Possible involvement of photosynthetic supply in changes of nodule characteristics of hypernodulating soybeans

The hypernodulating mutant of soybean En6500 has a larger number of small n odules than its parent, cv. Enrei, and the shoot size is relatively small a t the same time. It was shown that some characteristics of the nodules of E n6500 were different from those of the parent nodules such as, nodule size distribution, acetylene reduction activity (ARA), concentration of leghemog lobin (Lb) components and proportion of the infected region in the nodules. In this study, we attempted to identify the main factor(s) which control(s ) the changes in the characteristics of mutant nodules, by manipulating the source-sink relationship in relation to the photosynthetic supply. By decreasing the infection dose of Bradyrhizobium japonicum, the nodule nu mber of En6500 could be reduced to the level of nodulation normally observe d in Enrei. In addition a part of Enrei shoot was cut off for equalizing th e shoot size of Enrei with that of En6500 at 16 d after sowing (DAS). The p lants were hydroponically cultured until 30 DAS with or without 5 mM NO3- f or the last 14 d. ARA, bacteroid density, concentration of Lb components (L ba, Lbc1, Lbc2, and Lbc3), and volume ratio of the infected region in the n odules were determined. When the hypernodulating mutant En6500 which was treated to obtain a simila r nodule number to that of Enrei was compared with the parent Enrei which w as treated to obtain the same shoot size as that of En6500, nodule characte ristics such as, ARA per nodule dry weight, Lb concentration, Lb component ratios and volume ratio of the infected region in the nodules, were found t o be similar in both En6500 and Enrei. These results suggest that the speci fic characteristics of the nodules of hypernodulating mutant, such as low A RA, low Lb concentration and small infected region, may be caused by the in sufficient supply of photosynthates to each nodule. It is concluded that ma ny of the specific characteristics described above in the nodules of hypern odulating En6500 are not directly related to genetic factors induced by che mical mutation, but that a secondary effect is involved in that the mutants have an excess number of nodules with a relatively small shoot. On the oth er hand, the partial tolerance of nodule growth and ARA per plant to nitrat e may be involved in another mechanism than the secondary effect of photosy nthate deficiency. In soybean, autoregulatory control may play an important role in optimizing the nodule number where the nodule growth and total N-2 fixation activity can be maximized.