Nitrogenase activity and oxygen diffusion in nodules of soyabean cv. bragg and a supernodulating mutant: Effects of nitrate

A comparison was made of the growth, nodulation and nitrogenase activity of 42-45-d-old plants of soyabean (Glycine max) cv. Bragg and a supernodulating mutant (nts382) grown in controlled environment cabinets. Plants of cv. Bragg were approximately twice as heavy as plants of nts382 mutant but nodule dry weights were not significantly different. The nts mutant produced an average of 918 nodules compared with 254 for cv. Bragg, but individual nodules of the mutant had smaller masses and volumes.Following exposure to acetylene, nitrogenase activity of cv. Bragg rose during the initial mixing period and then showed a decline, recovery, decline pattern. A 48 h exposure to 15.7 mol m^-3 NO₃-N altered this to a two-phase decline pattern with a greater overall decline than for the - N plants. The nts382 mutant showed a much smaller acetylene-induced decline in nitrogenase activity, but this was also increased by exposure to NO₃-N. Maximum (pre-decline) rates of acetylene reduction activity of cv. Bragg were nearly 3 times greater than those of the nts mutant, when expressed on a nodule dry weight basis. Nitrate reduced activity of Bragg by 41% and that of nts382 by only 19%. Measurements of carbon costs indicated that there were no inherent inefficiencies in carbohydrate use within the nodules of the mutantThe oxygen diffusion resistance of nodules was characterized for all plant sets. Following exposure to acetylene, the total diffusion resistance of nts382 nodules was considerably larger than that of Bragg nodules, due mainly to a five times greater minimum resistance. Addition of nitrate caused this minimum resistance to increase 3-fold for Bragg and 2-fold for the nts mutant Both genotypes lost the ability to adjust their minimum resistance in response to changes in external oxygen concentration. Prior to acetylene exposure, however, the total diffusion resistance of nts382 nodules was five times that of Bragg nodules but nitrate caused increases of only 26% and 36% for nts382 and Bragg, respectively.