Elevated atmospheric CO sub(2) does not affect per se the preference for symbiotic nitrogen as opposed to mineral nitrogen of Trifolium repens L

• Published in: Plant, cell and environment cell and environment, 1998-06, Vol.21 (6), p.623-630
• Main Authors: Zanetti, S, Hartwig, U A, Noesberger, J
• Format: Article
• Language: English
• Subjects: Trifolium repens
• ISSN: 0140-7791

The objective of this investigation was to examine the effect of an elevated atmospheric CO sub(2) partial pressure (pCO sub(2)) on the N-sink strength and performance of symbiotic N sub(2) fixation in Trifolium repens L. cv. Milkanova. After initial growth under ambient pCO sub(2) in a nitrogen-free nutrient solution, T. repens in the exponential growth stage was exposed to ambient and elevated pCO sub(2) (35 and 60 Pa) and two levels of mineral N (N-free and 7.5 mol m super(-3) N) for 36 d in single pots filled with silica sand in growth chambers. Elevated pCO sub(2) evoked a significant increase in biomass production from day 12 after the start of CO sub(2) enrichment. For plants supplied with 7.5 mol m super(-3) N, the relative contribution of symbiotically fixed N (%N sub(sym)) as opposed to N assimilated from mineral sources ( super(15)N-isotope-dilution method), dropped to 40%. However, in the presence of this high level of mineral N, %N sub(sym) was unaffected by atmospheric pCO sub(2) over the entire experimental period. In plants fully dependent on N sub(2) fixation, the increase in N yield reflects a stimulation of symbiotic N sub(2) fixation that was the result of the formation of more nodules rather than of higher specific N sub(2) fixation. These results are discussed with regard to physiological processes governing symbiotic N sub(2) fixation and to the response of symbiotic N sub(2) fixation to elevated pCO sub(2) in field-grown T. repens.