Genetic Improvement in Short‐Season Soybeans: II. Nitrogen Accumulation, Remobilization, and Partitioning

Genetic improvement in yield is conditional on surmounting yield‐limiting factors. Nitrogen (N) has been considered an important limiting factor to soybean [ Glycine max (L.) Merr.] yield. The high demand for N by soybean seed was previously considered to lead to early leaf senescence through accelerated remobilization of N from the vegetative tissue. The consequent reduction in photosynthetic capacity was postulated to limit yield. The objectives of the current experiment were to determine the changes in N accumulation, remobilization, and partitioning associated with genetic yield improvement. Two groups of old, low‐yielding (‘Pagoda’ and ‘Mandarin Ottawa’) and new, high‐yielding (‘Maple Glen’ and ‘OAC Bayfield’) soybean cultivars of similar maturity were grown in side‐by‐side trials at the Elora Research Station, Ontario, in 1996 and 1997. Nitrogen and dry matter accumulation in leaf, stem + petiole, roots, and seeds were determined during the growing season. The newer cultivars had higher yields and higher seed N content. Contrary to the postulated association between leaf senescence and leaf N values, neither leaf N concentration nor leaf N content per unit leaf area (at R6) were association consistently with either yield or leaf area duration (LAD). Although most of the N in the seed was derived from N remobilized from vegetative tissue, the newer cultivars with their higher yields and LAD, remobilized no more N out of the vegetative tissue than did older, lower‐yielding ones. The newer cultivars were distinct from their older counterparts in their ability to accumulate more N during the seed filling period (SFP). Genetic improvement of the short‐season soybeans tested was a consequence of continued N accumulation during the SFP and was not due to differences in the genotype's capacity to remobilize or partition N to the seed.