northOctober. “Here are the leaves without sap/That fall on the grass,/Here is the wind that rises/And moans in the valley. » Alphonse de Lamartine (1790-1869) was not wrong: by force of circumstances (and gusts of wind), the plucked leaves are deprived of sap. But shortly before they fall, they remain connected to the rest of the tree thanks to the flow of this precious juice, even though the aging process is coming to an end.
“In animals, the phenomenon of senescence has a negative connotation, but in plants it is a mechanism of survival and adaptation to environmental stresses”, underlines Céline Masclaux-Daubresse, from Inrae de Versailles. With a key word here: recycling. In autumn, elements stored in the leaves during the summer are recycled and exported by the sap that descends from the foliage to the roots. This process allows deciduous trees to regain many of the nutrients that would otherwise be lost during leaf fall. “Especially minerals that the plant does not obtain easily, such as nitrogen, phosphorus, sulfur or iron”adds the physiologist.
In spring, the leaves grow. They build their photosynthetic factories (chloroplasts) and gorge themselves on nutrients until they mature: carbon (simple sugars, starch, etc.) and nitrogen, extracted from the soil by the roots and then accumulated in the form of proteins. Then they stop growing and storing reserves.
It is a turning point. Under the effect of the shortening of the day and the arrival of cold, it progressively stops its photosynthetic activity. And they produce a series of plant hormones (ethylene, abscisic acid, jasmonic acid, salicylic acid, etc.), which help induce leaf senescence.
Autophagy process
“But be careful, decline does not mean a generalized debacle: the tree still absolutely needs these leaves as long as it has not recovered much of its nutritional content”explains, in 2019, Gérard Guillot, naturalist, in his blog “Zoom nature”. The trees in the fall “They are caught in a dilemma”adds Céline Masclaux-Daubresse. When and how quickly to program the senescence of your leaves? Too soon, they shorten their nutrient storage period. Too quickly, the leaves die before they can return their contents to the rest of the tree. Too late, the plant is exposed to the risk of stress (early frosts, drought, storms, etc.): the leaves will fall without being able to export their contents.
You have 48.86% of this article left to read. The rest is reserved for subscribers.
