Influence de la durée d'acclimatation sur l'endurcissement à la sécheresse d'Acacia cyanophylla Lindl

Influence of acclimation duration on the drought hardening of Acacia cyanophylla Lindl. The effects of a acclimation treatment to a severely deficient watering regime on the drought hardening of Acacia cyanophylla Lindl. were studied in a set of 8-month old plants grown in plastic pots. Another set of plants watered with an optimal dose close to the maximum evapotranspiration loss served as a control treatment. The period of both watering treatments lasted 13 months. In addition, plants from both sets were submitted at 1,3, 5, and 13 months to a progressive dehydration by water withholding in order to allow for the expression of adaptive mechanisms under extreme drought conditions. Measurements of biomass, minimum water potential ($\Phi$ min), osmotic potential ($\Phi$$\pi$) and stomatal resistance were taken during different desiccation periods. Under extreme drought conditions, control plants showed signs of a satisfactory drought tolerance illustrated by a positive turgor and open stomata at fairly low levels of $\Phi$ min ($-2.5$ to $-3.0$ MPa). The acclimation to lack of water at a severe intensity during a period of 3 months was not sufficient to induce drought tolerance mechanisms in A. cyanophylla. Stressed plants responded by an early stomatal closure at $\Phi$ min of $-1.9$ MPa. With acclimation periods longer than 3 months, greater hardening signs were shown as a result of increased species tolerance to severe drought stress. Indeed, a relative growth rate level comparable to the control and a relatively high leaf $\Phi_{\rm p}$ were observed at very low $\Phi$ min levels ranging from $-3.2$ to $-4.7$ MPa, resulted from better stomatal regulation and increased osmotic adjustment capability. Les effets de la durée d'une acclimatation à une alimentation hydrique déficiente sur l'endurcissement à la sécheresse d'Acacia cyanophylla Lindl. ont été recherchés sur un lot de plants âgés de 8 mois, cultivés en pots. Un autre lot irrigué à une dose optimale proche de l'évapotranspiration maximale a servi de témoin. La durée des 2 traitements hydriques a été de 13 mois. Des plants des deux lots ont été soumis après l'expiration de 1, 3, 5 et 13 mois à un dessèchement rapide par privation d'eau afin de permettre aux traits physiologiques de l'acclimatation de s'exprimer. Des mesures de biomasse, de potentiel hydrique minimum ($\Phi$ min), de potentiel osmotique ($\Phi$ $\pi$) et de résistance stomatique ont été effectuées au cours des différentes périodes d