VARIATION AMONG SOYBEAN CULTIVARS IN DINITROGEN FIXATION RESPONSE TO DROUGHT

Dinitrogen fixation in soybean [Glycine max (L.) Merr.] is especially sensitive to soil drying, and this trait can have important negative a ffects on yield. An exception to this general response has been the id entification of 'Jackson' soybean as drought tolerant for N-2 fixation , The objectives of this research were to examine the response in addi tional soybean cultivars of N-2 fixation to soil drying under field co nditions, and to determine if there is any link between the drought se nsitivity among cultivars and the ureide levels in the plants. Two fie ld experiments were conducted to further examine the genetic variation in the sensitivity of soybean cultivars for biomass and N accumulatio n rates in response to soil drying. Substantial variation among soybea n lines was found, and the drought-tolerance trait was demonstrated ag ain in Jackson, Differences between cultivars in the response of N acc umulation to drought stress were associated with differences in petiol e ureide concentration found in well-watered plants, An experiment was conducted in the greenhouse with eight soybean cultivars grown in pot s and exposed to drought by soil dehydration over a 2-wk period. Diffe rences among cultivars in the sensitivity of N-2-fixation rates to soi l drying found from measurements of acetylene reduction activity (ARA) in the greenhouse correlated with differences in N accumulation in ti le field (r(2) = 0.86), The tolerance of N-2 fixation to drought stres s was associated (r(2) = 0.84) with maximum ARA in response to changes in O-2 concentration around nodules, Among the eight cultivars, there was a significant negative correlation (r(2) = 0.73) between the drou ght tolerance of N-2 fixation and ureide concentration in the xylem sa p, Ureide levels might be an efficient way to screen large numbers of genotypes for N-2-fixation drought tolerance.