Summary
Lucerne, red clover and white clover were grown at two atmospheric concentrations of CO2 (300 and 1000 μl l−1) and the effects on N2 fixation, nodule mass/number and root/shoot dry matter production determined. Pea plants were similarly evaluated as a comparison with grain legumes. CO2 enrichment increased N2 fixation activity in all cases but activity/unit nodule mass was significantly increased only in the pea. The enhancement of N2 fixation in herbage legumes by CO2 enrichment reflected an increase in nodule mass which in turn was attributed to increased nodule number, and results show that under the experimental conditions obtaining here photosynthate supply did not limit nodule N2 fixation in these plants though it was limiting in the case of peas.
White clover growing in a 6 and 14 hour photoperiod was studied for response of the N2 fixing system to light. Long photoperiod (14 hour) plants assayed at constant temperature (20°C) did not show a significant response to light at the end of the dark period either in terms of fixation per plant or per unit nodule mass, in contrast with short photoperiod (6 hour) plants which showed significant responses. Short photoperiod plants compensated for reduced photosynthates by maintaining only half the root nodule mass and fixation activity of 14 hour photoperiod plants though plants in both systems supported similar rates of N2 fixation per unit mass of nodule during the photoperiod. Comparison of N2 fixation activities in whole and decapitated plant systems indicates the importance of shoot reserves for sustaining nitrogenase activity in white clover during short-term interruption of photosynthesis. These results support the conclusion of the CO2 enrichment studies, that herbage legumes have the potential for supplying their nodule photosynthate requirements for sustaining optimum rates of N2 fixation and excess carbon supply is used solely to promote further nodulation.
Nodules of short photoperiod white clover plants were less efficient in N2 fixation in that they evolved more H2 relative to N2 (C2H2) reduced than did long photoperiod plants.
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Murphy, P.M. Effect of light and atmospheric carbon dioxide concentration on nitrogen fixation by herbage legumes. Plant Soil 95, 399–409 (1986). https://doi.org/10.1007/BF02374620
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DOI: https://doi.org/10.1007/BF02374620