TOLERANCE OF NITROGEN-FIXATION TO NITRATE IN SOYBEAN - A PROGENY (TOLERANT X NONTOLERANT) EVALUATION

The capacity to maintain high levels of N-2 fixation in high N soil, w hile exploiting mineral N through assimilation, should enable optimal utilisation of both pathways of legume nitrogen nutrition. Although ge netic variability for the tolerance of N-2 fixation to nitrate has bee n found in soybean, the agronomic advantage of this character has not been demonstrated. The present study was aimed at (i) characterizing t he expression of nitrate tolerance of N-2 fixation in different enviro nmental conditions, and (ii) evaluating the agronomic consequences of this character. Tolerant and non-tolerant lines resulting from the sam e cross (tolerant line Tielingbaime x cultivar Kingsoy) were used. Fou r lines were observed in four to five environments differing in nitrat e availability. Nitrate tolerance was only expressed in high N soils, and was related to a lower inhibition of N-2 fixation, rather than to a higher fixation potential on low N soils. Nineteen lines of the same progeny were further evaluated in the field, on a moderately high and a high N soil. Lines maintaining substantial N-2 fixation on the two soils (based on N-15 determination) had a greater seed yield (by an av erage of 12 to 15 per cent, than non-tolerant lines). The yield increa se was associated with a greater number of seeds per plant, a greater vegetative biomass, and on the high N soil, with no significant decrea se in seed N content. The data suggest that this nitrate tolerance of N-2 fixation allows for a better agronomic exploitation of N in high N soils.