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Recherche

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Agapanthus 'Blue Heaven' (La plante du mois)

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Comment les plantes utilisent l'eau

Schéma de la circulation de l'eau dans la planteChacun le sait, les plantes ont besoin d'eau pour vivre et croître : si l'état hydrique (en gros la quantité d'eau des tissus) n'est pas satisfaisant, la croissance, en taille, des organes va s'arrêter la première, suivie par les autres fonctions physiologiques (en particulier la photosynthèse). Si cet état se dégrade encore, le végétal risque de mourir rapidement. Or, nous allons le voir, l'état hydrique d'un végétal est extrêmement dépendant de deux quantités : l'absorption d'eau par les racines d'une part et la transpiration des feuilles principalement, d'autre part. Autrement dit, mis à part certains groupes comme les Cactées, la plupart des plantes n'ont pas d'autonomie hydrique : elles doivent puiser dans le sol ce qu'elles perdent au même moment par transpiration.

De quoi dépendent ces flux entrant (l'absorption) et sortant (la transpiration) ? Comment sont-ils reliés l'un à l'autre, et comment la plante « contrôle » son état hydrique en fonction du sol et du climat ?

1. La transpiration
La transpiration est la perte d'eau, sous forme vapeur, que subit un végétal par ses feuilles principalement (mais aussi, dans une bien moindre mesure habituellement, par les autres organes aériens comme les fleurs, les tiges, etc.) Cette transpiration dépend de facteurs climatiques et biologiques.
1.1 Facteurs climatiques
La transpiration est en fait une évaporation au sens physique c'est-à-dire une perte d'eau sous forme de vapeur. Que la surface qui évapore soit un drap, une surface d'eau, un sol ou une feuille, l'évaporation requiert de l'énergie. Dans les conditions naturelles, cette énergie provient principalement du soleil, mais aussi d'autres facteurs climatiques : température et humidité de l'air, vitesse du vent. Plus il y a de soleil, plus l'air est sec, plus la température est élevée, plus le vent est fort, plus la transpiration (on parle aussi d'évapotranspiration) « demandée » par le climat sera forte. C'est en effet le climat qui détermine l'intensité maximale (on parle d'évapotranspiration climatique ou potentielle) de la transpiration des végétaux du fait que c'est lui qui apporte l'énergie nécessaire à cette évaporation.

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Pierre Cruiziat
INRA Clermont-Ferrand

Journée à thème "Mieux arroser" Janvier 2008
Colloque "Gérer l'eau en horticulture" Mai 2007

acheter les actes du colloque Quand les plantes se parlent