Archive for the ‘Plant Biological Clock’ Category

2213262465_f22f073c8d_m.jpgThe Biology of Moonlight?

The moon may have effects on animal behavior (see here for example), but does it affect plants?

Last time I introduced the scientific literature on the subject by referring to a 1946 paper by Beeson (see ref. 1 below) published in the journal Nature. In this paper Dr. Beeson divided the information regarding the moon’s effects on plants into three categories: (a) myth or beliefs, (b) experiments by people believing in biodynamics, and (c) professional plant scientists with no ties to biodynamics.

Let’s see what’s out there from group (c), starting with the sunlight reflected by the moon.

Is Moonlight Bright Enough to Affect Plants?

moon_plant.jpgFrom laboratory experiments, it’s known that light intensities as low as 0.1 lux (approximately 0.01 foot-candle) during the night can influence photoperiodic time measurement in some plants and animals.

Yet the intensity of light from a full moon on a cloudless night may reach 0.3 lux at latitude of 50′, and more than three times this value in tropical regions.

This fact led E. Bunning and his colleagues (ref 2 below) to inquire whether moonlight can disturb time measurement. Surprisingly, their investigations revealed that some plants have adaptive mechanisms that apparently prevent moonlight from interfering with photoperiodism.

Photoperiodic perception occurs in the leaves. In the leguminous plants soybean, peanut, and clover, “sleep movements” change the position of the leaves from horizontal during the, day to vertical at night. This behavior reduces the intensity of light falling on the leaf surface from an overhead lamp (an “artificial moon”) by 85% to 95%, to an intensity below threshold for interference with time measurement.

In some nyctinastic plants such as Albizzia, Sainanea, and Cassia, leaflets not only orient vertically at night, but also rotate on their axes so that paired leaflets fold together, with the upper surfaces shading each other, an interesting behavior in view of the fact that the upper surface is more sensitive to light breaks than is the lower surface.

Albizzia.jpgSome long-night plants (a.k.a., short-day plants) flower most prolifically when grown with low intensity light (approximately 0.5 lux) rather than complete darkness during the night. In these plants, moonlight probably increases the number of flowers produced by a short-day regime.

However, flowering of Pharbitis nil (Morning Glory) plants was slightly inhibited by exposure to the light of the full moon for 8 or more hours with a single dark period of 16, 14 or 13 h. It is suggested that in the natural environment moonlight may have at most only a slight delaying effect on the time of flower induction in short-day plants (see ref. 3 below).

In a brief review, Wolfgang Schad (ref. 4) cites evidence for the effects of moonlight on biological rhythms in plants. He is co-author of the book Moon Rhythms in Nature: How Lunar Cycles Affect Living Organisms.

Bottom Line: Although it is not clear why low light intensities affect flowering more than darkness, these examples provide some rational basis for the belief of planting particular seeds by the light of the full moon. Another full moon one lunar cycle later could have effects on flowering.

1. Beeson, C.F.C. (1946) “The moon and plant growth.” Nature vol. 158, pp. 572-573. (PDF)

2. Bunning, E. and I. Moser (1969) “Interference of Moonlight with the
Photoperiodic Measurement of Time by Plants, and their Adaptive
Reaction.” Proceedings of the National Academy of Sciences (USA) vol. 62, pp. 1018-1022. (PDF)

3. Kadman-Zahavi, A. and D. Peiper (1987) “Effects of Moonlight on Flower Induction in Pharbitis nil, Using a Single Dark Period.” Annals of Botany vol. 60, pp. 621-623.

4. Schad, W. (1999) “Lunar influence on plants.”, Earth, Moon, and Planets vol. 85-86, pp. 405-411. (PDF)

Next Time: Does the moon’s gravity affect plants?

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Question_time.jpgLast time we had a taste of HOW plants tell time.

But what, if any, are the adaptive advantages to plants for doing so?

It has long been presumed that the ability to anticipate day/night cycles gives organisms a fitness advantage. For example, this would allow plants to anticipate daylight and adjust their photosynthetic metabolism accordingly, perhaps getting a “head-start” on plants that didn’t.

But the ability of plants to tell time also allows them to adjust their development to seasonal variations in the environment.

Time to Flower?cactus_clock.jpg

What factors influence when a plant flowers?

Unlike animals, plants don’t start out with their “naughty bits” – they have no sexual organs, a.k.a., flowers. They are said to grow “vegetatively”, that is, to produce just stems and leaves (and roots, of course).

It’s a very big deal (way bigger than puberty, for example) to the plant when it makes the transition from a vegetative plant to a flowering plant. This involves the “flipping” of some major genetic “switches”, that is, major changes in gene regulation.

Thus, perhaps a better question is: what factors influence the flowering transition in plants?

175262601_78dd6804c3_m.jpgThe Length of the Night

It turns out that one of the most important factors involved in the transition to flowering in many plants is photoperiodism.

By definition, the photoperiod is the duration of an organism’s daily exposure to light.

