All living things respond to stimuli in one way or another. Plant responses are subtle at times, but always very efficient—the solar tracking of leaves to maximize light capture is one example. Plants move, grow and change form in response to a variety of factors. Plant responses to changes in their world are regulated in many ways and on many levels, ranging from the molecular to that of the whole plant.
Tropisms are plant movements directed toward or away from a stimulus. Some familiar tropisms are:
- Phototropism (movement toward light)
- Gravitropism and geotropism (movement relative to a gravitational field, or toward the center of the Earth)
- Thigmotropism (plant growth in response to physical contact)
Other tropisms include:
- Chemotropism (movement in response to a chemical in the environment)
- Hydrotropism (growth or developmental response to water)
- Thermotropism (response dependent upon temperature)
It is helpful to also designate plant responses that are triggered by a definite stimulus, but not in a direction that is related to the direction of the stimulus. Such movements are called nastic movements. Examples of this are a mimosa folding its leaves in response to a touch or a Dionaea (Venus flytrap) closing in response to the presence of an insect.
A lot of tropism research has focused on how plant hormones known as auxins control plant growth by stimulating cell elongation. It is well accepted that phototropic and geotropic bending of shoots and roots results from cells on one side of a plant elongating faster than cells on the other side, thus causing the plant to bend and change the direction of its growth.
Photosynthesis is a popular way to make a living on Earth and plants—as well as some microscopic organisms—have finely tuned, light-controlled positioning systems. Auxins are important in regulating how plant organs move and grow toward a light source and photosynthesis is one of the most-studied topics in botany.
In the basic model of phototropism, the concentration of auxin is elevated on the unlit side of a plant shoot exposed to light, which causes the cell walls on that side to become less fixed in structure. The cells on the unlit side elongate and then re-solidify their cell walls—the effect of this deceptively simple-seeming process is that the elongating cells cause the shoot to bend toward the light.
Exactly how the concentration of auxin gets to be higher on the dark side is not perfectly understood, but current research suggests that auxin is transported from one side to the other and perhaps from other places in the plant. The biochemistry of auxin-regulated bending is still being worked out, even after many decades of research.
Two other pieces of the phototropism puzzle lie in the proteins phototropin and phytochrome. Although these two molecules are almost certainly on the light-receiving end of the phototropism system, the biochemical steps that go from there to the action of auxin are still under investigation.
Geotropism is a form of gravitropism, which is growth in a direction parallel to a gravitational field. Earth’s gravity results from its mass, is directed toward the center of the planet and diminishes in strength as one gets further from the Earth’s surface.
Plants are able to detect the presence of gravity and align themselves with it accordingly. Primary stems are negatively geotropic in most plants and have a strong tendency to grow upward away from the Earth, while primary roots are positively geotropic and grow toward the earth.
Secondary stems and roots are plagiogeotropic, meaning they grow at an oblique angle―not exactly straight down and not perfectly horizontally, either. And then there are the diageotropic rhizomes, which snake along perpendicular to the pull of gravity.
It is clear why plants benefit from having a gravitational sensor system―after all, roots need to get into the ground where the water and minerals are and shoots need to get into the air where the light, carbon dioxide, pollinators and the rest of the exciting world exists―but how exactly do they do it?
Early researchers suspected that auxins played a role in gravitropism and they were correct. Roots curve into the Earth because of differential cell elongation in the root. This is pretty much exactly the same mechanism that guides phototropism, but the sensing and signaling system is different.
As is the case with phototropism, the detection phase of the gravitational response is well known. Starchy grains called amyloplasts in root caps settle, under the influence of gravity, to the lower side of cells―this transmits a biomechanical signal of some kind that indicates which way is up (or down, as the case may be) and the auxin transport and regulation machinery takes it from there.
Plants cannot detect water at a distance and do not have the ability to direct their growth toward it. However, if they do detect water in their environment, plants are able to direct growth in the direction of greater water concentration. Plants also respond to water by rapidly growing when it is present and slowing growth when it is not. Roots can be sparse in a region of low moisture but suddenly explode in a riot of highly branched growth in a spot where water is plentiful.
Roots grow in all directions exploring the local substrate and when a good source of water is found it makes sense for a plant to take advantage of it by shifting resources away from non-productive regions to more promising ones. Some tree species have a reputation for finding their way into water pipes and sewer systems―they are simply taking maximum advantage of a lucky find.
Anyone who has seen a morning glory coiling around a fence post has observed thigmotropism in action. This response occurs following a force contact―the direction of curvature of an extending tendril when it comes in contact with a rigid surface is toward the rigid surface, which results in the tendril growing in a coiled fashion if contact is made with a suitable support.
The sensory input to elicit the growth response is mediated by blebs, which are cell membrane protrusions on the plant epidermis. Blebs operate by an unknown mechanism, but somehow transmit a signal that is acted upon very rapidly. Some tendrils will begin to curve within less than a minute of being subjected to a touch stimulus. At the cellular level, a combination of differential cell elongation and changes in cell turgor pressure are responsible for generating growth along or around a solid object.
Chemotropism is growth toward a chemical stimulus. Both positive and negative chemotropisms are well known to occur. Throughout the evolutionary history of plants the soil has provided a chemically diverse environment, which explains why roots are generally the most chemotropic plant organs.
Early research indicated that plant roots had a tendency to ‘turn away’ from a poor soil toward a healthier one, which suggested some sort of chemical sensing was occurring.
Plants roots tend to proliferate in regions of high nutrient ion concentration and they are also sensitive to organic compounds that can signal the presence of potential sources of nutrients or would-be attackers such as bacteria and fungi.
Pollen also exhibits a strong response to biochemical factors. When a pollen granule alights on the stigma above a clutch of waiting ovules, the growth of its pollen tube is guided inexorably downward by chemical signals from below.
Thermotropism is a plant movement in response to a temperature change. A typical nastic response is that of downward leaf curl in cold weather, which can often be seen in rhododendrons. This movement and change in leaf geometry is thought to be aimed at preventing water loss through stomata on the underside of the leaves.
In controlled experiments where a source of heat is directed at specific plants and plant organs, results are variable. The roots of some plants show positive thermotropism (bending toward the heat source) in one temperature range and negative thermotropism for another temperature range.
In general, higher temperatures (68 to 86°F) elicit a negative response and lower temperatures (59 to 68°F) result in a positive response. Plant thermotropic response can even cancel out the gravitropic tendency of young laboratory-grown corn roots, suggesting there is a complex interaction between the two tropic sensory systems.