Comportement des bourgeons de Pseudotsuga menziesii (Mirb.) Franco cultives en conditions controlees

English. The aim of this article was to describe and to measure the characteristics of the bud burst of 9 to 12 month old cuttings of Pseudotsuga menziesii as a function of variations in photoperiod, light energy and temperature. At the beginning of the experiments, all the buds were seemingly in a rest state. Compared with a short photoperiod (9 h) a 16 h lighting period (250 ụE m-2s-1) - accelerated and strongly stimulated the intensity of bud burst (fig. 1). The same result, although less intense, was observed when weak lighting (40 ụE m-2s-1) was given as a complement to the 9 h period (fig. 1). These results have been confirmed on 4 years old plants on which a high energy (430 ụE m-2s-1) - 16 h lighting period strongly enhanced the number of reactivated buds (fig. 2) as compared with the reactivated buds numbered on plants placed under the same photoperiod but with less intense light (250 ụE m-2s-1). On those plants placed in 9 h photoperiod, only intense light slightly enhanced the bud burst. A cold period (5 °C) accelerated the growth of all buds of the cuttings if the treatment lasted 9 weeks. For shorter cold periods, the intensity of bud burst depended upon the duration of the stay at cold temperature (fig. 3). The elongation of the shoot of the first generation which occured immediately after the cold period was higher when the cold period was longer (fig. 4). Finally, high temperatures given alternatively (35 °C during 9 h and 25 °C during 15 h) for 2 weeks enhanced appreciably the bud burst. 1 or 3 weeks of this treatment were less effective (fig. 5). These effects were obtained in non limiting conditions of water and mineral nutrition.Whatever the culture conditions, a slow growth phase took place immediately after a rapid elongation period. This was morphologically visible by the formation of scaly buds or, in some cases, by a few scales surrounding green buds. We discuss the physiological heterogeneity of the population of the buds of a cutting as revealed by their different photoperiodic or temperature needs for their bud burst. These factors are not essential for the growth of all the buds, but they synchronize and hasten the renewal of the growth. According to the results high temperatures associated with long summer days could explain the formation of lamma shoots by trees growing outside.

French. Des Douglas issus de boutures âgées de 9 il 12 mois ont été soumis à divers traitements lumineux et thermiques afin d’observer la capacité de débourrement de l’ensemble des bourgeons. En début d’expérience, tous les bourgeons étaient en repos apparent. Par rapport à une photopériode courte (9 h) un éclairement de 16 h (250 ụE m-2s-1)stimule fortement le débourrement des bourgeons. Cette stimulation s’observe également pour un éclairement faible (40 ụE m-2s-1) donné en complément de la période de 9 h. Ces résultats ont été confirmés chez des plants issus de semis et âgés de 4 ans où, de plus, un éclairement pendant 16 h accroît fortement le nombre de bourgeons réactivés. Une période de froid (5 °C) réactive tous les bourgeons d’une bouture si le traitement a duré 9 semaines. L’allongement de la pousse de première génération qui se produit dès la fin de la période froide est d’autant plus important que le froid a duré plus longtemps. Enfin, des températures élevées données en alternance (35 °C pendant 9 h et 25 °C pendant 15 h) durant 2 semaines accroissent notablement le débourrement des bourgeons. 1 et 3 semaines de ce traitement stimulent également, mais de façon moindre, le débourrement. Ces effets sont obtenus en régime d’alimentation hydrique et minérale non limitante. La discussion met l’accent sur l’hétérogénéité physiologique de la population de bourgeons d’un jeune plant révélée par des exigences différentes pour leur débourrement, en photo-périodes longues ou en températures froides ou en températures chaudes.