How Plant Tropisms Guide Growth

A tropism is growth towards or away from a directional stimulus. A positive tropism grows towards the stimulus, while a negative tropism grows away from it. Because the response changes growth, it is usually slow and cannot be reversed as easily as an animal turning around.

Phototropism is a response to light. Shoots are often positively phototropic because growing towards light helps leaves receive more energy for photosynthesis. Roots usually show little positive response to light and are guided more strongly by gravity and moisture.

Gravitropism is a response to gravity. Roots usually grow in the direction of gravity, helping them enter soil and gain water and minerals. Shoots usually grow against gravity, carrying leaves upward even when a seed begins in an awkward position.

Plant hormones help coordinate tropisms. Auxin is produced in growing regions and can move through plant tissues. In a shoot exposed to light from one side, auxin becomes more concentrated on the shaded side.

In shoots, higher auxin concentration encourages cells to elongate. Cells on the shaded side grow longer than cells on the lit side, so the shoot bends towards the light. The plant has not pulled itself sideways. One side has simply grown more than the other.

Roots respond differently to high auxin concentrations. In roots, extra auxin can slow cell elongation. When gravity causes auxin to gather on the lower side of a root, the upper side grows faster and the root bends downward. The same signal can therefore produce a different result in different tissues.

Roots can show hydrotropism by growing towards areas with more available water. Climbing plants may show thigmotropism, a response to touch, when tendrils coil around a support. These responses do not guarantee success, but they improve the chance that growth is directed towards useful resources.

When analysing a tropism, ask:

• •What directional stimulus is present?
• •Which plant part is responding?
• •Is the growth towards or away from the stimulus?
• •Which side grows faster?
• •How does the response help the plant?

The auxin explanation has a neat experimental history. In 1880 Charles Darwin and his son Francis showed that a grass seedling bent towards light only if its tip was uncovered, so the tip must sense the light and send some signal downwards. Later researchers cut the tip off, placed it on a block of gelatine, and found the block alone could make a headless shoot bend, proving the signal was a chemical rather than an electrical impulse.

In the 1920s Frits Went collected that chemical in small blocks of agar and showed that placing a treated block on one side of a shoot made it bend the other way; the substance was soon named auxin. The story is a good template for exam answers on experimental design: one variable changed at a time, a control kept in darkness, and a conclusion that stops exactly where the evidence stops.

For a root to bend downward, something must detect which way is down. Cells in the root cap contain dense, starch-filled bodies called statoliths. When a root is turned on its side, these grains settle toward the new lower side of the cells, much like sand in a tilted jar. This shift is linked to auxin being redirected toward the lower side of the root, which slows growth there. Experiments that remove the root cap support this idea, because roots without it lose much of their ability to respond to gravity.

Not every plant movement is a tropism. A Venus flytrap snapping shut or the leaflets of Mimosa pudica folding when touched are nastic movements. Their direction is fixed by the structure of the plant, not by the direction the stimulus came from, and many are driven by rapid changes in water pressure inside cells rather than by slow, uneven growth. That is why they can happen in seconds and then reverse. A tropism, by contrast, bends toward or away from where the stimulus actually is.

To study gravitropism, scientists need a way to cancel gravity's direction. A clinostat is a slowly rotating holder that keeps turning a plant so that gravity pulls on it equally from every side, and seedlings grown on one fail to bend in any consistent direction. Space offers a more complete test. On the International Space Station, plants grown in near weightlessness have been observed to orient their roots and shoots using light and other cues instead, showing that gravity is one important signal among several rather than the only guide.

Our understanding of auxin has practical uses too. Rooting powders sold to gardeners contain synthetic auxins, which encourage cuttings to grow new roots at their cut ends. At high doses, some synthetic auxins act as selective weedkillers. Compounds such as 2,4-D cause broadleaved weeds like dandelions to grow in a rapid, uncontrolled and twisted way that kills them, while grasses are much less affected. That is why these chemicals can be sprayed on lawns and cereal crops. A hormone that gently steers growth towards light becomes lethal when the plant cannot regulate it.

Many people believe that sunflowers turn their open flowers to follow the Sun across the sky. The truth is more interesting. Young sunflower plants, before they bloom, do track the Sun, facing east in the morning and west by evening, then swinging back to face east overnight. They do this through growth, like a tropism: the east side of the stem elongates more during the day, bending the top westward, and the west side grows more at night. An internal daily clock helps time these movements so the plant is ready for sunrise.

Once the flower head opens, the stem largely stops elongating and the movement ceases, so most mature sunflowers end up fixed facing roughly east. Researchers tested why by turning some potted plants to face west. East-facing heads warmed up faster in the morning sun and received more visits from bees and other pollinators than west-facing ones. So a field of sunflowers in full bloom usually stares in one direction all day, while the sun tracking you may have heard about belongs to the younger plants still growing towards their full height.

Not all light is equal for phototropism. Experiments shining different colours at seedlings found that blue light produces strong bending, while red light, which matters greatly for photosynthesis, causes little or none. The sensors responsible are proteins called phototropins, found in the membranes of cells in growing tissues. When blue light strikes them they change shape and set off the signalling that shifts auxin towards the shaded side. A shoot lit from one side receives more blue light on that flank than on the other, and that difference across the stem is what tells the plant which way to grow.

You can put this to use at home. A houseplant on a windowsill gets most of its light from one direction, so over a few weeks its stems lean towards the glass. Turning the pot a quarter or half turn every week or so lets each side take its turn facing the window, and the plant grows more evenly. Do not confuse that lean with a different response. A seedling kept in darkness grows unusually tall, thin and pale, using its stored food to search for light. That stretching, called etiolation, happens even when the light has no direction at all, so it is not a tropism.

Tropisms guide plant growth through unequal cell elongation. Light, gravity, water and touch can all provide directional information, while hormones help translate that information into bending. Identify the stimulus and compare growth on the two sides, and the movement becomes clear.