Ann. For. Sci. 47 (1990) 451-460
DOI: 10.1051/forest:19900504

Nodulation in vitro d'Acacia mangium Willd (Leguminosae)

A Galiana^a, J Alabarce^a and E Duhoux<sup>b

a</sup>  CTFT/ORSTOM, biotechnologie des systèmes symbiotiques forestiers tropicaux
^b  CTFT/ORSTOM, université Paris V et biotechnologie des systèmes symbiotiques forestiers tropicaux, 45 bis, avenue de la Belle Gabrielle, 94736 Nogent-sur-Marne Cedex, France

Résumé - Un nouveau dispositif de culture in vitro a permis d'étudier les conditions de la nodulation de jeunes plantes d'Acacia mangium inoculées avec la souche de Bradyrhizobium sp Aust13c. La cinétique de la nodulation a été suivie pendant plus de 3 mois et a permis de mettre en évidence 3 vagues successives de formation de nodules. La répartition des nodules le long du système racinaire, l'activité réductrice de l'acétylène et la croissance des plantes nodulées ont été étudiées en fonction de l'âge de la plante au moment de l'inoculation et de la présence ou non d'une source d'azote minéral (KNO₃, 0,2 mmol/l). Le dispositif utilisé a permis d'optimiser les conditions de la nodulation in vitro et a facilité les observations de la nodulation au cours de la croissance des plantes. Ce dispositif expérimental est désormais utilisé pour sélectionner de manière précoce les 2 partenaires de la symbiose Acacia mangium - Bradyrhizobium sp et pourrait être adapté à l'étude de la nodulation d'autres Légumineuses ligneuses.

Abstract - In vitro nodulation in young plants of Acacia mangium Willd (Leguminosae). A new in vitro method was developed to compare the effect of early versus late inoculation on nodulation of Acacia mangium seedlings by Bradyrhizobium sp strain Aust 13 c. During the 3-month culture period, the time course of nodulation exhibited 3 active phases separated by 2 stationary phases (the first one 10 d, the second 45 d-long). When plants were inoculated early (ie immediately, T(I) = 0 d after germination or T (15) = 15 d after germination) in the absence of combined N, nodulation was restricted to the basal part of the root system and nodule number was the highest. In the presence of combined nitrogen (KNO₃, 0.2 mmol/l) nodulation shifted slightly towards the root apex. When plants were inoculated later (T(I) = 30 d after germination) in the absence of combined nitrogen, nodulation occurred at the apex of the root system and nodule number was the highest when nitrogen was added to the culture medium. After 5 months of growth, nodule number and dry weight, shoot dry weight and acetylene reduction activity (ARA) were significantly higher (P = 0.05) in plants inoculated early (immediately or 15 d after germination) than in plants inoculated later. Application of combined nitrogen to the medium did not significantly (P = 0.05) affect shoot dry weight, nodulation and ARA. The method used proved to be the most convenient to study nodulation during plant growth because it allowed continuous and non destructive observations to be made. In the future, it could probably be used for early selection of the 2 partners of the Acacia mangium-Bradyrhizobium sp symbiosis and would also be suitable in nodulation studies of other woody legumes.

Key words: in vitro nodulation / Acacia mangium / Bradyrhizobium sp / nitrogen fixation

Mots clés : nodulation in vitro / Acacia mangium / Bradyrhizobium sp / fixation biologique de l'azote