11.4 Tropisms and Photoperiods
Key Takeaways
In shoot phototropism, auxin on the shaded side makes that side elongate more, so the shoot bends toward the light.
Roots are positively gravitropic and shoots are negatively gravitropic, while thigmotropism is a coiling growth response to touch, as in a vine.
Short-day plants flower when the uninterrupted night is longer than a critical length, so they are really long-night plants.
Long-day plants flower when the night is shorter than a critical length, and day-neutral plants do not use photoperiod.
Red light converts Pr to Pfr, far-red light converts Pfr to Pr, and Pfr is the active form in many responses, so a far-red flash can reverse a red night break.
11.4 Tropisms and Photoperiods
A tropism is directional growth toward or away from a stimulus. The organ bends because one side elongates more than the other. Photoperiodism is a separate decision, usually about flowering, and it depends on the length of the night. A bend is a tropism. A change in flowering is often a photoperiod response. Light can appear in either kind of item, so name the stimulus before you name the response.
Phototropism
Phototropism is growth toward or away from light. Ordinary shoots are positively phototropic and bend toward the light. In a coleoptile or young shoot lit from one side, auxin is redistributed to the shaded side. Cells on that shaded side elongate more than cells on the lighted side. The faster side pushes the tip over, so the shoot curves toward the lamp. The lighted side does not shrink. It simply lengthens less. The hormone is auxin, and the side that elongates more is the shaded side.
Gravitropism
Gravitropism is growth with respect to gravity. Roots are positively gravitropic and grow downward. Shoots are negatively gravitropic and grow upward. Lay a seedling on its side and the root curves down while the shoot curves up. Dense starch-filled plastids called statoliths settle on the low side of cells in the root cap and in shoot tissue and help the plant tell which way is down.
Auxin participates in both curves, with different results. In a horizontal shoot, extra auxin on the lower side makes those lower cells elongate more, so the shoot bends up. In a horizontal root, auxin also collects on the lower side, but that concentration inhibits root cells. The upper side of the root elongates more, so the root bends down. Roots are more sensitive to auxin than shoots are. A claim that roots grow down because the lower side elongates faster has used the shoot pattern on the root.
Thigmotropism
Thigmotropism is directional growth in response to touch. A pea tendril or a twining stem that meets a stake grows more slowly on the touched side than on the free side, so the organ coils around the support. The cue is contact. Grape tendrils and morning-glory stems are typical examples. A coil around a string is thigmotropism when the plant is responding to touch. It is phototropism only when the unequal stimulus is light.
The plant measures the night
Photoperiodism times development by the relative lengths of night and day. Flowering is the case to learn. The plant measures uninterrupted darkness. A short-day plant flowers when the night is longer than a critical length. It is really a long-night plant. Chrysanthemum, poinsettia, and cocklebur are standard examples. They flower as nights lengthen in late summer and autumn, and a greenhouse can induce them by giving a long enough dark period.
A long-day plant flowers when the night is shorter than a critical length. It is really a short-night plant. Spinach, lettuce, and many cereal grasses flower when nights are short, which in the field is late spring and early summer. The critical night is not a single hour-count for every species. Each species has its own threshold.
Day-neutral plants do not use photoperiod to time flowering. Tomato, cucumber, and corn are common examples. They flower when they are mature enough, on long nights or short nights. A flash in the dark does not switch them the way it switches a short-day or long-day species.
The midnight flash
A brief light flash in the middle of a long night can prevent a short-day plant from flowering. The flash splits one long night into shorter dark stretches, so the plant does not receive one uninterrupted night longer than its critical length. The same flash can let a long-day plant flower under nights that would otherwise be too long, because that plant no longer passes through one continuous long night. Day-neutral plants ignore the flash. Midnight light is not a universal stop for flowers.
Note
Short-day is older language about day length. The plant measures the night. A short-day plant flowers when the uninterrupted night is longer than its critical length, and a light flash can break that night.
Phytochrome and the last flash
Phytochrome is the pigment that reads a night break. It switches between two forms. Pr absorbs red light and becomes Pfr. Pfr absorbs far-red light and becomes Pr. In many responses, Pfr is the active form. Daylight leaves much of the pigment as Pfr, and Pfr declines over a long dark period. A red flash at midnight converts Pr to Pfr and interrupts the night. Far-red light given immediately afterward converts Pfr back to Pr and can reverse that interruption. The last flash sets the form. Red followed by far-red can leave a short-day plant able to flower. Far-red followed by red finishes as Pfr and can prevent that flowering.
This pigment belongs on flowering-time items. A coleoptile bending toward a lamp is auxin and unequal elongation, not a phytochrome story. Red light, far-red light, and whether a short-day plant flowers are the phytochrome story.
| Response | Stimulus | Result |
|---|---|---|
| Phototropism | Light from one side | The shoot bends toward the light because the shaded side elongates more |
| Gravitropism | Gravity | Roots grow down, and shoots grow up |
| Thigmotropism | Touch | A vine or tendril coils around a support |
| Short-day flowering | An uninterrupted night longer than a critical length | The plant flowers, and a midnight flash can prevent flowering |
| Long-day flowering | A night shorter than a critical length | The plant flowers when the night is short enough |
| Day-neutral flowering | No photoperiod requirement | Maturity, not night length, times the flowers |
Light from one side is phototropism, and gravity is gravitropism. Contact with a support is thigmotropism. Flowering controlled by night length is photoperiodism. Red light converts Pr to Pfr, far-red light converts Pfr to Pr, and Pfr is the active form in many of these responses.
A bean seedling is lit only from the right, and the shoot bends right. What growth difference explains the curve?
The root is showing negative phototropism and shoves the shoot toward the light.
Auxin moves to the shaded side, that side elongates more, and the shoot bends toward the light.
The lighted side elongates faster because gravity pulls auxin into the lamp.
Air moving past the stem causes thigmotropism, so the shoot coils without any light difference.
Cocklebur flowers only when the uninterrupted night is longer than its critical night. A grower turns on a bright lamp for a few minutes at midnight. Why do the plants fail to flower?
Cocklebur is day-neutral, and day-neutral plants flower only if they receive a midnight flash.
Cocklebur is a long-day plant, and a midnight flash lengthens the night enough to block flowering.
Cocklebur is a short-day plant, so it actually requires a long uninterrupted night, and the flash breaks that night into shorter dark periods.
The lamp is a gravity cue, so flowering now depends only on whether the pots are upright.
A short-day plant is kept in a long night that would normally allow flowering. It receives a brief red flash and, immediately afterward, a brief far-red flash. What does phytochrome do?
Far-red light converts Pr into a permanent Pfr form, so the plant can no longer measure night length.
Phytochrome only steers phototropism, so red and far-red flashes cannot change a night-length response.
Pfr absorbs red light and is the only form present in darkness, so far-red light cannot change it.
Red light converts Pr to Pfr, and the following far-red light converts Pfr back to Pr, which can reverse the interruption and still allow flowering.
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