin was better known for his earlier books on evolution (The Origin of Species and The Descent of Man ), this book was an important contribution to plant physiology.

Darwin studied phototropism in canary grass and oat coleoptiles. The coleoptile is a hollow sheath of tissue which surrounds the apical axis (stem) of these and other grasses. Darwin demonstrated that coleop-tiles are phototropic in that they bend toward a light source. When he covered the tips of the coleoptiles, they were not phototropic but when he covered the lower portions of the coleoptiles, they were photo-tropic. Darwin concluded from these and other experiments that (a) the tip of the coleoptile is the most photosensitive region; (b) the middle of the coleoptile is responsible for most of the bending; and (c) an influence which causes bending is transmitted from the top to the middle of the coleoptile.

The Dutch-American botanist Frits Went built upon Darwin’s studies and began his own research on phototropism as a student in the 1920s. In particular, Went attempted to isolate the chemical influence that Darwin described. He took tips of oat coleoptiles and placed them on small blocks of agar, a special type of gel. Then, he placed these agar blocks on the sides of other coleoptiles whose tops he cut off. Each coleoptile bent away from the side that had the agar block. Went also performed important control experiments. He

observed that plain agar blocks, which were placed beneath the lower portions of coleoptiles had no effect on coleoptile bending. Went concluded that the coleoptile tips contained a chemical substance which diffused into the agar blocks and he named this substance auxin. The auxin that Went studied was subsequently identified by chemists as indole-3-acetic acid (IAA). IAA is one of many plant hormones which control a number of aspects of plant growth and development.

Cholodny-Went theory

These and other experiments by Went led to what has become known as the Cholodny-Went theory of tropic curvature. In terms of phototropism, the Cholodny-Went theory proposes that (a) auxin is synthesized in the coleoptile tip; (b) the coleoptile tip perceives the asymmetric illumination and this causes auxin to move into the un-irradiated side; (c) auxin moves down the coleoptile so that lower regions develop an auxin asymmetry; and (d) the higher auxin concentration on the un-irradiated side causes the coleoptile to bend toward the light source.

There is currently vigorous debate among plant physiologists about the Cholodny-Went theory. Critics have noted that Went and other early researchers never actually measured the auxin concentrations but only relied on bioassays performed with agar blocks. Furthermore, the early studies relied on small sample sizes which were statistically unreliable, and the researchers may have wounded the coleoptiles during tip removal.

In addition, numerous recent experiments indicate that the coleoptile tip is not always necessary for tropic responses and that auxin gradients form in the tissue more slowly than the development of curvature.

Despite these criticisms, many plant physiologists maintain that the basic features of the Cholodny-Went theory have been upheld. The debate about the Cholodny-Went theory has stimulated much new research in phototropism and gravitropism. Many researchers currently are investigating tropic curvature using modern time-lapse photography. Others are examining the role of additional plant hormones in regulating phototropism and gravitropism.

The photoreceptor pigment

There has also been an active search for the identity of the photoreceptor pigment, an aspect of photo-tropism not covered by the Cholodny-Went theory. In the 1930s, many researchers believed the photoreceptor was a carotenoid, a class of mostly orange plant pigments. They argued that carotenoids strongly absorb blue light and phototropism is most effectively elicited by blue light. Furthermore, retinal, a carotenoid derivative, was identified as the photoreceptive pigment controlling vision in humans and other animals.

However, more recent experiments appear to rule out a carotenoid as the photoreceptor. In particular, when seedlings are treated with norflurazon, a chemical inhibitor of carotenoid synthesis, they still exhibit phototropism. In addition, mutants of plants and fungi which have greatly reduced amounts of carotenoids are unaffected in their phototropic responses.

A great variety of different experiments now indicate that a flavin (vitamin B-2) is the photoreceptor pigment. Like carotenoids, flavins strongly absorb blue light. However, unlike most carotenoids, they also strongly absorb radiation in the near-ultraviolet (370 nm) region. Radiation in the near-ultraviolet region of the spectrum is also highly effective in phototropism.

Phototropism in other organisms

While phototropism has been most intensively studied in vascular plants, many other organisms also exhibit phototropism. Phototropism occurs in the filaments and rhizoids of algae, germ tubes and protonemas of mosses, rhizoids and protonemas of ferns, spore-bearing stalks of certain fungi, and numerous other organisms.

Many phototropism experiments have been performed on Phycomyces blakesleeanus, a zygomycete fungus. Phycomyces has slender spore-bearing stalks, referred to as sporangiophores, which bend in response to light and other external stimuli. Incredibly, the sporangiophore of Phycomyces is about as photosensitive as the eyes of humans and about one thousand times more photosensitive than a grass coleoptile. Furthermore, the sporangiophore has the ability to adapt to a one hundred million-fold change in ambient light intensity. These and other interesting characteristics of Phycomyces have made it an excellent model organism for investigation of phototropism.

Phototropism in nature

Laboratory studies of phototropism have a bearing upon the life of plants in nature. It is advantageous for a young seedling, such as a coleoptile, to bend toward the light so that its leaves can intercept more sunlight for photosynthesis and grow faster. Phototropism is also related to solar tracking, the orientation of a plant’s leaves in response to the sun.

KEY TERMS

Agar— Carbohydrate derived from a red alga which biologists use in a gel form for culture media or other purposes.

Bioassay— Estimation of the amount of a substance, such as a hormone, based upon its effect on some easily measured response of an organism.

Coleoptile— Hollow sheath of tissue which surrounds the stem of young grass plants.

Gravitropism— Orientation of an organism in response to gravity.

Nastic movement— Growth movement controlled by external or endogenous factors in which the orientation of the movement is not determined by an external stimulus.

Tropism— Orientation of an organism in response to an external stimulus such as light, gravity, wind, or other stimuli, in which the stimulus determines the orientation of the movement.

Unlike the response in coleoptiles, which is caused by differential stem growth, solar tracking responses in most species are caused by pressure changes in special cells at the leaf base. Depending on the species and other factors, the blades of a mature leaf may be oriented perpendicular to the sun’s rays to maximize photosynthesis or parallel to the sun’s rays to avoid over-heating and desiccation.

See also Geotropism.