Comparison of the growth and nitrogen fixation activity of the hypernodulation soybean mutant NOD1-3 and its parent cv. Williams in field cultivation

The hypernodulation soybean mutant NOD1-3 and its parent cv. Williams were transplanted in the sandy dune field of Faculty of Agriculture of Niigata University at planting density 2 plantsm**-2. The plants and root bleeding xylem sap were sampled at 54, 61 (R1), 81 (R3), 102 (R5), and 130 (R7) days after sowing. The concentration of major nitrogenous compounds (ureide-N, amide-N, nitrate-N) in xylem sap was measured by colorimetric method and capillary electrophoresis. The concentration of ureide-N was consistently higher in NOD1-3 than Williams. The amide-N concentration was similar between Williams and NOD1-3, and it increased at R7. The nitrate-N concentration in NOD1-3 was higher than Williams after R1, showing a peak at R5. According to the simple relative ureide method, nitrogen fixation rate and nitrogen absorption rate were estimated. The highest nitrogen fixation activity and nitrogen absorption rate was observed from R3 to R5 stage, and it decreased after R5 in both Williams and NOD1-3. Throughout the growth period, nitrogen absorption rate and N2 fixation rate was always higher in Williams than NOD1-3. After flowering dry weight of NOD1-3 was severely depressed and almost a half of Williams. This way be due to the imbalance of photosynthate supply due to excess nodulation, as well as inferior water and nutrient absorption by small root system. The 58% and the 65% of N was derived from N2 fixation in Williams and NOD1-3 respectively. One example of Williams was carefully dug out without loosing the roots and nodules. It had 17 branches, 178 total nodes, 3.44 pods per node, 2.74 seeds per pod and 1687 total number seeds. The plant didn't show a succulent growth in spite of the vigorous growth. The length of main stem was 87.5 cm and the diameter of the stem was 25 mm. Because the basement of the stem and root were extremely strong, it didn't lodge. There were 1874 nodules per plant and the total nodule fresh weight was 45 g. The dry weight per plant reached 564 g and the seed yield was estimated at 7.16 t ha**-1. However, the final yield was 2.2 t ha**-1 due to the severe damage by insects.