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CROP PHYSIOLOGY & METABOLISM

Genetic Improvement in Short-Season Soybeans

II. Nitrogen Accumulation, Remobilization, and Partitioning

^a Plant Science Dep., Rutgers Univ. Philip Marucci Center, 125 Lake Oswego Rd., Chatsworth, NJ 08055
^b Office of Research, Univ. of Guelph/Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), 1 Stone Road West, Guelph, ON N1G 4Y2 Canada
^c Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, N1G 2W1 Canada

* Corresponding author (kumudini{at}aesop.rutgers.edu)

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.

Abbreviations: N, nitrogen • NHI, Nitrogen harvest index • LAI, leaf area index • LAD, leaf area duration • SFP, seed-filling period

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