EFFECTS OF SHOOT REMOVAL ON N-2 FIXATION AND ASSIMILATION IN NODULATION MUTANT AND WILD-TYPE SOYBEAN

The effects of a decapitation (shoot removal) treatment on soybean (Glycine max L. Merr.) N2 fixation, transport and assimilation were determined in a partially nitrate-tolerant nodulation mutant line (NOD1-3) and the parent (Williams) using a N-15 tracer technique. The root systems of intact plants were exposed to N-15(2) and the shoots from half of the plants were removed 1 h after the onset of N-15(2) exposure. Decapitated and intact plants were harvested at 2 h after decapitation. N2 fixation activity was markedly depressed by the decapitation treatment and the amount of fixed N-15 in soluble and insoluble N in the nodules was decreased. Simultaneously, the concentration of amino-N, ureide-N, and soluble carbohydrate in the nodules also declined following shoot removal. Although the NOD1-3 line fixed N2 more actively than Williams, the response to decapitation was very similar in the two lines. From the results obtained it is suggested that the rapid decline in N2 fixation following decapitation was primarily due to the interruption of the carbohydrate supply from shoot.