Let there be light

5 April 2013 by Malcolm Campbell, posted in Biology

“At first the plant bent so much towards the light that it was useless to attempt to trace the movement” from The Power of Movement in Plants (1880) by Charles Darwin & Francis Darwin.

Light is returning to this part of the world. Daylight is slowly but assuredly winning its battle with night. Daylight is asserting its dominance over the dark, and assuming a majority position as the days pass from winter toward summer.

The increase in daylight occurs on account of a phenomenon with which we are all well familiar. As the Earth continues on its solar orbit, the tilt of the planet on its axis places the northern hemisphere in closer proximity to the sun. The North Pole will soon bask in continuous sunlight, with the rest of the hemisphere following suit, enjoying longer daylight. In keeping with this, the flora and fauna of these northern latitudes are making themselves ready to capitalise on the increased daylight hours – to feed, to migrate, to grow, to develop, to mate, to reproduce.

Plants are particularly attuned to changes in day length. Consider perennial plant species, like forest trees, for example. Perennial species persist from one year to the next, with a distinct growing season, and a dormant period over the winter months. In the autumn, perennial species enter the dormant state contingent on external cues. Day length is a crucial cue.

As daylight decreases in duration, perennial species cease growth and enter a dormant state. Acquisition of dormancy involves shifting plants’ development, physiology and metabolism, to prepare it, and indeed protect it, from the ensuing harsh conditions of winter. For example, in entering dormancy, plants will form a hard bud at their growing tips, to protect the special pools of dividing cells that reside at those tips over the winter, so that growth can commence anew the following year. Dormancy will also invoke the formation of special metabolite reserves. These reserves will serve as ready nutrient sources when the plant initiates spring growth, before new leaves have been made to provide sugars from photosynthesis.  Thus, day length functions as an essential cue to protect plants from winter and prime them for spring.

In the springtime, provided they have experienced critical chilling conditions, increased daytime temperatures associated with longer light periods can function to release perennial plants from their winter dormancy.  Dormancy release will involve mobilisation of the food reserves laid down in the autumn, and stored over the winter. It will also involve opening, or “breaking”, the hard bud at the shoot tips, enabling the specialised stem cells that lie beneath to reinitiate division, and create new growth.  Depending on the latitude or altitude at which they evolved, perennial plants may require greater periods of warm days, or longer daylight hours, to exit dormancy. Either way, it is the increased day lengths, and the warmth they bring, that ushers in a new growth period for an incredibly diverse array of perennial plants.

As with perennial plants, seeds from both perennial and annual plants will lay dormant over the winter months, and may be set on the course to growth and development by daylight. In fact, some seeds have an obligate requirement for light in order to germinate. Once they have germinated, some plants have additional day length requirements in order to undergo maturation, from a juvenile to reproductively viable state. Simply put, longer days are required for these plants to flower. Strictly speaking, from the perspective of the molecular machinery that promotes flowering, such plants are actually sensitive to shorter nights than to longer days. That is, short nights invoke flowering. This is a very useful evolutionary adaptation, as it ensures that the flowering, pollination, seed and fruit production occur at a time of the year when conditions are ideal –when photosynthesis can proceed for the longest amount of time, so that the carbon fixed through this process can be dedicated to the production of flowers, pollen and eggs, and then to the developing seeds and related fruits. As such, increased daylight can be viewed as not only inducing new growth, but the renewal of life itself.

Given the importance of light in plant growth, development and reproduction, it should come as no surprise that plants are active light seekers. It has long been known that plants exhibit phototropism – the directed reorientation of growth toward a light source. Charles Darwin and his son Francis famously investigated phototropism together. Perhaps one of the most striking illustrations of plant phototropism is a field of sunflowers. As the sun passes overhead, the sunflowers orient their apices toward the light source, capturing as much energy as they can to feed the growth of their remarkable seed factories.

Light seeking behaviour is certainly not limited to plants. Humans share an acute case of phototropism with the planet’s flora. Our mythology, our culture, our languages - all are infused with either direct reference or allusions to light. We seek enlightenment. We see things in a better light. We see novelties in old experiences when viewed in a new light. We highlight that which we deem important. Light is used as metaphor, and also holds appeal as a tangible feature of our day-to-day existence. We care about the hours of daylight, and the amount of sunlight that will feature over that time. We, like plants, bend towards to light.

