IRRADIANCE AND NITRATE EFFECTS ON GROWTH AND SYMBIOTIC PARAMETERS OF THE SUPERNODULATING AND NITRATE-TOLERANT SOYBEAN MUTANT EN6500 AND ITS PARENT CULTIVAR ENREI

The supernodulating and nitrate-tolerant soybean (Glycine max [L.] Merr.) mutant <<En6500>> and its parent cv. <<Enrei>> were compared in their response to combined irradiance and nitrate treatments. The mutant exhibited substantially less overall dry matter (DM) production but had markedly more nodules plant-1, larger nodule DM, and higher acetylene reduction activity (ARA) plant-1 compared to Enrei at all nitrate (KNO3) concentrations of 0.5, 5.0, and 15.OmM and irradiance levels of 150 and 850 mumol m-2 s-1. In both genotypes, high irradiance increased DM of the shoot and root, and leaf apparent photosynthetic rate, while symbiotic activity and nodule mass were increased in the mutant but essentially unaffected in Enrei. Dry matter production increased with nitrate level in the mutant but tended to be reduced by the highest level in Enrei. Reduction in nodule DM in favor of the roots and shoot at high levels of nitrate and irradiance was more pronounced in Enrei. High nitrate levels stimulated nodule formation only in the mutant regardless of irradiance, but it consistently depressed specific nodule activity, indicating that the tolerance of the mutant to nitrate is essentially limited to nodule formation and does not extend to nitrogenase function. The failure of high irradiance to overcome the depressive effect of high nitrate on specific nodule activity suggests that carbon is preferentially used for nodulation rather than nodule function in the mutant. At high nitrate levels, the mutant, in contrast to Enrei, appeared to rely more on symbiotically-fixed N than on mineral N. Nodule N concentrations were surprisingly similar across genotypes, irradiance, and nitrate levels despite substantial differences in specific nodule activity, indicating the presence in both genotypes of a common mechanism(s) regulating nodule N metabolism.