doIt’s autumn, the season of flurries and extravagance. Before putting on its sad winter clothes, nature sheds its last lights; the foliage is adorned with rubies and gold. But the trees are already 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 – They usually live in areas exposed to frost in winter.
“By losing their leaves in autumn, these trees prevent them from freezing during the cold season”indicates Jérôme Chave, ecologist at the CNRS (University of Toulouse). They also prevent the risk of breaking large branches that, if covered in leaves, would retain so much snow or ice that they could give way. In contrast, other trees maintain evergreen foliage, even during winter. They live in southern regions less exposed to cold, where their needles are protected by a waxy cuticle, like pine trees.
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.
It is a combination of signals, in autumn, that causes the leaves to enter senescence: the accelerated shortening of days, which the tree measures through photoreceptors (or phytochromes). foliar, coupled with nocturnal cooling and, sometimes, also drought.
The first sign of this entry into senescence is the progressive loss of the emblematic green of the leaves. In fact, as the duration of daylight shortens, the famous pigment that dyes them green degrades. This pigment is chlorophyll, responsible for the alchemy of photosynthesis: taking advantage of the energy of sunlight, it converts water (extracted from the soil) and carbon dioxide (taken from the air) into sugar molecules, valuable nutrients for cells. and oxygen.
But this chlorophyll is an unstable molecule and expensive to produce. When autumn arrives, the leaves, which have less solar energy, stop “recharging” themselves with chlorophyll. And its green fades, revealing the red, yellow, orange pigments… until then hidden in the tissues of the leaves.
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