WE can't hear it, but scientists are finding that in the apparently silent world of plants, a whole lot of communication is going on.
At the University of Western Australia (UWA), researchers recently found that seeds recognise "good" or "bad" neighbouring seeds - even when there is a plastic sheet between them.
And in the United Kingdom, researchers have found that plants also maintain sophisticated underground signalling networks through mycorrhizal fungi.
If a plant is attacked by a pest, the attack is telegraphed to other plants connected by the fungi. Those "alerted" plants can then produce repellent compounds ahead of the pests' arrival.
The new information helps explain why the long-established gardener's practice of companion planting, or the permaculture principle of planting in plant "guilds", has an effectiveness that goes beyond visible evidence of competition and community.
Mixed plant communities compete for resources like water and light, but helpful interactions also play out: nitrogen fixing, pest control or the attraction of desirable insects like pollinators.
The interactions we know about are the ones we can see, like the shading effects of taller plants on those below.
Monica Gagliano and colleagues at UWA looked at whether some other form of invisible communication was going on. To their interest and bemusement, it seems there is.
In an exploratory study, the researchers first looked at the interactions between chilli and fennel.
Chilli doesn’t like fennel: when grown next to each other, the volatile chemicals of fennel hinders the germination rate of chilli seeds.
But when the researchers blocked all known light, chemical and touch signals between the plant seeds - using a sheet of black plastic - the same suppression of chilli germination occurred.
"This demonstrated that plants were able to sense their neighbours even when all known communication channels are blocked, and most importantly, recognise the potential for the interfering presence of a 'bad neighbour' and modify their growth accordingly," Dr Gagliano wrote.
The next step was to look at the effect of "good" plant neighbours: in this case, basil and chilli.
Basil, Dr Gagliano wrote, has the capacity to act as a natural insecticide, and to produce secondary and organic volatiles inhibiting germination and root growth of common competitive weeds like barnyard grass and lambsquarter.
"Besides, gardeners commonly regard it as the ideal companion to chilli plants by virtue of its ability to keep the soil moist and act as organic living mulch."
In their experiments, basil proved to have a positive effect on chilli germination and growth - and again, it didn't matter whether the plants were in visible contact or all known sources of communication had been masked.
Dr Gagliano speculated that even at the seed stage, plants have evolved the ability to sense their neighbours in the soil.
"Because the presence and specific identity of neighbours influence germination timing and success, the existence of an adaptive mechanism that allows a plant to detect its neighbours (and potentially its forthcoming competitive environment), and hence to regulate its developmental responses accordingly at the very onset of its life (ie. seed stage) is clearly advantageous."
How this works remains a mystery, although Dr Gagliano theorises that "nanomechanical oscillations of various components in the cytoskeleton can produce a spectrum of vibrations" - basically, seeds produce nano-sounds detectable by other seeds.
Less mysterious, but of great significance, is the discovery by UK researchers from several universities of an Avatar-like signalling network broadcast through mycorrhizae.
These fungi form a symbiotic relationship with plant roots. In exchange for compounds the plant makes during photosynthesis, mycorrhizae transports nutrient and moisture through its vast networks of hyphae to the host plant.
In this sense, the fungi act as root extenders, often doubling or more the reach of a plant's roots. Many Australian trees can't thrive in Australia's poor soils without them.
American mycorrhizae specialist Dr Jim Trappe, a regular visitor here, speculates that widespread Australian tree dieback is a result of mycorrhizae networks being killed off by fertiliser and soil compaction on farmland.
The UK research shows that the fungal networks also form an underground communications network.
The researchers used plots of broad beans, with each plant covered with a bag so it wasn't possible for the beans to communicate with chemicals released into the air.
Some plots were connected through mycorrhizal networks; in others, the fungi wasn't allowed to grow.
When aphids were introduced to one of the mycorrhizae-connected bean plants, the levels of defensive chemicals in bean plants on the same fungal network quickly rose.
In the unconnected beans, introducing aphids to a plant provoked no response in other plants.
John Pickett of Rothamsted Research told the BBC that one possible use for this knowledge would be to include particularly aphid-prone sacrificial plants in a crop. When aphids attacked, the fungal signalling system could raise the natural defences of more economically valuable plants.
It could prove a robust method of switching on plant defences when needed - without demanding the plant produce defensive chemicals all the time - and reduce development of resistance, Professor Pickett said.
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