Phototropism, Gravitropism, and Photoperiodism

Phototropism is growth of a plant in response to light direction, gravitropism is growth in response to gravity, and photoperiodism is a response to the relative lengths of day and night, which controls processes such as flowering. In brief: shoots grow toward light (positive phototropism) and away from gravity (negative gravitropism), while roots grow downward (positive gravitropism). Short-day plants, such as chrysanthemums, flower when the night is longer than a critical length, and long-day plants, such as spinach, flower when the night is shorter than a critical length. The light-sensing pigment phytochrome measures the length of the night.

These responses show how plants sense their environment without nerves. The earlier article on tropisms introduced the basics. This advanced article explains the photoreceptors and the cellular mechanisms, the statolith hypothesis, how phytochrome controls flowering, the practical applications, a worked example on interpreting flowering experiments and common mistakes.

How does phototropism work?

The blue-light photoreceptors, called phototropins, are proteins in the plasma membrane of cells at the tip and upper part of the shoot. They absorb blue light (about 450 nm), and the most effective wavelengths for bending are in the blue range. The photoreceptor triggers a redistribution of the transport proteins that carry auxin (the PIN proteins), which moves auxin to the shaded side of the stem.

Cells on the shaded side receive more auxin, so they elongate more than those on the lit side, and the shoot bends toward the light. This explains the Cholodny-Went hypothesis. The bending increases the leaf area exposed to light. Roots are generally less sensitive and some bend slightly away from light. The response to blue light also helps to open stomata. Red light does not cause much bending, which was shown using filters.

How does gravitropism work?

Plants detect gravity with specialized cells containing dense starch-filled organelles called amyloplasts, or statoliths. In roots, these cells are in the root cap (the columella cells), and in shoots, they are in a layer around the vascular tissue (the endodermis).

1. Sensing: in a vertical root, the statoliths settle to the bottom of the cells. If the root is turned on its side, they fall to the new lower side (statolith hypothesis).
2. Signal: the sedimentation triggers a relocation of PIN auxin transporters, and auxin is transported to the lower side of the root.
3. Response in roots: roots are very sensitive to auxin, and a higher concentration inhibits elongation. The lower side grows more slowly than the upper side, and the root bends downward (positive gravitropism).
4. Response in shoots: in shoots, a higher auxin concentration stimulates elongation, so the lower side grows faster and the shoot bends upward (negative gravitropism).

The different sensitivity to auxin explains why the same hormone causes opposite responses. Experiments on the International Space Station have shown that plants grown in microgravity develop in random directions, and that light can then become the main guide for the orientation of shoots.

What is photoperiodism?

Photoperiodism is the response of an organism to the length of day and night. Many plants flower at a particular time of year. Temperature changes from year to year, but the length of the day is a reliable signal of the season. Surprisingly, it is the length of the uninterrupted night, and not the day, that is measured. A flash of light in the middle of a long night can stop short-day plants from flowering, but a flash of darkness in the middle of a day has no effect.

• Short-day plants flower when the night is longer than a critical length. Examples: chrysanthemum, poinsettia, soybean, rice. They typically flower in autumn or spring.
• Long-day plants flower when the night is shorter than a critical length. Examples: spinach, lettuce, wheat, barley, irises. They typically flower in summer.
• Day-neutral plants flower independently of day length. Examples: tomato, cucumber, dandelion.

The “critical night length” differs among species. A short-day plant with a critical night of 10 hours flowers only when the night is longer than 10 hours. A long-day plant with a critical night of 10 hours flowers only when the night is shorter than 10 hours. The signal is perceived in the leaves, and a flowering signal, called florigen (a protein known as FT, FLOWERING LOCUS T), travels through the phloem to the shoot tips and switches them from leaf production to flower production.

How does phytochrome work?

Phytochrome is a blue-green pigment protein that exists in two interconvertible forms:

• P_r: absorbs red light (about 660 nm) and is converted into P_fr. It is the inactive form.
• P_fr: absorbs far-red light (about 730 nm) and is converted back to P_r. It is the biologically active form.

Sunlight has more red than far-red light, so during the day P_r is converted into P_fr. In the dark, P_fr slowly reverts to P_r or is broken down. The amount of P_fr at the end of the night therefore tells the plant how long the night has been. In short-day plants, P_fr inhibits flowering, so flowering needs a long night in which P_fr drops below a threshold. In long-day plants, P_fr promotes flowering. Phytochrome also controls seed germination (for some seeds, red light stimulates it, and far-red inhibits it), and the “shade avoidance” response: plants under the canopy of other plants receive light that is rich in far-red, and they grow taller to reach the light. Cryptochromes and other blue-light receptors also help to set the biological clock.

Photoperiodic plant types compared

Worked example: interpreting flowering experiments

The critical night length of a plant is 10 hours. Plants are exposed to the following 24-hour cycles, and the flowering is observed. Decide whether the plant is a short-day or long-day plant.

1. Treatment A: the night is 16 hours, longer than the critical 10 hours, and the plant flowers.
2. Treatment B: the night is 8 hours, shorter than 10 hours, and there is no flowering. This points to a short-day plant.
3. Treatment C: the flash divides the 16-hour night into two nights of 8 hours each (8 + 8), both shorter than 10 hours. The plant does not flower, which confirms that it measures an uninterrupted night.
4. Treatment D: interrupting the day with darkness does not make a night of sufficient length, so no flowering. This also fits.

The conclusion is that the plant is a short-day plant, which flowers only when the uninterrupted night is longer than 10 hours. The flash of light acts through phytochrome: red light converts P_r to P_fr, which resets the night measure. If the flash was followed by a flash of far-red light, the effect of the red flash would be reversed, and flowering could be restored, which demonstrates the involvement of phytochrome.

How are these responses used in practice?

Florists and growers control flowering by manipulating the length of day. Chrysanthemums and poinsettias are covered with black cloth to give long nights, so that they flower on time for the market, and long-day plants are given artificial light at night in winter. Understanding photoperiod is also important in crop breeding, since a variety adapted to one latitude may not flower when it is grown at another, because of the different day lengths. Greenhouses use lights of specific wavelengths to control growth, and vertical farms often use red and blue LEDs.

How were these responses discovered?

Wightman Garner and Harry Allard discovered photoperiodism in 1920 when studying a mutant tobacco (“Maryland Mammoth”), which flowered only in short winter days. Karl Hamner and James Bonner showed in 1938 that the length of the night is the critical factor. Harry Borthwick, Sterling Hendricks and colleagues at the US Department of Agriculture discovered phytochrome in the 1950s, using the reversal of red and far-red light. Winslow Briggs identified phototropin in the 1990s. The structure of florigen as the FT protein was established in 2005 to 2007.

Which mistakes are common?

Mistake 1: Saying that the length of the day controls flowering

The length of the uninterrupted night is the factor that is actually measured.

Mistake 2: Assuming that auxin goes to the lit side in phototropism

Auxin accumulates on the shaded side, which grows faster.

Mistake 3: Mixing up P_r and P_fr

P_r absorbs red light and becomes P_fr, the active form. P_fr absorbs far-red light and returns to P_r.

Mistake 4: Saying that roots and shoots respond to gravity by different hormones

The same hormone, auxin, is involved. The difference is in the sensitivity of the tissues.