The Chemistry Column

Sensing sunlight

Claire Gormley

I am dreaming of warmer, sunnier days; reading in the park, the sun and a soft breeze on my shoulders, the sounds of children playing, the texture of grass on my toes, the smell of barbeques. Our incredible nervous system allows us to experience the world in so many different ways, and to recall these detailed sensations years or even decades later.

Our nervous system is made up of two parts: the central nervous system and the peripheral nervous system. The central nervous system consists of our brain and spinal cord— the two most important structures for controlling our bodily functions, and retaining our memories and consciousness. The peripheral nervous system consists of receptors— sensory neurons, intermediate neurons and motor neurons —which together enable us to detect and respond to stimuli; a hot plate, a bright light, or a bird’s song. This is the part of our nervous system that actually allows us to experience and engage with our surroundings.

While many other organisms have a nervous system, none are believed to be quite like ours (although it is currently being debated if some molluscs have consciousness as we know it). But what is clear from studying the nervous systems of other organisms is that sensing the world around us is critical for survival. So how do organisms that don’t have a nervous system— like plants —experience and engage with their surroundings? One sure answer is heliotropism.

Heliotropism is the movement of a plant with the sun during the day (Vanderbrink et al, 2014). It is also referred to as ‘solar tracking’ and is performed by numerous plants— most notably Sunflowers (Helianthus annuus) —in order to maximise photosynthesis, and thereby plant growth (Sherry and Galen, 2002; Vanderbrink et al., 2014). In contrast, phototropism— the better-understood phenomenon —is when a plant grows towards a fixed light, resulting in the sustained curvature of a plant. Think of the oddly shaped branches of young trees searching for light under a crowded canopy of mature trees. Technically, heliotropism and phototropism are the processes of plants responding to their surroundings, but I think the dynamic and continual movement of heliotropic plants draws a stronger parallel to how our peripheral nervous system engages with its surroundings. Feel free to make up your own mind…

The mechanism that causes a plant to track the sun can be either turgor-mediated or growth-mediated (Vanderbrink et al, 2014).

Turgor can be defined as the resulting pressure found in plant cells due to the absorption of water. It is key to maintaining plant rigidity. To create leaf movement, specialised organs in leaves— called pulvini —change their turgor pressure (Koller, 2001; Taya, 2003). Pulvini are located at the base of the leaf stalk and act a little like our shoulder joints would, if our bodies were stems and our arms were leaves. They consist of two types of motor cells; flexor cells which are located on the bottom, and extensor cells which are found at the top. These cells undergo visible swelling and shrinking, which corresponds to the lifting and drooping of the leaf (Taya, 2003). Most importantly, pulvini have photoreceptors which allow them to sense changes in light. Potassium ion (K + ) channels in the extensor cells are opened when the cell senses light*. These ions flow into the cell, causing the increase in turgor pressure which lifts the leaf (Taya, 2003). In darkness, the K + channels in the extensor cells close, but open in the flexor cells— causing turgor pressure to drop, cells to shrink, and the leaf to droop (Taya, 2003).

However, many solar tracking plant structures, such as stems and flowers, do not have pulvini— so how do they do it and why is it different? Growth-mediated heliotropism has not been studied as extensively as turgormediated heliotropism. I couldn’t find all the answers, and there are a few different ideas about what may be happening, but what is clear is that growth-mediated heliotropism results in irreversible cell expansion (Vanderbrink et al., 2014). Auxins— which are a large class of hormones associated with plant growth —are thought to be involved. The Cholodny-Went hypothesis proposed that auxins were responsible for the bending of plants towards light by moving laterally from illuminated areas to shaded areas of the plant (Vanderbrink et al., 2014). These results, however, could not be replicated in repeated studies. More recent studies have shown an increase in auxin inhibitors in the cells on the illuminated side of the plant, disproportionately inhibiting cell growth and causing the stem to bend towards the light (Bruinsma and Hasegawa, 1990). As the sun moves from east to west, subtle changes in illumination create changes in growth patterns and thereby the overall heliotropic effect.

What this idea doesn’t necessarily explain, though, is how a Sunflower (and some other heliotropic plants) re-orients itself after dark to be facing east again for the morning sun. An intrinsic circadian clock has been suggested, but heliotropism may be governed by numerous mechanisms working together (Vanderbrink et al., 2014).

And tracking the sun is not the only way plants engage with their surroundings. Dionaea muscipula (Venus Flytrap) fold their leaves in response to touch in order to catch prey. Mimosa pudica (Sensitive Plant) behaves similarly. So, plants may not have a nervous system as we know it, yet we know that they thrive, react, and respond in so many different ways.

Perhaps they do have their own kind of consciousness?

References

Bruinsma, J. and Hasegawa, K. (1990) ‘A new theory of phototropism – its regulation by a light-induced gradient of auxin-inhibiting substances’, in Physiologia Plantarum: 79(4); 700-704

Koller, D. (2001) ‘Solar navigation by plants’, in: D.-P. Häder, M. Lebert (eds.), Photomovement. Elsevier: Amsterdam; pp. 833-896

Sherry, R. A. and Galen, C. (2002) ‘The mechanism of floral heliotropism in the snow buttercup, Ranunculus adoneus’, in Plant, Cell & Environment: 21(10); 983-993.

Taya, M. (2003) 'Bio-inspired design of intelligent materials’. Proc. SPIE 5051, Smart Structures and Materials 2003: Electroactive Polymer Actuators and Devices (EAPAD). Vanderbrink, J. P.; Brown, E. A.; Harmer, S. L. and Blackman, B. K. (2014) ‘Turning heads: The biology of solar tracking in sunflower’, in Plant Science: 224; 20-26

*See my article in the December 2021 issue for more on how ion channels work