GROWTH DYNAMICS AND NITROGEN-FIXATION IN YOUNG PROSOPIS-CHILENSIS (MOL) ST PLANTS

The reestablishment of Prosopis forests is the principal means for inc reasing productivity in the semi-arid Chaco region of Argentina. In or der to achieve this, it is necessary to insure a N source and to know the growth dynamics of young plants. These two points were the basis o f this study which investigated the relationship between growth and N fixation in young Prosopis chilensis plants during the first months fo llowing emergence. Changes in nodular dry weight and nitrogenase activ ity showed a biphasic response, with an ascending phase up to about 90 days and then a descending phase, suggesting a loss in N fixing capac ity after 3 months. The entire nodular biomass was not responsible for N fixation; only large size nodules were found to play a part. When N fixation decreased, the N redistribution pattern and dry weight varie d towards the root, indicating an adaptive growth pattern towards new N sources. For Prosopis chilensis, there is a strong link between grow th dynamics and nodular activity for up to about 90 days; from this po int on the plant must depend on alternative N sources.