4 Phototropism and Its Relationship to Gravitropism
Jack L. Mullen and John Z. Kiss*794.1 Phototropism: General Description and Distribution
That sunlight can affect the development of plants has been known for thousands of
years, and one of the more striking effects is the growth of plants toward areas of more
intense light. This response, phototropism, is caused by a difference in growth rate across
part of the plant with the direction of the growth differential determined by the gradient
in light intensity. Phototropism is widespread among plants and is found in algae, mosses,
and ferns, as well as seed plants. In flowering plants, stems are generally positively pho-
totropic, curving toward the direction of highest light intensity. However, there are some
species, including climbing plants with tendrils, which have negatively phototropic
shoots, allowing them to grow toward neighboring plants (Darwin 1875; Strong and Ray
1975). Some recent reviews of phototropism include Liscum and Stowe-Evans (2000),
Kimura and Kagawa (2006), and Whippo and Hangarter (2006).
Leaves are also frequently phototropic, though the effect of light on leaf growth is
more complex than for stems. Leaves of many species engage in a diurnal phototropic re-
sponse, following the movement of the sun. Much of the curvature occurs in the leaf peti-
ole, which is capable of sensing the light directly or, if shaded, responding to light sensed
by the leaf blade (Haberlandt 1914). Some species have specialized turgor-driven motor
tissues, called pulvini, which allow for greater reversibility in the response. The leaf blade
itself may also reorient to be normal to the incident light and can track the sun across the
sky in this position (Lang and Begg 1979; Koller and Levitan 1989). Since the direction
of the light is nearly perpendicular to the leaf surface throughout the day, this response
requires impressive sensitivity to changes in photostimulation.
Although its importance is not clear, phototropism has also been observed in root sys-
tems, with roughly half of species examined showing some response (Hubert and Funke
1937). Root phototropism could be useful in positioning lateral roots near the soil sur-
face, where light may penetrate, though the response is generally smaller than the grav-
itropic response of roots (Kiss et al. 2002). The majority of roots showing a response are
negatively phototropic (Hubert and Funke 1937; Okada and Shimura 1992). However,
roots of Arabidopsiscan respond either positively or negatively to unidirectional light,
depending on the light quality (Kiss et al. 2003). Thus, it appears that multiple light-
signaling pathways interact with other growth responses such as gravitropism, even be-
lowground in the root systems.
*Corresponding author