Plant movements and photoperiodism

1A plant that shies away, and plants that keep a calendar

Brush the feathery leaves of the sensitive plant, Mimosa pudica, a common weed of roadsides and fields, and within a second or two its leaflets fold together and the leaf droops. A few minutes later it slowly opens again. This is far too fast to be growth. Other plants move so slowly that you need days to see it: a shoot bending towards a window. And many plants "know" the time of year: chrysanthemums grown for flower markets flower only when the nights are long enough. This lesson explains these movements and how plants measure the length of the night.

2Three kinds of movement

3Photoperiodism: measuring the night

Temperature and rainfall vary from year to year, but day length changes in exactly the same way each year, so many plants use it as a calendar. Experiments showed that what really matters is the length of the uninterrupted night:

• Short-day plants (better called long-night plants) flower only when the night is longer than a critical length; examples: chrysanthemum, poinsettia, soybean.
• Long-day plants (short-night plants) flower only when the night is shorter than a critical length; examples: spinach, radish, wheat.
• Day-neutral plants flower whatever the day length, once they are old enough; examples: tomato, cucumber.

The light detector is phytochrome (Figure 1). Sunlight is rich in red light, so by day phytochrome is mostly in the active form, ${\mathrm{P}}_{\mathrm{f}\mathrm{r}}\backslash mathrm\{P\_\{fr\}\}$. In the dark, ${\mathrm{P}}_{\mathrm{f}\mathrm{r}}\backslash mathrm\{P\_\{fr\}\}$ slowly changes back to ${\mathrm{P}}_{\mathrm{r}}\backslash mathrm\{P\_r\}$, so the amount of ${\mathrm{P}}_{\mathrm{f}\mathrm{r}}\backslash mathrm\{P\_\{fr\}\}$ left at dawn measures the length of the night. In short-day plants ${\mathrm{P}}_{\mathrm{f}\mathrm{r}}\backslash mathrm\{P\_\{fr\}\}$ inhibits flowering, so they need a long night to remove it; in long-day plants ${\mathrm{P}}_{\mathrm{f}\mathrm{r}}\backslash mathrm\{P\_\{fr\}\}$ promotes flowering. When conditions are right, the leaves make a signal (often called florigen) that travels in the phloem to the shoot tips and turns them into flowers.

Figure 1. Phytochrome is switched by red and far-red light, and slowly reverts in the dark.

Figure 2. Night-break experiments. A short flash of red light in the middle of a long night makes it act like a short night; far-red straight afterwards cancels the red.

Rwanda lies just south of the equator, between latitudes 1° and 3° S, where day length stays close to 12 hours all year. So strongly photoperiodic crops bred far from the equator may flower at the wrong time or not at all here, and growers choose day-neutral varieties or ones bred for the tropics. Flower growers can make short-day plants flower out of season by covering them with black cloth to lengthen the night, or stop them flowering with a few minutes of light in the middle of the night.