|Bacanamwo, Methode - U OF ILLINOIS, URBANA|
Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 2, 1996
Publication Date: N/A
Interpretive Summary: It has long been known that application of fertilizer nitrogen to legume species, such as soybean, results in inhibition of the ability of the plant to obtain a portion of its nitrogen needs through the process termed symbiotic nitrogen fixation. This process occurs in root nodules, which are specialized structures which form in response to a specific bacterial invasion of the root, and are capable of converting atmospheric nitrogen to a usable form. This study established that nitrogen deficient plants are less susceptible to nitrogen fertilizer application, in terms of continued ability of the plant to utilize the atmospheric source of nitrogen. This suggested that a nitrogen compound, which is synthesized and builds up in response to nitrogen fertilizer application, may serve to limit the process of symbiotic nitrogen fixation. Amino acids are being evaluated as possible signal compounds which serve to control symbiotic nitrogen fixation.
Technical Abstract: Regulation of N2ase is not sufficiently understood to engineer symbioses that achieve a high N2 fixation rate under high levels of soil N. Biochemical processes implicated, which may involve N and C compounds, are poorly understood. This study evaluated the hypothesis that besides carbohydrate involvement, N2ase activity and the extent of its inhibition by NO3- may be related to N levels in plant tissues. A wide range of C:N ratios in various plant tissues (9.6 to 39.8, 1.9 to 3.4, and 1.1 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO2 levels in three independent experiments, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was significantly (p<0.05) negatively correlated to N2ase activity and positively correlated to the extent of N2ase inhibition by NO3-. Furthermore, N2ase activity was significantly (p<0.05) positively correlated to total nonstructural carbohydrates (TNC) in nodules and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO3- was significantly (p<0.05) negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. These results indicate that both C and N levels in plant tissues are involved in regulation of N2ase activity. We suggest that the level of activity achieved by N2ase may be determined by (i) N needs (as determined by shoot C:N) and (ii) availability of carbohydrates in nodules. Modulation of the N2ase activity may occur through sensing changes in plant N, i.e., changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).