For plants – particularly in temperate zones – it’s conceivably a way for them to tell the time of year.

The discovery of photoperiodism and flowering in plants is attributed to Garner and Allard. In 1920 they published their findings on how day length affected flowering in certain varieties tobacco.

From their research, they determined that the plants could be divided into three general groups by how they flowered in response to relative day lengths:

Short-Day Plants (SDP) flowered after exposure to relatively short days; Long-Day Plants (LDP) that flowered after relatively long days; and Day-Neutral Plants (DNP) that didn’t seem to flower in response to photoperiod.

From experiments that interrupted the night with a brief period of light, we now know that it’s the night length that is critical in the photoperiodic control of flowering.plant_clock.jpg

This means that SDP are really Long-Night Plants! (confusing?…more about this at another time, or see here).

Photoperiod + Circadian Rhythm

To further complicate our attempts at understanding of the photoperiodic control of flowering, it’s clear that the photoperiodic time-keeping mechanism is coupled with the plant’s internal circadian clock .

Though this complex mechanism is currently not fully understood, a simplified explanation may be as follows.

In plants that flower in response to photoperiod, a flowering signal (called florigen) may fluctuate in the leaves with a circadian rhythm. (LDP and SDP may differ in how the level of florigen is coupled to the circadian rhythm.)

When the external photoperiod (sensed by the leaves) is coincident with a certain phase of the internal clock (i.e., level of florigen in the leaves), then leaves send enough florigen to the apical meristem (via the phloem) to trigger the floral transition. (For a more thorough explanation see here.)

Much recent progress has been made in identifying florigen and how it triggers the massive changes in gene regulation that lead to the floral transition. (Much more on this in upcoming weeks.)

Bottom line: The ability of plants to tell time allows them to adjust their metabolism and development on a daily, as well as seasonal, basis.

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412791223_79498a7b2f_m.jpgHow Do We Know Plants Can Tell Time?

The daily opening and closing of flowers and the rhythmic leaf movement of some plants suggests, even to the casual observer, that plants have an internal clock.

To more careful observers, such as Carl Linnaeus and Charles Darwin, the evidence was clear that plants can tell time.

For example, in 1751 Linnaeus published Philosophia Botanica in which he noted what time of day flowers of various species open and close.

And also in this book, Linnaeus conceived the idea of a floral clock (“horologium florae“) garden by which one could estimate the time of day by observing which flowers were open and which had closed. (Click on photo of floral clock below for more information.)

Darwin, assisted by his son Francis, studied the diurnal movement of leaves (sometimes called “sleep movements”, a.k.a., nyctinasty). In his book The Power of Movement in Plants Darwin argued that the plants had an internal clock that generated the observed rhythms, rather than them being solely imprinted by the diurnal cycle.

Of course, we now know that these “sleep” movements in plants are manifestations of the circadian rhythm, which is evident in most organisms.

leaves.jpgWhat Sets the Clock?

Think about it…what happens during the course of a typical 24-hr period on Earth? In simplest terms, it cycles between light/warm and dark/cool.

So, what sets (entrains) the biological clock of plants are mainly light/dark transitions, augmented or reinforced by diurnal cycles in temperature. In other words, light (dawn/dusk) acts to reset the clock, but temperature also has an effect, albeit not very well defined.

It turns out that, in most plants, the leaves play a central role in sensing the light that entrains the biological clock. But it’s not chlorophyll that is the light-sensing pigment, but two other non-photosynthetic pigments called phytochrome and cryptochrome. (Much more about these two photoreceptors another time.)

How Does the Clock Work?

Research on the cellular mechanisms of circadian (“about a day”) rhythms in plants has greatly advanced our understanding of how the clock works at the molecular level. (For an excellent review from an historical perspective see here.)

Briefly, the clock works at the individual cell level and consists of three basic components as shown in the diagram below.


It turns out that plants likely have three such mechanisms, all interlocked in a complex system, working inside leaf cells. As mentioned above, phytochrome and cryptochrome are the photoreceptors. These modify other proteins involved in a transcription/translation feedback loop that serves as the central oscillator.

The collective output consists chiefly of proteins, and maybe even RNA, that serve to modify the plant’s metabolism and development. These output signals may even travel from the leaves through the phloem to other parts of the plant.

Some Recent News About Plant Circadian Rhythms

Leaves may have three interlocking clocks, but there may be only one root clock, and it’s apparently a slave of the leaf clocks.

The circadian rhythm also apparently results in the rhythmic growth of plants.

Researchers at the University of Texas at Austin have shown that modifying the internal clock may result in bigger plants.

Much has been learned about clock genes in plants and how they relate to clock genes in animals.


Bottom line: For hundreds of years people have recognized that plants have an internal clock, but only recently have plant molecular biologists discovered the complex inner workings of this timepiece.

Next Time: Why Plants Tell Time

HowPlantsWork © 2008-2011 All Rights Reserved.

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