Under Pressure: How Plants Grow – Part 2

Pressure Gauge (CC BY-SA 2.0) by Michael Coghlan

Do Plants Grow More During The Day Or At Night?

A silly question?

Whenever I posed this question in my plant science classes, most students thought I was joking. They presumed that plants grow only during the daytime, when the plants are photosynthesizing.

If you’re like them, then it may surprise you to find out that plants grow during both the day and night, and that often plants actually grow MORE at night.

This is not new information. Indeed, nineteenth-century scientists studying plant growth, including the Darwins, recognized this.

But, of course, the main question is how can this be so?

Don’t plants need sugars and energy produced by photosynthesis to grow?

Indeed, they do. But plant cells can metabolize stored starch during the night to obtain the sugars and energy needed for growth.

But perhaps another important factor driving plant cell enlargement at night is positive hydrostatic pressure, i.e., turgor pressure.

It’s important to keep in mind that growth in plants is defined as an irreversible increase in cell volume. And the largest component of plant growth is cell expansion driven by turgor pressure.

And when is the turgor pressure typically the highest in plant cells?

During the night!

Please allow me to explain…

Day/Night (Diurnal) Fluctuation in Plant Cell Turgor Pressure

The subject of the osmotic driving force for water uptake by plant cells was explored in a previous post, so I won’t repeat all of it here.

And you probably already know that, simply put, there are two opposing forces that determine the water status (water potential) of plant cells: (1) the osmotic force and (2) the force of turgor pressure.

  • The osmotic force (solute potential) is the attractive force for water uptake into plant cells. Generally, the more solutes a plant cell accumulates, the stronger the attractive force for water flow into the cell.
  • The opposing (repulsive) force is the plant cell’s turgor pressure (or pressure potential). Because plant cells are encased in a (typically) rigid cell wall, as water flows into the cell, the turgor pressure increases. (Again, think pumping air into a bike tire.)

    But why is there usually a diurnal fluctuation in turgor pressure, with the pressure being the greatest during the night?

    You can probably guess the answer (especially if I give you a one-word hint: stomates).

    Yep, the answer resides in the fact that, in most land plants, stomates (a.k.a., stomata) are open during the day and closed at night. They’re open during daytime to allow for CO2 uptake from the air for photosynthesis, but this also allows for water loss from the plant. To prevent this water loss, the stomates are typically closed from sunset until dawn since the plant doesn’t need CO2 for photosynthesis at night.

    So, the reason that plant cell turgor pressure is often highest at night is because there is simply more available water to osmotically flow into the plant cells at night. (Please see Ref. 1 below for more information.)

    Returning to the bike tire analogy: What if your bike tires had a hundred tiny leaks during the daytime, but NOT during nighttime. It would surely be a lot easier to keep the tires inflated at night.

    So, since plant cell turgor pressure is the driving force for plant cell expansion (growth), and since relative turgor pressure is likely greater during the night than during the day, it seems reasonable to expect that plants may indeed grow more at night.

    Are Plants Programmed To Grow More When Turgor Pressure Is Highest?

    One of the more fun places to visit on the inter-webs is Indiana University’s “Plants in Motion” website, courtesy of Prof. Roger Hangarter.

    Among all the time-lapse videos of plants is a movie of pumpkins growing that clearly shows night-time growth spurts. Included in the legend to this video is the following:

    Most plants grow faster in the evening and at night than they do during the day. The same holds for pumpkins. In recent years, research on circadian rhythms in plants has shown that the night-time growth spurts of plants is under control of the plant’s biological clock.

    There is abundant scientific evidence that plant growth is affected by a plant’s circadian rhythm. Much of such evidence has come from studies of the growth of plant seedlings, especially of hypocotyl growth.

    A noteworthy such study was published in 2011 (see Ref. 2 below).

    This study was described in a UCSD press release:

    “Farmers and other astute observers of nature have long known that crops like corn and sorghum grow taller at night. But the biochemical mechanisms that control this nightly stem elongation, common to most plants, have been something of a mystery to biologists—until now.
    In this week’s early online publication of the journal
    Nature [Ref. 2 below], biologists at the University of California, San Diego report their discovery of a protein complex they call the “evening complex” that regulates the rhythmic growth of plants during the night.”

    This news report further states: “While most people assume that plants grow at a slow and steady rate throughout the day and night, Charles Darwin and others more than a century ago observed that they actually grow in spurts late at night, with plant stems elongating fastest in the hours just before dawn.”

    Is it a coincidence that the hours before dawn are often when the diurnal turgor pressures of plant cells are the highest?

    The circadian clock may also serve to anticipate regular changes in plant physiology and the environment; growth would be timed to occur when growth material is most likely to be abundant.” (from Ref. 3 below)

    As we’ll see in the next part of this story about how plants grow, for plant cell enlargement to occur, a certain threshold level of turgor pressure force is required. And, especially in water-stressed plants, this threshold turgor pressure may occur in plant cells most often at night.

    References

    1. Kramer, P. J. (1983) Water Relations of Plants, Chapter 12: Water Deficits and Plant Growth, pp. 342-389.

    2. Nusinow, D. I, et al. (2011) “The ELF4–ELF3–LUX complex links the circadian clock to diurnal control of hypocotyl growth.” Nature, Vol. 475, pp. 398–402. doi:10.1038/nature10182 (Abstract)

    3. Nozue, K. And J. N. Maloof (2006) “Diurnal regulation of plant growth.” Plant, Cell & Environment, Vol. 29, pp. 396-408. doi: 10.1111/j.1365-3040.2005.01489.x (Full Text)

    4. Beauzamy, L., N. Nakayama and A. Boudaoud (2014) “Flowers under pressure: ins and outs of turgor regulation in development.” Annals of Botany, Vol. 114, pp. 1517–1533. doi: 10.1093/aob/mcu187 (Full Text)

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    ZZ Top – Got Me Under Pressure (Live)

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