Artificial light for bonsai

It is possible to keep a bonsai alive with artificial light such as a lamp as long as the lamp emits the spectrums of light needed by plants to grow. This starts with their need to perform photosynthesis. Chloroplasts are the organelles within plant cells which are responsible for photosynthesis, and they do this using photosynthetic pigments – substances which absorb photons of light.

You may be surprised to learn that there are actually multiple photosynthetic pigments in chloroplasts and that land plants always have two forms of chlorophyll – chlorophyll a (C55H72MgN4O5) and chlorophyll b (C55H70MgN4O6)ref, and six carotenoids – neoxanthin, lutein, β-carotene, violaxanthin, antheraxanthin and zea. The role of chlorophylls are to be “light collectors, whereas carotenoids, apart from participating in light harvesting, are also involved in photoprotection”.ref Different levels of these pigments are contained in angiosperms vs gymnosperms – to see the differences you can review the data in this article (figure 4) but in summary gymnosperms have more chlorophyll (a + b) and deciduous angiosperms have the least chlorophyll b. The different chlorophylls and carotenoids absorb different spectrums of light as shown in the chart at the bottom of this post.

But it’s not just the process of photosynthesis which uses light in plants. Actually many of their growth responses depend on light – including seed germination, flowering, leaf senescence, stomatal and chloroplast development, cell differentiation and root growth. Plants also have a circadian clock which controls when processes happen – such as stomatal closing & opening, flower closing & opening, leaf position changes and so on. The circadian clock uses light sensors to keep the clock in time with the world.

Plants use phototropins, phytochromes, cryptochromes and UV detectors to perform different light sensing tasks and to trigger signals throughout the plant.

Phytochromes are red and far-red light photoreceptors, whose role in plants is to detect favourable conditions for growth and to signal within the plant accordingly – five have been identified, known as phytochromes A to E.ref They function not just as light detectors but as temperature sensors since they are able to detect infrared light which is associated with heat.ref I was interested to read that phytochromes A, B and E stimulate lateral root production while phytochrome D inhibits it.

Phototropins are responsible for the phenomenon of phototropism whereby a plant grows towards the light – they contain blue light photoreceptors.ref Cryptochromes are another form of blue light photoreceptor only recently discovered, they’ve been shown to inhibit elongation of the germinating stem and to form an input to the circadian clock.ref

Several plant growth processes have been found to be initiated by UV-B light, summarised in this table but including stomatal closing and the production of compounds such as anthocyanins which protect the plant from damaging UV levels. UV-B light has been found to be detected in plants by a receptor known as UVR8ref.

So below there is a nice diagram put together by a producer of LED grow lamps (they’ve left the UV receptor off but everything else is there). As is clear to see, there is a wide range of spectra required by the different photoreceptor apparatus in a plant. So back to the whole point of this post – can you use artificial light? Well yes but for full functioning of your tree it should provide the spectra needed – which is basically most of the range of light from UV to infrared excluding green and yellow. The LED grow lamp seller has an article on finding lights which meet the requirements below (I do not know them, I just think their website makes sense).

The other factor to consider is that leaves are adapted to respond to the parallel rays of light from the sun. Their internal cell structure, for example, is organised to encourage the scattering of light to the spongy mesophyll cells which are underneath the palisade cells.ref In general a light bulb or LED light is not going to produce parallel rays of light so the photosynthetic efficiency of the light will be reduced (since less light will make it into the spongy mesophyll cells) and more power will be needed from the light.ref

Personally I’ve never used artificial light for a bonsai – the one indoor bonsai I have (a Ficus retusa) lives quite happily on a table near a window in my kitchen where it gets natural light throughout the day and copes with the large variations in photoperiod experienced in the UK. But theoretically it should be possible to use a light instead, assuming you have one which meets the requirements outlined in this post in terms of spectrum and intensity.

If you end up with a light which doesn’t reflect the spectra needed, this may impact your tree’s growth. For example “low blue light from warm white LEDs increased stem elongation and leaf expansion, whereas the high blue light from cool white LEDs resulted in more compact plants”ref – you can end up with ‘leggy’ plants if you don’t have enough light.