Nitrogen fixation in nodulated legumes in relation to the assimilation of carbon

Lawrie, Ann Campbell (1973) Nitrogen fixation in nodulated legumes in relation to the assimilation of carbon. PhD thesis, University of Glasgow.

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Abstract

There is a considerable volume of work which suggests that photosynthetic assimilates are important for nitrogen fixation and, in the work described in this thesis, attempts have been made to clarify the relationship between nitrogen fixation and photosynthetic assimilates in the nodules of two leguminous species - Pisum sativum L. and Vicia faba L. Simultaneous measurements of acetylene-reducing activity and radioactivity of C-labelled photosyhthates in the nodules have been made over a wide range of experimental conditions. In order to verify that the conditions used in the acetylene reduction assay were suitable for the species studied, the parameters thought most likely to affect acetylene-reducing activity in the nodules - those of temperature of incubation, oxygen and acetylene concentrations and the nature of the biological material assayed - were investigated. Optimum conditions were broadly similar for both species and differences between the species were probably due to differences in the biological materials themselves, such as nodule size and whether nodules were attached to or detached from the host plant. Nitrogenase activity changed during growth of both species and differences between the two species were thought to be due to different patterns of flowering and fruiting. Studies of pea plants during growth showed that nodules active in nitrogen fixation accumulated most C-labelled photosynthesis, but that a substantial proportion was not used in nitrogen fixation, but in respiration and growth of the nodule. Both nitrogenase activity and the accumulation of C-labelled photosynthates in nodules declined when flowering and fruiting took place and this was believed to be due partly to a redirection of the photosynthetic assimilates within the plant to the developing seed. However, removal of the shoot apex did not result consistently in increases in nitrogenase activity, even though the accumulation of photpsynthates increased, and so it is believed that, in this instance, control of nitrogenase activity is exerted by some factor other than, or additional to, the supply of photosynthates to the bacteroids. Light and photpperiod were shown to play important rules in the regulation of nitrogenase activity of pea nodules. Darkening pea plants resulted in large reductions in both nitrogenase activity and lAe ethanololuble carbohydrajfces in the nodules in 24 hours, but there was no corres- ponding reduction in the accumulation of C-labelled photosynthates manufactured just prior to darkening. However, the accumulation in ihe nodules of 14C-labelled photosynthates manufactured after darkened plants were returned to the light was much reduced and recovery of accumulation of photosynthates preceded recovery in nitrogenase activity. Although changes in the artificial light intensity to which pea plants were exposed for a short period had little effect on nitrogenase activity, diurnal fluctuations in nitrogenase activity were observed. However, maxima in nittogenase activity did not correspond with maxima in light intensity, but occurred in the evening due to the accumulation of photosynth ates translocated to the nodules after a delay of 4 to 8 hours. An additional mid-day maximum was observed sometimes and was believed to be due to changes in airibient temperature. In both species, a large proportion of the 14C-labelled photosynthates in the nodules were recovered as sugars, but there was some indication that ethanol-insoluble compounds were formed in pea nodules. Some photosynthates were rapidly utilised, as basic compounds were formed in bean nodules only 30 minutes after exposure of the shoots to CO2. Accumulation of basic compounds in detached bean nodules exposed to CO2 did not inhibit nitrogen fixation, and it is likely that organic acids formed by carboxylation reactions were aminated directly, as sugars were unlabelled. Microautoradiography indicated that the main site of accumulation of C-labelled photosynthates was the young infected cells near the meristematic tip of pea nodules, whereas histochemical tests with tetrazollum salts suggested that most nitrogen fixation took place in older cells. Thus the site of maximum accumulation of C-labelled photosynthates does not appcirently correspond with the site of maximum nitrogen fixation and therefore this observation supports the suggestion that a substantial proportion of the photosynthates which accumulate in the nodules are iiot used directly in nitrogen fixation. It is suggested that nitrogen fixation is not supported by reserve products, such as starch and poly-g-hydroxybutyric acid, in pea nodules, but by a supply of photosynthates which are only available when the requirements of the actively growing regions of the nodule have been met and which, after amination, are exported rapidly from nitrogen-fixing cells.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: C T Wheeler
Keywords: Plant sciences, Biogeochemistry
Date of Award: 1973
Depositing User: Enlighten Team
Unique ID: glathesis:1973-73816
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56
URI: http://theses.gla.ac.uk/id/eprint/73816

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