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Publication Date

1981

Description

Photosynthate supply is known to limit nitrogen fixation in legumes, and the resulting shortfall in energy supply necessary for maximum crop production may be aggravated by the energy-wasteful process of hydrogen (H2) evolution by the legume N2 fixing system. There is less information available on these aspects of the nitrogen-fixing process in forage legumes, such as white clover (Trifolium repens). The objectives of the present work were to determine how N2(C2H2) fixation and H2 production are in­fluenced by carbohydrate supply in forage legumes.

Plants were grown under controlled environmental conditions, and nodule energy supply was varied by adjusting either photoperiod or atmospheric carbon-dioxide concentration. The effect of these treatments on N2(C2H2) fixation and H2 evolution both by intact plants and by nodulated root segments was determined. Light-induced diurnal variation in N2(C2H2) fixation by intact SlOO white clover (at a constant temperature of 20°C) is related to the duration of the prior photoperiod. Short-photoperiod (6-hour) plants show a diurnal effect. Plants in a 14-hour photoperiod showed a constant N2(C2H2) fixation rate throughout the light and dark periods. Fixation rate of these plants was double that of the 6-hour plants during the photoperiod, and this difference was related to increased nodule mass. Specific ac­tivity (n moles N2(C2H2)/mg nodule/hr) was similar for both treatments during the light cycle. Activity of nodulated root segments from both light treatments showed diurnal variation. Response to light was greater for the long-photoperiod plants. Plants growing in the short photoperiod evolved more H2 to N2(C2H2) fixed than did long-photoperiod plants.

Atmospheric enrichment with carbon dioxide did not affect the N2(C2H2) fixation of a number of forage legume species when measurements were made within days of commencement of treatment. After 3 weeks of enrichment, activity/plant was doubled. This increase was associated with an increased nodule mass. Specific N2(C2H2) fixation was not affected. It is concluded that length of photoperiod and integrity of the plant system are important determinants of light-induced diur­ nal vadation in N2(C2H2) fixation. Photosynthate supply limits nodulation rather than N2(C2H2) fixation per se. Short­photoperiod plants have reduced efficiencies of N2(C2H2) fixation in terms of H2 evolved relative to N2(C2H2) fixed. The results show that constant rates of Nz(C2H2) fixation in forage legumes can be maintained during the light and dark periods by carbohydrate reserves in the plant and that photosynthate supply is not limiting for nodule activity. Conditions that deplete these reserves decrease rates of dark fixation. Energy loss through H2 evolution is not reduced even in carbon-stressed plants.

DOI

nitrogen fixation, forage legume, photosynthate supply, hydrogen production.

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Photosynthate Supply and Nitrogen Fixation in Forage Legumes

Photosynthate supply is known to limit nitrogen fixation in legumes, and the resulting shortfall in energy supply necessary for maximum crop production may be aggravated by the energy-wasteful process of hydrogen (H2) evolution by the legume N2 fixing system. There is less information available on these aspects of the nitrogen-fixing process in forage legumes, such as white clover (Trifolium repens). The objectives of the present work were to determine how N2(C2H2) fixation and H2 production are in­fluenced by carbohydrate supply in forage legumes.

Plants were grown under controlled environmental conditions, and nodule energy supply was varied by adjusting either photoperiod or atmospheric carbon-dioxide concentration. The effect of these treatments on N2(C2H2) fixation and H2 evolution both by intact plants and by nodulated root segments was determined. Light-induced diurnal variation in N2(C2H2) fixation by intact SlOO white clover (at a constant temperature of 20°C) is related to the duration of the prior photoperiod. Short-photoperiod (6-hour) plants show a diurnal effect. Plants in a 14-hour photoperiod showed a constant N2(C2H2) fixation rate throughout the light and dark periods. Fixation rate of these plants was double that of the 6-hour plants during the photoperiod, and this difference was related to increased nodule mass. Specific ac­tivity (n moles N2(C2H2)/mg nodule/hr) was similar for both treatments during the light cycle. Activity of nodulated root segments from both light treatments showed diurnal variation. Response to light was greater for the long-photoperiod plants. Plants growing in the short photoperiod evolved more H2 to N2(C2H2) fixed than did long-photoperiod plants.

Atmospheric enrichment with carbon dioxide did not affect the N2(C2H2) fixation of a number of forage legume species when measurements were made within days of commencement of treatment. After 3 weeks of enrichment, activity/plant was doubled. This increase was associated with an increased nodule mass. Specific N2(C2H2) fixation was not affected. It is concluded that length of photoperiod and integrity of the plant system are important determinants of light-induced diur­ nal vadation in N2(C2H2) fixation. Photosynthate supply limits nodulation rather than N2(C2H2) fixation per se. Short­photoperiod plants have reduced efficiencies of N2(C2H2) fixation in terms of H2 evolved relative to N2(C2H2) fixed. The results show that constant rates of Nz(C2H2) fixation in forage legumes can be maintained during the light and dark periods by carbohydrate reserves in the plant and that photosynthate supply is not limiting for nodule activity. Conditions that deplete these reserves decrease rates of dark fixation. Energy loss through H2 evolution is not reduced even in carbon-stressed plants.