Christmas trees stay green in winter due to photosynthesis shortcut

The reason why pine trees are so popular at Christmas is because they stay green through the winter. But how come coniferous trees keep their green needles in winter while other trees lose all their leaves? An international team of scientists, including researchers from the Biophysics of Photosynthesis Department at VU Amsterdam, has finally found an answer to this question.

12/17/2020 | 12:06 PM

The team found that a shorter pathway in the photosynthetic mechanism is what keeps pine needles green all year round. The study was published in the journal Nature Communications.

Excess light energy
In photosynthesis, light energy is absorbed by the green chlorophyll molecules. In winter, however, this energy cannot be used properly in the photosynthetic process because the low temperatures bring most biochemical reactions to a stop. The excess light can cause damage, which can even be fatal to the tree or plant. That’s why most species shed their leaves to get rid of the chlorophyll molecules as the weather conditions turn colder. The leaves turn yellow, orange and brown and eventually fall to the ground. Coniferous trees, on the other hand, keep their green needles. It seems that their photosynthetic system remains intact even during extreme winters.

Shorter route
How is this possible? Under normal circumstances, two photosystems work side by side in plants and trees. The systems absorb light energy and convert it into chemical energy. Although these two photosystems work together, they are kept strictly separate from each other to allow for more efficient photosynthesis. However, in coniferous trees, the membrane in which these systems are located is structured differently, allowing the two systems to come into direct contact with each other. The researchers showed that one system then donates energy directly to the other system. This shorter route protects the chlorophyll and green needles as the conditions turn colder.

Ultrafast fluorescence
To demonstrate this, the research team combined biochemical analysis methods with highly advanced, ultra-fast fluorescence measurements and analysis. This enabled them to demonstrate how the pine needles deal with the excess light energy to protect their sensitive photosynthetic mechanisms. “We had to adjust the equipment in order to analyse the pine needles,” says Volha Chukhutsina of the Biophysics of Photosynthesis group at VU Amsterdam, which carried out many of the fluorescence measurements and analyses.

The importance of coniferous trees
The research focused on pine trees, but the scientists expect the mechanism to be similar in other types of coniferous trees because they have a similar photosynthetic mechanism. “We all enjoy a nice pine tree at Christmas time of course, but at the same time this adaptation has been hugely important to humanity,” says Stefan Jansson from Umeå University. “If coniferous trees were not able to survive in extremely harsh winter climates, vast areas of the northern hemisphere might never have become habitable for humans. Coniferous trees provide firewood, material for housing and other necessities, and they still form the basis of the economy in most of the taiga region, for example.”


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