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Eucalyptus plants are sensitive to abiotic stresses in their initial growth, and nutritional deficiency is one of the most recurrent among them. Trinexapac-ethyl, which is a plant growth regulator, can positively affect eucalyptus, a response known as hormesis, possibly providing plants with greater tolerance to stress. The objective of this study was to evaluate the effect of trinexapac-ethyl at two application times, before planting (BP) or after planting (AP), in Eucalyptus urophylla under conditions of nutritional deficiency (NPK). Two experiments (one for each application time) were conducted simultaneously during 81 days after planting of eucalyptus in 15 L pots. The treatments consisted of three doses of trinexapac-ethyl (0, 30, and 60 g a.i. x [ha.sup.-1]) and four variations of nutrient supply: complete solution (NPK) and solutions without nitrogen (-N), without phosphorus (-P), and without potassium (-K). The variables of gas exchange, growth, and dry matter were evaluated. For both application times, trinexapac-ethyl had a positive effect on the root-shoot ratio of plants grown in -N and also positively affected some eucalyptus photosynthetic characteristics. In the AP application, the compound provided gains in height and dry matter, regardless of the nutrient supply. Under phosphorus deficiency, trinexapac-ethyl provided gains in total dry matter (BP) and leaf area (AP). Key words: Eucalyptus urophylla, nitrogen, phosphorus, plant growth regulator, hormesis. Durant la periode initiale de croissance, les plants d'eucalyptus sont sensibles aux stress abiotiques dus le plus souvent a des deficiences nutritionnelles. Le trinexapac-ethyle, un regulateur de croissance des plantes, peut avoir un effet positif chez l'eucalyptus en procurant probablement une plus grande tolerance aux stress chez les plantes, une reaction connue sous le nom d'hormese. L'objectif de cette etude consistait a evaluer l'effet du trinexapac-ethyle applique a deux moments differents, soit avant (BP) ou apres (AP) la plantation chez Eucalyptus urophylla soumis a des deficiences nutritionnelles (NPK). Deux experiences, une pour chaque moment d'application, ont ete menees simultanement durant 81 jours suivant la plantation d'eucalyptus dans des pots de 15 L. Les traitements comprenaient trois doses de trinexapac-ethyle (0, 30 ou 60 g de substance active a l'hectare) et quatre solutions d'apport d'elements nutritifs: solution complete (NPK) et solutions sans azote (-N), sans phosphore (-P) ou sans potassium (-K). Les variables ayant trait aux echanges gazeux, a la croissance et a la matiere seche ont ete evaluees. Peu importe le moment ou il a ete applique, le trinexapac-ethyle a eu un effet positif sur le rapport racines-tige des plants cultives avec le traitement -N ainsi que sur certaines caracteristiques photosynthetiques de l'eucalyptus. Applique apres la plantation, le compose a entraine des gains en hauteur et en matiere seche peu importe l'apport de nutriment. En presence d'une deficience en P, le trinexapac-ethyle a entraine des gains de matiere seche totale (BP) et de surface foliaire (AP). [Traduit par la Redaction] Mots-cles: Eucalyptus urophylla, azote, phosphore, regulateur de croissance des plantes, hormese., Introduction Eucalyptus is the most important genus in the Brazilian forestry sector, being the main source of wood for cellulose production and energy, among other uses (Industria Brasileira de Arvores [...]