How Christmas trees keep their green needles over the boreal winter?

Christmas trees can be green because of a photosynthetic short-cut.


Evergreen conifers in boreal forests can survive extremely cold (freezing) temperatures during long dark winter and fully recover during summer. What is the science behind this?

Scientists believe that a phenomenon called sustained quenching offers photoprotection and enables their survival, but its precise molecular and physiological mechanisms are not understood.

An international team of scientists, including researchers from Umeå University, has deciphered that a shortcut in the photosynthetic machinery allows the needles of pine trees to stay green.

In winter, green chlorophyll molecules harness light energy, but the energy cannot be used by downstream reactions in the photosynthetic machinery as freezing temperatures stop most biochemical reactions.

This problem arises especially in early spring when temperatures can still be very low. Still, sunlight is already strong, and the excess light energy can damage the photosynthetic machinery’s proteins.

Scientists show that photosynthetic apparatus is wired in a special way that allows pine needles to stay green all year long.

Photosystems are functional units where light energy is absorbed and converted into chemical energy. Under normal conditions, the two photosystems are kept apart to prevent a shortcut and allow efficient photosynthesis. In winter, the thylakoid membrane structure, where the two photosystems are located, is reorganized, which brings the two photosystems in physical contact.

Scientists showed that photosystem II donates energy directly to photosystem I, and this shortcut mode protects the green chlorophyll and the needles when conditions become harsh.

Pushan Bag, a Ph.D. student at Umeå University, who had collected samples all around the year and made many of the analyses, said, “We have followed several pine trees growing in Umeå in northern Sweden over three seasons. It was essential that we could work on needles “straight from outdoors” to prevent that they adjusted to the higher temperatures in the lab environment before we analyzed them for example with electron microscopy which we used to visualize the structure of the thylakoid membrane.”

All plants have safety valves to manage the excess light energy, either dispersed as heat or as fluorescence light. In any case, only conifers appear to have such amazing valves that they can keep the photosynthetic apparatus intact over the extreme boreal winter.

Scientists combined biochemistry and ultrafast fluorescence analysis, which can resolve chlorophyll fluorescence light at a picosecond time scale. Like this, they could demonstrate how the pine needles deal with excess light energy to protect their sensitive photosynthetic apparatus from damage.

Volha Chukhutsina from Vrije Universiteit Amsterdam, who has performed much of the ultrafast fluorescence analysis, said, “We needed to adjust the equipment to study pine needles in cold temperatures to trap the unique mechanism. We also tried spruce needles, but they were hard to fit in a good way into the equipment.”

Alfred Holzwarth, who has developed the time-resolved fluorescence measurements, adds: “The pine needles allowed us to study this shortcut mechanism – also called spill-over – as they show an extreme adaptation.”

Scientists conducted this study on pine trees, but scientists believe that the mechanism is probably similar for other conifer species – like the typical Christmas trees spruces and firs – because their photosynthetic apparatus is similar.

Professor Stefan Jansson from Umeå University said“This remarkable adaptation not only enjoys us during Christmas but is extremely important for mankind. Hadn’t conifers survive in extremely harsh winter climates, vast areas in the northern hemisphere may not have been colonized as conifers provided firewood, housing, and other necessities. Still today, they form the basis of the economy in most of the circumpolar taiga region.”

Journal Reference:
  1. Bag, P., Chukhutsina, V., Zhang, Z. et al. Direct energy transfer from photosystem II to photosystem I confer winter sustainability in Scots Pine. Nat Commun 11, 6388 (2020). DOI: 10.1038/s41467-020-20137-9
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