Dry Matter and Nitrogen Accumulation in Male‐Sterile and Male‐Fertile Soybeans1

Abstract

AbstractOne explanation for the rapid decline in N fixation of soybeans [Glycine max (L.) Merr.] during pod‐fill, is that deveioping seeds compete so strongly for photosynthate that inadequate amounts are available to nodules. Nitrogen fixation by nodulated legumes may be enhanced during the pod‐filling period by increased photosynthate supply to the root system. A reduced number of pod sinks presumably would allow more of the available photosynthate to be translocated to the root system and nodules. In the field, genetic male‐sterile (ms1) soybean plants naturally produce between 80% and 90% fewer pods than their fertile siblings. In addition, their leaves remain green until a killing frost which occurs usually well after their fertile siblings have completey matured. The objective of this study was to determine if these secondary effects of male sterility are also associated with dry matter and N accumulation patterns which are different from those of male‐fertile plants.Sterile and fertile plants from a population of the male‐sterile (ms1) maintainer line, N69‐2774, were sampled in a replicated experiment at 10day intervals from flowering to maturity. Dry weights and Kjeldahl N determinations were made on stems plus petioles, leaf blades, and pods.We found patterns of dry matter and N accumulation in sterile plants indicating that with reduced pod set, leaves, stems, and roots were sinks for N and photosynthate during the pod‐filling period. At peak N accumulation, 130 days after emergence (DAE), 70y0 of the total N in the aboveground portion of fertile plants was in the pods, 8% in stems plus petioles, and 22y0 in leaf blades. In sterile plants at 130 DAE, 11% of the N was in pods, 390/, in stems plus petioles, and 50y0 in leaf blades.Although the sterile plants had 87% fewer pods per plant than the fertile plants and did not show leaf yellowing, sterile and fertile plants showed little difference in the rate of dry matter accumuation per pod or in the total accumulation of plant dry matter. These genotypes also contained similar amounts of total N throughout the pod‐filling period. The results suggest that the retention of green leaves and increased carbohydrate supply to the roots may not increase N2 fixation in the absence of a strong seed sink.