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Isolation and nitrogenase activity of Burkholderia vietnamiensis, a nitrogen-fixing bacterium associated with rice (Oryza sativa L) on a sulphate acid soil of Vietnam. To study the dominant diazotrophs associated with rice in Vietnam, young rice plants were grown on a Vietnamese sulphate acid soil in a phytotron; under these conditions, a maximum nitrogenase activity (estimated with acetylene reduction activity technique) of ca 360 nmol C2H4h-1plant-1, was measured on day 8. At this stage, N2-fixing bacteria were counted and isolated, using the 'spermosphere model' technique, in which exudates of an aseptically grown rice plantlet are the only C source for bacteria. The contribution of N2-fixing bacteria to the microflora (total bacteria, 6.5 x 108colony forming units g-1soil) was shown to be close to 40% in the initial rhizosphere soil. Twelve representative isolates were further studied and proved to be Burkholderia vietnamiensis. They exhibited a low pH-adapted nitrogenase activity. Four of them were selected for comparison with six other strains of Azospirillum lipoferum, A brasilense and Enterobacter cloacae for their nitrogenase activities in the presence of axenically grown rice plantlets. Strain TVV75 of B vietnamiensis was more efficient (1 900 nmol C2H4tube-1day-1) than the others, with the exception of strain MRB16 of A lipoferum (from Bangladesh). Consequently, strain TVV75 of B vietnamiensis has been retained for subsequent inoculation trials in Vietnam where yield increases were obtained. L'?tude de la microflore bact?rienne fixatrice d'azote dominante associ?e au riz a ?t? men?e sur un sol sulfat? acide du Vi?t-Nam. Une premi?re exp?rience a montr? que l'activit? nitrog?nasique (estim?e par mesure de l'activit? r?ductrice d'ac?tyl?ne) du syst?me sol-plante intact ?tait maximale apr?s huit jours de croissance, en chambre climatis?e (360 nmoles de C2H4h-1plante-1). Dans une deuxi?me exp?rience, le comptage et l'isolement des bact?ries fixatrices d'azote les plus abondantes de la rhizosph?re apr?s 8 jours de culture, ont ?t? r?alis?s ? l'aide du mod?le spermosph?re. Dans ce syst?me les exsudats d'une plante ax?nique de riz constituent l'unique source de carbone pour la croissance des bact?ries. Le peuplement des bact?ries fixatrices d'azote a ?t? ?valu? dans le sol de d?part ? 40 % de la microflore totale cultivable qui atteint 6,5 108UFC (unit?s formant colonies) par gramme de sol sec. Douze souches fixatrices parmi les plus abondantes ont ?t? particuli?rement ?tudi?es. Ces souches appartiennent ? la nouvelle esp?ce Burkholderia vietnamiensis. Elles ont une fixation d'azote adapt?e aux bas pH. Le mod?le spermosph?re a ?t? utilis? pour comparer l'activit? nitrog?nasique de quatre isolats de B vietnamiensis et de six souches de collection appartenant aux genres Azospirillum et Enterobacter, isol?es d'autres sols. La souche B vietnamiensis TVV75 s'est av?r?e plus efficiente que toutes les autres souches test?es (1 900 nmoles de C2H4tube-1jour-1) exception faite de la souche MRB16 d'A lipoferum (isol?e au Bangladesh). La souche TVV75 de B vietnamiensis avait ?t? retenue sur ce crit?re pour inoculer le riz au Vi?t-nam dans des essais montrant des augmentations de r?colte.

Des essais d'inoculation du riz par une souche bact?rienne fixatrice d'azote, Burkholderia vietnamiensis TVV75, ont ?t? men?s en pots sur un sol sulfat? acide du Vi?t-nam, en conditions ext?rieures. L'effet de cette bact?rie a ?t? test? ? 3 niveaux d'azote : 0, 45 et 90 U. La souche utilis?e avait ?t? au pr?alable isol?e parmi les bact?ries fixatrices d'azote les plus abondantes de la rhizosph?re du riz cultiv? sur ce m?me sol, et s?lectionn?e pour son activit? r?ductrice d'ac?tyl?ne et son activit? de promotion de croissance sur jeune plante. Dans cet essai, l'azote ?tait bien le facteur limitant, comme le montre clairement la r?ponse de la culture du riz ? la fertilisation azot?e. Dans ces conditions, l'inoculation bact?rienne a entra?n? des augmentations significatives du tallage (12%) et de la hauteur des plantules (8%) du peuplement herbac? en p?pini?re, et un accroissement significatif de la production de mati?res s?ches racinaires et a?riennes tout au long de la culture. Cette augmentation de la biomasse v?g?tale s'est accompagn?e d'une augmentation du rendement final de 20% ? la r?colte, expliqu?e par un effet de l'inoculation sur toutes les composantes du rendement. Ces r?sultats ouvrent la voie ? des applications agronomiques tr?s prometteuses. Growth promotion of rice after inoculation with a nitrogen-fixing bacteria Burkholderia vietnamiensis isolated from an acid-sulfate soil in Vietnam. Inoculation of rice with TVV75, an N2-fixing strain of Burkholderia vietnamiensis, was conducted in a Vietnamese acid-sulfate soil in pot trials. The effect of this bacterial strain was tested for 3 nitrogen fertilizer rates, 0, 45 and 90 U under outdoor conditions. The inoculated strain has been isolated as a member of the dominant N2-fixing microflora in the rhizosphere of rice growing on the above-mentioned soil. It was selected because it exhibited a high acetylene reduction rate and high growth promotion on young plants. In the present experiment, nitrogen availability was the limiting factor for the growth of rice, which is clearly shown by the response of the plant to nitrogen fertilizer application in control pots. Under these conditions, bacterial inoculation resulted in significant increases in tiller number (12%) and plant height (8%) in nursery beds, and significant increases were also observed in both shoot and root dry weights, throughout the growth cycle, leading to a higher final yield (20%). These results are promising for further applications.

Nitrogenase activity was determined in situ by repeated assays at regular intervals to compare the patterns of diurnal variations occurring in a rice field, a grassland, and a peanut field. In the rice field only one peak occurred (mid-day), whereas the other systems exhibited typical two-peak patterns, which were suspected to be induced by climatic conditions or possibly by a specific rhythm of exudation. Observed data on diurnal variations in nitrogenase activity were fitted to different models of diurnal variation comprised of computed periodic curves.