Illuminating the mesocosm…

Of all the abiotic factors one could imagine, what’s the one that should always be good for plants? Light gets my vote. In fact it’s so good it features as the prefix of some of the most important plant processes – e.g. photo-synthesis, photo-tropism, photo-morphogenesis, photo-periodism…

But most of those phenomena concern light during the day – from the good old sun itself. Being such a powerful force in plant biology, what happens to plants if they get light at the wrong time of day, say during the night-time? Idle curiosity?

No, plants – and other biota – are increasingly exposed to night-time illumination and any effects this has on their biology need to be understood. The complexity of the multifaceted interactions that ensue from nocturnal illumination have been underlined by Jonathan Bennie et al. who examined the effects of artificial light at night in a grassland ecosystem.

Low-intensity illumination was provided by LEDs (light-emitting diodes). In the case of the ‘amber’ treatment this was intended to mimic illumination from low-pressure sodium lighting (LPS), which although not as prevalent as it once was in the UK, is still the most common form of lighting in many regions.

The grassland ecosystem was modelled as small experimental plots – mesocosms – which were exposed to the illumination outdoors. Each mesocosm, of area 0.5 m² and 0.2 m deep, consisted of 72 individual plants, four each of 18 common grassland species. Studying the effects of artificial light on the population density of the pea aphid (Acyrthosiphon pisum*) – a specialist herbivore of legumes** – they were found almost exclusively on Lotus pedunculatus, a legume.

Although potential predators of the aphid were also included in the experiment, and which might be expected to influence prey – aphid – numbers via a top-down interaction, the University of Exeter (UK)-based team deduced that control over numbers of the insect was principally bottom-up, determined by available plant resources.

In particular, they propose that the near-continuous illumination – albeit at very low levels during the night-time illumination period – depressed flowering of L. pedunculatus (rather curiously since it is a long-day species, but also thereby showing that ecology isn’t straightforward…) and consequently reduced numbers of flower heads and developing seed pods, which provide the main source of nutrition for the sap-feeding insects.

Their overall conclusion is suitably all-embracing and holistic: ‘these results suggest that physiological effects of light on a plant species within a diverse plant community can have detectable demographic effects on a specialist herbivore’. And far from being an abstract study with no real-world relevance, it has as its impetus the prevalence of night-time illumination around the globe, particularly in built-up areas. So a reasonable question is what effect is this comparatively recent phenomenon having on the planet? A consideration that’s about as big as they get on Earth!

* Which organism has been proposed as an emerging genomic model system for ecological, developmental, and evolutionary studies and whose genome has been sequenced.

** Just out of curiosity, what about the other legumes in the mesocosms – L. corniculatus, Trifolium dubium, and T. pratense – what happened to them…?

[For more on the biological effects of nocturnal artificial light (but animal-biased…), see the Philosophical Transactions’ themed issue thereon and the editorial thereto by Kevin Gaston et al. and references therein – Ed.]