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Carbon Costs of Dinitrogen Fixation Associated with Dry Matter Accumulation in Alfalfa
Author(s) -
Twary S. N.,
Heichel G. H.
Publication year - 1991
Publication title -
crop science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.76
H-Index - 147
eISSN - 1435-0653
pISSN - 0011-183X
DOI - 10.2135/cropsci1991.0011183x003100040029x
Subject(s) - biology , nitrogen fixation , nodule (geology) , nitrogenase , dry matter , symbiosis , photosynthesis , legume , medicago sativa , agronomy , botany , horticulture , bacteria , paleontology , genetics
Dinitrogen fixation of modulated legumes theoretically may limit crop yield because of nodule needs for products of photosynthesis; however, this concept is poorly supported by experimental evidence. To determine how nodule C use might limit legume dry‐matter accumulation, the C costs of N2 fixation of alfalfa ( Medicago sativa L.) were investigated in the glasshouse daring two harvest‐regrowth cycles. Plants were inoculated with either of two contrasting rhizobiai strains and grown in nil‐N hydroponics to provide differing symbioses. Dry matter accumulation, total reduced N accumulation, and nodule gaseous exchange (respiration and nonphotosynthetic CO 2 fixation) were measared, and C costs were calculated. Dinitrogen fixation and nodule gaseous exchange varied with plant development and harvest. Plants nodulated by Strain I02F51 averaged a 68% higher rate of N 2 fixation on a unit nodule‐mass basis than those nodulated by strain P207. Total N 2 fixation per plant, however, was similar due to the contrasting nodulation characteristics of the two symbioses. Nodule CO 2 fixation on a unit nodule‐mass basis was greater in nodules formed by strain I02F51 than in those formed by strain P207; nodule CO 2 fixation per plant was similar for the two symbioses. Nodule CO 2 fixation apparently assimilated 25 to 30% of the CO 2 respired by nodules. Over the course of the experiment, the average C cost of N 2 fixation in nodules formed by strain P207 (9.4 mol C mol ‐~ N) was 59% greater than that of nodules formed by strain I02F51 (5.9 mol C mol −1 N). Total dry matter accumulation during the experiment by plants nodulated by Strain P207 (with higher C costs) was 21% greater than that of plants nodulated by Strain I02F51 (with lower C costs). The results indicated that, in our experimental system, dry matter accumulation was unrelated to the C costs of N 2 fixation. These results contrast with what theory suggests, despite the association between specific activity of N 2 fixation and C costs. The results have implications to understanding symbiotic limitations to legume productivity and to transforming nonlegumes to fix N 2 .