Sunlight is certainly deserving of a special place in our imaginations. The substance of light, photons, are born in nuclear reactions in the sun’s core. The photons begin their existence as gamma rays, which, after billions of collisions within the sun’s active core, escape into space and reach us as light. Before this escape, photons have a lengthy incubation within the sun, where they undertake a random walk between collisions that may take between 10k and 170k years. Once they emerge from their random walk, photons take an 8 minute journey from the Sun to the Earth. The warmth we feel as we turn our faces to the sun is the product of nuclear reactions, billions of collisions, thousands of years, and an approximately 150M km journey across our solar system in mere minutes. No small wonder it fires our imagination!

Our fascination with light is perhaps most sublimely underscored by the Planck project. The Planck project examines the remnants of the oldest light in the Universe, the cosmic microwave background radiation. Planck itself refers to a space observatory run by the European Space Agency. Planck was launched in 2009, and sits 1.5M km from Earth. The Planck observatory spacecraft carries two instruments that detect the total intensity, as well as polarization of the substance of light, photons. Together the two instruments cover a frequency range of 30 to 857 GHz, ensuring that they include the cosmic microwave background spectrum, which peaks at 160.2 GHz. Importantly, they examine this spectrum over the entirety of space. The name of the Planck spacecraft pays homage to the Nobel prize-winning physicist Max Planck.

The Planck project is akin to photon palaeontology. It digs up the fossil remnants of light from when the Universe was a mere 380k years old.  At that time, the Universe was a hot and dense mixture of protons, electrons and photons, all interacting at about 2700ºC. This mixture gave rise to hydrogen, as electrons and protons fused. This, in turn, liberated the photons, which were flung across the Universe as it expanded. In far less than the time required to blink an eye, the universe expanded 100 trillion trillion times.  Due to this phenomenal expansion, the original photons were stretched out to microwave wavelengths. These “photon fossils” have taken 13.8B years to reach us. They comprise what is known as the cosmic microwave background (CMB) radiation. This is what Planck measures.

After fifteen and a half months, the Planck project released a comprehensive picture of the CMB. This picture is rendered as a map, with various colours representing intensities in the CMB. Strikingly, there is an “unevenness” in the CMB.  This unevenness, corresponding to fluctuations in the CMB, reflects the fact that the voyage of photons across the universe was contingent on densities in the original photon, electron, and proton mixture. Densities varied in regions of new structure – the seeds of stars and galaxies that would eventually populate the Universe. In essence, Planck provides an exquisite snapshot of the germination of phenomenal cosmic features that make up our universe.

The features of our Universe revealed by Planck’s portrait of ancient light are fascinating.  Amongst the most fascinating is the discovery that the Universe contains more dark matter than had previously been thought. Dark matter is a mysterious, invisible substance. It can only be observed indirectly through the effects of its gravity.

It is striking, and perhaps somewhat ironic, that our efforts to characterise the Universe’s primordial light should reveal so much dark.  This said, lovers of light that we are, we can be assured that our future will involve much effort to illuminate the darkest recesses of our Universe. Just as we seek out light with the lengthening of days, we will certainly seek enlightenment with the lengthening of our view of the Universe.

References:

Franklin K (2009) Light and temperature signal crosstalk in plant development. Current Opinion in Plant Biology 12: 63-68

Heide OM (1993) Daylength and thermal time responses of budburst during dormancy release in some northern deciduous trees. Physiologia Plantarum 88: 531-540

Holland JJ et al. (2009) Understanding phototropism: from Darwin to today. Journal of Experimental Botany 60:1969-1978.

Olsen JE (2010) Light and temperature sensing and signaling in induction of bud dormancy in woody plants. Plant Molecular Biology 73: 37-47

Planck Collaboration (2013) Planck 2013 results. I. Overview of products and scientific results. arXiv preprint arXiv:1303.5062

Rohde A, & Bhalerao RP (2007) Plant dormancy in the perennial context. Trends in Plant Science 12: 217-223

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