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Can a limitation in pholem supply to nodules account for the inhibitory effect of nitrate on nitrogenase activity in soybean?
Author(s) -
Vessey J. Kevin,
Walsh Kerry B.,
Layzell David B.
Publication year - 1988
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 - 10.1111/j.1399-3054.1988.tb04954.x
Subject(s) - nitrogenase , bradyrhizobium japonicum , leghemoglobin , root nodule , glycine , starch , nitrogen fixation , nodule (geology) , respiration , bradyrhizobium , chemistry , nitrate reductase , biology , botany , horticulture , zoology , food science , rhizobium , symbiosis , inoculation , nitrogen , biochemistry , rhizobiaceae , enzyme , bacteria , amino acid , organic chemistry , genetics , paleontology
Within 48 h of exposure of nodulated soybean [ Glycine max (L.) Merr. cv. Harosoy 63 x Bradyrhizobium japonicum USDA 16] to 10 m M NO 3 , significant decreases were observed in nodule‐specific nitrogenase (EC 1.7.99.2) activity and CO 2 evolution and in the proportion of [ 14 C]‐labeled photosynthate partitioned to nodule biomass and respiration. These trends continued over the subsequent 3 days of the study period. Concomitant with these events was an 137% increase in the relative growth rate of the whole plant and a cessation in nodule growth. Although the concentration of total soluble sugar in nodules was not affected by NO 3 treatment, the concentration of starch declined to 13% of the control level after 2 days exposure to NO 3 − . In contrast to the effects of NO 3 − , nodules in which nitrogenase activity was partially inhibited by a 30 min exposure to 100% O 2 , showed a 52% increase over control in the starch pool over a 72 h period. The results were compared with recent studies of NO 3 − inhibition of nitrogenase activity in legumes, and in contrast to these studies it was concluded that the inhibitory effects of NO 3 − could be accounted for by alterations in photosynthate partitioning to nodules. A hypothesis is proposed which attempts to account for the recent observation (J. K. Vessey, K. B. Walsh, and D. B. Layzell 1988. Physiol. Plant. 73: 113–121) that nitrogenase activity in phloem‐limited and nitrate‐inhibited nodules is limited by O 2 diffusion. This hypothesis separates the concepts of photosynthate partitioning and phloem supply from that of carbohydrate deprivation and related effects on the size of the carbohydrate pools in nodules.