Xylogénèse chez les dicotylédones arborescentes. III. Transport de l'auxine et activité cambiale dans les jeunes tiges de Hêtre

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Abstract
The pattern of [3H]-labelled auxin transport has been studied in the beech (Fagus sylvatica L.) (a diffuse-porous tree), in young shoots where the slow basipetal cambial reactivation depends on the buds. At 20 °C, most [3H]-labelled IAA molecules proceed downwards in active shoots as a wave, whose average speed is 13 to 15 cm a day; the velocity of the front being at least 18 to 19 cm a day. The speed of the [3H]-labelled IAA wave depends on the temperature; it is about 7 cm a day at 9 °C, which is the average temperature in the forest during the cambial reactivation period. In natural conditions, the auxin pulse velocity is comparable to that of the cambial reactivation. The ability of tissues to transport auxin at 20 °C depends on the season. During winter rest, the pattern of [3H]-labelled IAA movement is diffusive and very slow. During the 2 months preceding cambial reactivation, the tissues become gradually capable of active auxin transport, the speed and the intensity of which increase at the same time. Microautoradiographs show that the cambium and its differentiating derivatives are the site of auxin transport and also of its utilization: the hormone occupies a preferential site for xylogenesis regulation.