Rapid And Reversible Nitrate Inhibition On Nodule Growth And N2 Fixation In Soybean

H. Fujikake1, H.I. Yashima1, T. Suganuma1, T. Sato2, N. Ohtake1, K. Sueyoshi1, T. Ohyama1

'Faculty of Agriculture, Niigata University, Niigata 950-2181, Japan

2Faculty of Bioresource Sciences, Akita Pref. University, Akita 010-0195, Japan

In soybean plants, the development and function of root nodules are markedly depressed when the nodulated roots are in direct contact with a high level of NO3" a main form of inorganic nitrogen in field. A short-term effect of presence or absence of 5 mM NO3" in culture solution on the nodule growth of hydroponically grown soybean was observed by a computer microscope. In addition, a split roots 14CC>2 tracer experiment was carried out to investigate the involvement of photosynthate partitioning to nodules and roots in relation to the NO3" inhibition of nodule growth and N2 fixation.

Under N-free conditions the nodules on the primary roots grew continuously from 11 to 25 d after planting (DAP) and the diameter of nodules became about 5 mm. After 5 mM NO3" addition to the nutrient solution, the nodule growth was quickly depressed from the next day as shown in Figure 1. On the other hand, the depression of the nodule growth was rapidly recovered by withdrawal of NO3" from the medium. It appears that the inhibitory effect of 5 mM NO3" on the nodule growth is rapid and reversible under the experimental conditions. All the nodule tissue cells (infected, uninfected, sclerenchyma and inner cortex cells) enlarged in the N-free medium, especially the infected cell growth was prominent. The 5 mM NO3" decreased the cell expansion in any tissue. The acetylene reduction activity (ARA) of the plants supplied with 5 mM NO3" for the last week were significantly lower than those of the control plants, however the ARA were almost recovered during the cultivation with the N-free solution for a week after 5 mM NO3" cultivation.

The lateral roots were split into two compartments, and 5 mM NO3" was supplied to one side and N-free solution to the other for 3 d. Then after the exposure of 370 kBq 14C02 to a whole shoot for 2 h, the effect of 5 mM NO3" supply on the 14C partitioning to each organ was measured. The percentage distribution of 14C in the underground parts was about 8.2% in the nodules and about 17.4% in the lateral roots in N-free compartment. Whereas in 5 mM NO3" compartment 14C distribution was much lower in the nodules (2.3%) and higher in the roots (29.5%) than those in N-free side. This result indicates that the photosynthate transported from shoot might be predominantly utilized in the nitrate-fed roots and consequently the photosynthetic supply to the nodules was decreased, then it retarded the nodule growth and depressed N2 fixation.

11 13 15 17 19 21 23 25 Days after planting


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Figure 1. Changes in nodule diameter in culture solution with 5 mM N03" or N-free solution.

fTTl 5 mM NO," treatment |_| N-free treatment

11 13 15 17 19 21 23 25 Days after planting

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