Ct’s autumn, the season of gusts and flamboyance. Before putting on its dreary winter clothing, nature sheds its last lights; the foliage is adorned with rubies and gold. But already, the trees are becoming bare. Torn off by the gusts, the leaves fly away.
The ancient Greeks called this fall « apoptosis »from which the current term “apoptosis” is derived, which designates a phenomenon of programmed cell death. In fact, leaf senescence is an active process, programmed in plant cells. At least, for deciduous trees – whose leaves fall at the end of the season, usually in autumn – who often live in areas exposed to frost in winter.
“By losing their leaves in the fall, these trees prevent them from freezing during the cold season”indicates Jérôme Chave, ecologist at CNRS (University of Toulouse). They also prevent the risk of breaking large branches which, if they were covered with leaves, would retain so much snow or ice that they could give way. In contrast, other trees keep evergreen foliage, even during winter. They live in southern regions less exposed to cold, where their needles are protected by a waxy cuticle, like pines.
Signal combination
« Through the autumn mist/The leaves of the garden fall./Their fall is slow. We can follow them/With our eyes recognizing/The oak with its copper leaf,/The maple with its blood leaf”, noted the poet François Coppée (1842-1908). Science now tells us why.
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It is a combination of signals, in the fall, which triggers the leaves to enter senescence: the accelerated shortening of the days, which the tree measures using photoreceptors (or phytochromes). foliar, coupled with night cooling and, sometimes also, drought.
The first mark of this entry into senescence is the progressive loss of the emblematic green of the leaves. As the length of daylight shortens, in fact, the famous pigment which colors them green degrades. This pigment is chlorophyll, responsible for initiating the alchemy of photosynthesis: exploiting the energy of sunlight, this process converts water (drawn from the ground) into oxygen, and carbon dioxide (taken from the air) into sugar molecules , valuable nutrients for cells.
But this chlorophyll is an unstable molecule and expensive to produce. When autumn comes, the leaves, which have less solar energy, stop “recharging” with chlorophyll. And their green fades, which reveals the red, yellow, orange pigments… until then hidden in the leaf tissues.
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