Plants, scientists say, transmit information about light intensity and quality from leaf to leaf in a very similar way to our own nervous systems.
These “electro-chemical signals” are carried by cells that act as “nerves” of the plants.
And the response, which took the form of light-induced chemical reactions in the leaves, continued in the dark.
This showed, they said, that the plant “remembered” the information encoded in light.
“We shone the light only on the bottom of the plant and we observed changes in the upper part,” explained Professor Stanislaw Karpinski from the Warsaw University of Life Sciences in Poland, who led this research.
He presented the findings at the Society for Experimental Biology’s annual meeting in Prague, Czech Republic.
“And the changes proceeded when the light was off… This was a complete surprise.”
In previous work, Professor Karpinski found that chemical signals could be passed throughout whole plants – allowing them to respond to and survive changes and stresses in their environment.
But in this new study, he and his colleagues discovered that when light stimulated a chemical reaction in one leaf cell, this caused a “cascade” of events and that this was immediately signalled to the rest of the plant via a specific type of cell called a “bundle sheath cell”.
The scientists measured the electrical signals from these cells, which are present in every leaf. They likened the discovery to finding the plants’ “nervous system”.
What was even more peculiar, Professor Karpinski said, was that the plants’ responses changed depending on the colour of the light that was being shone on them.
“There were characteristic [changes] for red, blue and white light,” he explained.
He suspected that the plants might use the information encoded in the light to stimulate protective chemical reactions. He and his colleagues examined this more closely by looking at the effect of different colours of light on the plants’ immunity to disease.
“When we shone the light for on the plant for one hour and then infected it [with a virus or with bacteria] 24 hours after that light exposure, it resisted the infection,” he explained.
“But when we infected the plant before shining the light, it could not build up resistance.
“[So the plant] has a specific memory for the light which builds its immunity against pathogens, and it can adjust to varying light conditions.”
He said that plants used information encrypted in the light to immunise themselves against seasonal pathogens.
“Every day or week of the season has… a characteristic light quality,” Professor Karpinski explained.
“So the plants perform a sort of biological light computation, using information contained in the light to immunise themselves against diseases that are prevalent during that season.”
Professor Christine Foyer, a plant scientist from the University of Leeds, said the study “took our thinking one step forward”.
“Plants have to survive stresses, such as drought or cold, and live through it and keep growing,” she told BBC News.
“This requires an appraisal of the situation and an appropriate response – that’s a form of intelligence.
“What this study has done is link two signalling pathways together… and the electrical signalling pathway is incredibly rapid, so the whole plant could respond immediately to high [levels of] light.