A tale of two chromophores

We took to the woods.

I said, I have things to do. We won’t be long, she replied.

I had seen a bit of green clubmoss on our last walk, a sure sign of things to come. Clubmosses are tough, determined, and among the first to break winter dormancy and to begin the inexorable journey which leads to reproduction. I said, find me some nice clubmoss. Not just any clubmoss. I wanted to photograph clubmoss that was particularly vigorous. Vigorous clubmoss that was well illuminated. Vigorous clubmoss that was well illuminated, and which had a pleasing bit of snow surrounding it. You’re not the least bit picky, she said. We walked on.

I found some, she called. Found some what, I said. She was pointing to a nice bit of Lycopodium. Beautiful. I lay on the ground. The frame counter skipped from 2250 to 2255. We walked on.

I can’t resist this opportunity to describe how it is that, having been dormant for several months during winter, plants know when winter is over and when to begin to grow again. Wait too long, and the weather may change before you flower and set seed. Wait not long enough, and run the risk of frost. The quick answer to the question, How do plants know, is that they don’t, in fact, know anything at all but they do respond, biochemically, in ways which switch their physiological state from one of deep dormancy to another of intense activity.

Like the lives of most other organisms, those of plants are controlled, in part, by a circadian rhythm, an internal clock, the cadence of which is determined by the rotation of our planet on its axis. Cycles of growth and reproduction are complex and influenced by cycles of day and night, by the movement of our planet about the sun, and by changes in the relative durations of light and dark.

So, back to the question of how is it that plants are able to determine day length. We learned in grade school that plants rely on photosynthesis for nutrition. This would lead one to predict that plants should grow and reproduce when sunshine (and access to nutrient) is plentiful and that they should go dormant when the days are short. Surely. But again, how are plants able to determine the length of a day? The answer is found in pigment.

Although some plant pigments are responsible for making plants colorful, others do more than make these organisms pleasant to look at. A number are sensitive to light and are, at the same time, physiologically active. Light sensitive plant pigments provide a link between environmental cue and physiological response. One in particular, phytochrome, is responsible for the response to day length and belongs to a large class of pigments called chromophores (molecules capable of absorbing light energy). The thing about phytochrome is that the very act of absorbing a photon at a particular wavelength changes its three dimensional structure, and this allows it to then respond to a photon of a different wavelength. The phytochrome which absorbs red light is written Pr and is called the red form, while the form which absorbs in the far-red is written Pfr, and is called the far-red form.

The P form of phytochrome is physiologically inactive while its Pfr form is active. When a photon of red light strikes Pr  the phytochrome changes shape to form Pfr and, as you might have guessed, when a photon of far-red light strikes Pfr the phytochrome shifts back to the Pr form. These forms are interconvertible and differentially active. One other thing you need to know to make sense of all of this is that, in darkness, Pfr shifts to Pr. What really matters to a plant, in terms of detecting both annual and daily cycles, is the balance between the two forms of phytochrome. Let’s simplify and say that a plant will do its thing if Pfr > Pr and that it will go dormant if the reverse is true, and Pr > Pfr. So, during that time of the year when there is more light (short nights, less Pfr being converted to Pr, there will be more active phytochrome around) the plant will grow and reproduce. Alternately, during that time of year when there is less light (long nights, more Pfr being converted to Pr, more inactive phytochrome around) the plant will enter dormancy. To end this discussion, remember that Pfr is a physiologically responsive protein. It acts as a transcription factor and is therefore responsible for driving cellular mechanisms which result in tissue growth and reproduction.

So now, here is an image of a clubmoss, shown responding to an increase in day light or, more precisely and as you and I now know, a decrease in the length of night.


17 thoughts on “A tale of two chromophores

  1. My own clock is telling me it’s soon time to wake up. I’ve been writing very little over the winter as my free time has been taken up by other things. Among other things I’ve been learning to play the Irish Bouzouki and that takes a good hour out of each day. With more daylight, though, so too will come more time for thought. Weird–I imagine it’s the opposite for most.
    I imagine you’re looking forward to the change of the season. You’ve done a nice job of exploring the winter side of your new environs. I wonder what spring will bring?

  2. Deep science amazes me. I can never understand the resistance to it that is growing in unfortunate popularity among some segments of our society. It is easy to react to things in our anthropomorphic conceit, but once the intimate nature of things is revealed we realize that none of it is for our entertainment, rather for the furthering of nature itself. Remove us from the equation and the earth sadly improves. Sadly because it should not be that way … but we resist not being the center of our universe.

    • Agreed. Someone one said something like … The Earth was here for more than 4 billion years before we arrived and couldn’t have cared less about our arrival. How arrogant for us to assume that it was made for us.’ And, how about the following, by Twain …

      Man has been here 32,000 years. That it took a hundred million years to prepare the world for him is proof that that is what it was done for. I suppose it is. I dunno. If the Eiffel tower were now representing the world’s age, the skin of paint on the pinnacle-knob at its summit would represent man’s share of that age; and anybody would perceive that that skin was what the tower was built for. I reckon they would. I dunno.

      Thanks for your support of scientific inquiry. D

  3. I read a lot about chronobiology and sleep research recently. What impressed me was for example the story of the discovery of the internal clock by a French astronomer – when he noticed that a certain flower (mimosa) open an closes about every 24 hours even when it is not exposed to light at all (he locked the plant in a dark drawer of his desk), just as human beings in caves or the like tune in to a cycle typically a bit longer than 24 hours. If I recall there are different kinds of clocks and different processes in living beings might be desynchronized (like digestion versus sleep-wake) and that the internal clock that does not depend on light works via a ‘counter’ or clock (in the electronics sense) based on sort of a micro assembly line of a biochemical process whereas external stimuli like light are able to re-adjust the phase-response curve every day so that the whole cycle is entrained to the 24 hours. So what I learn from your post is that among all these different processes and ways life form run them there are indeed beings whose inner clock depend primarily on light.

  4. Amazing how you can manipulate a photo of something so common as moss and turn it into a piece of art. Love this color green and it certainly POPS on the white background.

    • If I thought there was interest, I would spend much more of my time writing about similar sorts of things. Having experimented with the genre before, however, I can report that there’s just about next-to-zero interest in it. My most popular posts are those which presented pretty pictures … period (how alliterative). With a bit of a nod to Gilbert and Sullivan … a photographer’s lot is not a happy one (happy one) … especially one that also likes to write.

      • I’ve had similar results. A flower (or colony of wildflowers) or a closeup of a butterfly or bird usually trumps everything else. These are blogs, after all, so there’s little interest in the abstract and the abstruse.

        Coincidentally, we have an active Gilbert and Sullivan society in Austin.

      • I beg, no make that demand, to differ, good sir. My favorite posts are indeed those that have you waxing scientific. Whilst I am not that deep an intellect to converse on those topics, I enjoy learning (with some deep doubts about my ability to retain) the subjects which you have put years into studying. So I am, I believe, one of a few folks who are in that “next to” part of being just above zero interest. 🙂 And, I envy two parts of that “pretty picture” … the club moss and the snow. Two things with which I have little current visual familiarity.

        • As a fellow blogger you know the value of words of support and of encouragement. Thanks for your support, in particular. As I told (the other) Steve, I’ll keep at it.

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