Photosynthate Partitioning in Relation to N2 Fixation Capability of Alfalfa1
- Beat C. Boller and
- G. H. Heichel2
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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