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Nitrogenase activity and root nodule metabolism in response to O 2 and short‐term N 2 deprivation in dark‐treated Frankia ‐ Alnus incana plants
Author(s) -
Lundquist PerOlof,
Näsholm Torgny,
HussDanell Kerstin
Publication year - 2003
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1034/j.1399-3054.2003.00177.x
Subject(s) - frankia , nitrogenase , botany , biology , darkness , respiration , symbiosis , horticulture , root nodule , nitrogen fixation , bacteria , genetics
Inhibition of nitrogenase (EC 1.18.6.1) activity by O 2 has been suggested to be an early response to disturbance in carbon supply to root nodules in the Frankia‐Alnus incana symbiosis. Intact nodulated root systems of plants kept in prolonged darkness of 22 h were used to test responses to O 2 and short‐term N 2 deprivation (1 h in Ar:O 2 ). By using a Frankia lacking uptake hydrogenase it was possible to follow nitrogenase activity over time as H 2 evolution in a gas exchange system. Respiration was simultaneously recorded as CO 2 evolution. Dark‐treated plants had lower initial nitrogenase activity in N 2 :O 2 (68% of controls), which declined further during a 1‐h period in the assay system in N 2 :O 2 at 21 and 17% O 2 , but not at 13% O 2 . When dark‐treated plants were deprived of N 2 at 21 and 17% O 2 nitrogenase activity declined rapidly to 61 and 74%, respectively, after 20 min, compared with control plants continuously kept in their normal light regime. In contrast, there was no decline in dark‐treated plants at 13% O 2 , and only a smaller and temporary decline in control plants at 21% O 2 . When dark‐treated plants were kept at 21% O 2 during 45 min prior to N 2 deprivation at 17% O 2 the decline was abolished. This supports the idea that the decline in nitrogenase activity observed in N 2 :O 2 at 21% O 2 and during N 2 deprivation was caused by O 2 , which affected a sensitive nodule fraction. Nodule contents of the amino acids Gln and Cit decreased during N 2 deprivation, suggesting decreased assimilation of NH 4 + . Contents of ATP and ADP in nodules were not affected by short‐term N 2 deprivation. ATP/ADP ratios were about 5 indicating a highly aerobic metabolism in the root nodule. We conclude that nitrogenase activity of Alnus plants exposed to prolonged darkness becomes more sensitive to inactivation by O 2 . It seemed that dark‐treated plants could not adjust their nodule metabolism at higher perceived pO 2 and during cessation of NH 4 + production.