Blog - Using Robots to Understand How Plants Sense Light

(Author: Phil Butlin, Editor: Tea Reinart)

Plants rely on light to produce sugars using photosynthesis and these sugars fuel their growth. However, the importance of light for plants doesn’t stop there. As well as being a crucial source of energy, light also acts as a valuable source of information about the plant’s surroundings. Prof. Karen Halliday’s lab here in IMPS studies how plants can use light as an information source and the impacts it can have on their growth. Excitingly, they have recently developed a unique robot to help their experiments more accurately replicate the sun. 

In nature, plants experience constantly changing light environments. For example, as the Sun’s location in the sky changes throughout the day, this not only alters the direction of incoming light but also its intensity. In addition, seasonal changes due to the Earth tilting on its axis can alter the day lengths we experience – something we especially notice in Edinburgh. Plants can detect these fluctuations and use them to gauge what time of day and what time of year it is. This information is vital as it can, for example, initiate flowering, help plants prepare for cold-snaps and even brace themselves for increased risk of infection from seasonal disease.^[1]  

Image

To add even more complexity, a plant’s physical surroundings can also impact its light environment. For example, a plant growing on a forest floor will not only receive less light than one growing in an open field, but the makeup of the light itself will differ. Although we perceive light as one colour (white light), it actually consists of a range of colours, each with their own wavelength, known as the spectrum of light. While light’s true colours are revealed to us when it is split by a prism or through the appearance of a rainbow, plants use specialised proteins to monitor changes to light’s spectrum. We can also measure this difference in light using sensors that detect all wavelengths– you can see the difference between an open field (no shade) and woodland floor (shaded) in Fig. 1. It is important to understand wavelengths because for plants, not all wavelengths are equal. While blue and red light are efficiently absorbed by chlorophyll in plant leaves and used for photosynthesis, others, such as green and far-red light (not visible to us), are not. Instead, these wavelengths pass through and reflect off of plants, which is why plants appear green to our eyes. Plants are able to detect changes in the proportions of these wavelengths and use this to determine how much competition for light there is from their neighbours. To try and gain the upper hand – or leaf – in this ‘fight for light’, plants will divert resources towards stems rather than leaves, helping them to outgrow their competitors. This means that plants that sense shade and are competing with their neighbours have an elongated appearance which we call the Shade Avoidance Syndrome – you can see an example of this is Fig. 1.^[1] 

Image

However, while light can drastically alter plant development, most growth systems used in the lab are limited in their ability to vary aspects of light environments. For example, labs may have systems that can vary two characteristics of light (usually intensity and day length). Spectral variability can also be introduced through the addition of extra lighting or using specialist light colour filters. Unfortunately, this means that plant science labs are usually unable to closely replicate natural settings, and the responses that we then see in plants may not be representative of what happens in nature. While in an ideal world experiments would be conducted outside, the many external factors present in nature, like weather changes and insects, can have unintended impacts on results, making them difficult to interpret. This means that for now, experiments largely remain indoors. In an attempt to recreate some of the subtle complexities of real-world environments, the Halliday Lab have recently created a new robotic system called SOLAR or Sunlight Orbiting Long Arm Robot (you can see SOLAR in Fig. 2). 

SOLAR consists of an LED panel (produced by OSRAM) that can be programmed to produce light of specific intensities and spectral compositions at different time points during the day. By basing these light ‘recipes’ on data collected from real world environments, they are able to more accurately mimic the light that plants experience outside, while still growing them in the controlled environment of a lab. Additionally, thanks to work by the School of Biological Sciences’ Workshop, the light source is attached to a robotic arm making it capable of gradually raising and lowering over the course of the day, mimicking the rising and setting of the Sun. With this amazing bit of machinery, the next step is to test the differences between plants grown using SOLAR and compare to plants grown with traditional lab lighting systems. 

Studying how light shapes plant growth is important for developing our understanding of crop growth defects. For instance, while the Shade Avoidance Syndrome has evolved to help plants ‘escape’ shade environments in nature, in agriculture – where shading is common due to planting crop plants very close together– it can lead to resources being used up by the stem, instead of valuable parts of crops like leaves, roots and grains. With SOLAR, the Halliday lab can better understand the regulation of the shade response, test how different characteristics of light interact to trigger it, and find ways to prevent the shade response from happening in crops. They hope that buy testing the more realistic SOLAR lighting system they can help improve indoor or ‘vertical’ farming practices which are becoming very important for agriculture. Vertical farms are designed to be closed systems where all aspects of a plant’s environment, including the light, are controlled. These systems even provide the prospect of manipulating certain aspects of light depending on what crops a farmer grows and even what characteristics they desire from them. This will help to achieve not only enhanced yields, but also optimised taste and nutritional profiles of crops, as the biochemical processes behind these traits are often heavily light regulated.^[2] This flexibility offered by SOLAR not only enables the testing of novel ideas but will also unlock new avenues for improving plant growth in agriculture. 

The Halliday lab would like to acknowledge the amazing work carried out by the School of Biological Sciences’ Workshop on this project, and the University of Edinburgh Student Experience Grant for funding it. 

References 

1. Ballaré, C.L. and Pierik, R. (2017). The shade-avoidance syndrome: multiple signals and ecological consequences. Plant Cell Environ., 40, pp.2530–2543. doi:https://doi.org/10.1111/pce.12914. 
2. Paradiso, R. and Proietti, S. (2021). Light-Quality Manipulation to Control Plant Growth and Photomorphogenesis in Greenhouse Horticulture: The State of the Art and the Opportunities of Modern LED Systems. J. Plant Growth Regul., 41, pp.742–780. doi:https://doi.org/10.1007/s00344-021-10337-y.