Near the tree, I handle the tree pruner. “Clac”, the sample is taken. In the laboratory, Chantal cuts it very finely with a microtome and mounts it between slide and coverslip… After a few adjustments, Romain places everything on the stage of the epifluorescence microscope. A few moments of suspense and, on the black screen, we discover, as always with joy and astonishment, the interior beauty of the branches… here, before your eyes, a cross section of a young stem taken from a sycamore maple.
Where is chlorophyll hiding in tree stems?
On these sections, we are interested in the location of the red color, which here reflects the presence of chlorophyll, a plant pigment allowing photosynthesis, i.e. the absorption of light energy and its transformation into energy. chemical.
Although it is usually green, chlorophyll is a molecule that has the particularity of being “autofluorescent” and emitting red light when illuminated by… blue. This red, unnoticed under the luminosity of the sun and the intensity of the reflected green, is clearly visible here under the epifluorescence microscope, which sends only an exciting wavelength of chlorophyll onto the sample.
We distinguish the different tissues constituting the stem, arranged concentrically: at the periphery, the epidermis which will become a bark and in the center (dark oval), the medullary parenchyma. Between the two we see red rays which correspond to the ligneous rays bringing together the living cells of the wood, surrounded by the vessels (which ensure the transport of the raw sap) and the fibers (dead cells responsible for the solidity of the wood) appearing here in green.
The arrangement of chlorophyll observed in maple is not unique, but it varies markedly between species : some species such as beech or alder have chlorophyll to the heart of the stem, while this is not the case with oak. To produce chlorophyll, the cell must receive at least some light – a few photons are enough. In beech, whose bark remains thin and allows a little light to penetrate, chlorophyll is produced even by the trunks of adult trees.
So far, we have not found any tree species without chlorophyll in the stems. Why does the tree therefore retain the ability to produce chlorophyll in its stems, which has an “energy cost”, while its leaves, organs specialized in photosynthesis, are generally sufficient to ensure its “autotrophy”, that is to say its capacity to manufacture all its organic matter from mineral elements?
The rod recycles its carbon
Stems do assimilate carbon through photosynthesis. Surprisingly, this does not only come from the surrounding air, but also from CO2 from the respiration of living cells inside the stem…and sometimes even CO2 from the roots and transported in dissolved form by the raw sap. In addition to this recycling of CO2the photosynthesis of stems ensures release of oxygenwhich would prevent the branches from suffering from hypoxia.
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Moreover, the presence of chlorophyll in the branches is probably very useful in species whose leaves are reduced (for example for trees of the genus Acacia), or at certain times of the year when the leaves are absent or the needles less active.
More recently, the idea has emerged that the role of chlorophyll could also be decisive during prolonged hot dry summers, during which the leaves close their stomata to limit water loss, thereby also limiting carbon inputs. The photosynthesis of the stems by its recycling of CO2 internal could partly compensate for carbon needs, avoid “starvation” and ensure growth or storage.
Stem photosynthesis could also play a role in maintaining hydraulic integrity and/or the repair of damage caused by droughts and thus avoid “death of thirst”.
Indeed, when little water is available, the functioning of the wood (xylem) can be disturbed by the appearance of air bubbles in the water columns of the raw sap, stopping its circulation and the water supply. leaves. Through the production of sugars in the ligneous rays in particular, the photosynthesis of the stems could allow the local repair of the “embolized” vessels, and thus put the sap back into circulation.
Understanding the functioning mechanisms of trees in their environment, a field of plant ecophysiology, is essential for characterizing their adaptations to a changing environment. Concerning the chlorophyll in the branches and the associated photosynthesis, let us remember that the “apparently negligible” should not be neglected. Even if, in general, their quantitative role in the carbon balance of trees and forests is probably moderatetheir qualitative role could prove to be very useful in the event of repeated setbacks by limiting mortality and forest decline.