HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Boller, B. C. Articles by Heichel, G. H. Search for Related Content PubMed Articles by Boller, B. C. Articles by Heichel, G. H. Agricola Articles by Boller, B. C. Articles by Heichel, G. H.

Photosynthate Partitioning in Relation to N₂ Fixation Capability of Alfalfa¹

Although N₂ fixation is held to be strongly dependent upon the products of current photosynthesis, the allocation of photosynthate to nodules of forage legumes in comparison with allocation to other metabolic sinks is unknown. Field experiments were undertaken to determine the partitioning of ¹⁴C-photosynthate among various organs over an annual growth cycle of effectively and ineffectively nodulated alfalfa (Medicago sativa L.). Alfalfa genotypes were examined during vegetative growth and at 5% flower at two rates of supplemental N (0 and 100 kg ha^–1 as NH₄NO₃). Across all entries and growth stages, priority for allocation of photosynthate (measured as relative specific activity of tissue 24 h after pulse-labeling, RSA₂₄) was greatest for young leaves. The RSA₂₄, of effective nodules averaged 44% and of ineffective nodules 11% of that of young leaves over all stages of growth. During vegetative growth of effectively nodulated entries, the RSA₂₄ values of old leaves, stems, and roots for photosyntheate were similar and averaged about 44% that of young leaves. In comparison, allocation of photosynthate to effective nodules averaged only 16% of that to young leaves. At 5% flower of effective entries, RSA24, of old leaves, stems, and roots were similar to those of nodules, or about 54% that of young leaves. Thus, allocation of photosynthate to nodules was greater late in a growth cycle (2 d before harvest) than early in the cycle (2 weeks after harvest). Comparison of effectively and ineffectively nodulated alfalfa showed that capability for N₂ fixation was without effect on the partitioning of recent photosynthate to all organs except nodules. Throughout the season photosynthate was mobilized from old and young leaves and partitioned to stems, roots, and nodules. Priority for photosynthate mobilization varied strongly with growth stage in all entries. The results of photosynthate partitioning suggest a relatively weak coupling between supply of current photosynthate and nodule activity.

Key Words: Symbiotic N₂ fixation • Source-sink relations • Allocation of photosynthate • Nodule function • ¹⁴CO₂-labeling • Medicago sativa L.

² Former research specialist, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota (present address: Inst. für Pflanzenbau, ETH Zentrum, Universitätstrasse 2, 8092 Zurich, Switzerland), and plant physiologist, USDA-ARS in the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1509 Gortner Ave., St. Paul, MN 55108.

Received for publication August 27, 1982.