Trials and tribulations: a review of the role of assimilate supply in soybean genetic yield improvement

The contribution of genetic improvement to overall soybean (Glycine max (L.) Merrill) yield improvement has been estimated to range from 0.5 to 0.7% per year in continental North America. Therefore, breeding has had and potentially will play an important role in yield improvement. An understanding of the physiological traits associated with genetic improvement in yield holds the promise of increasing the efficiency of further advances in yield improvement. A number of theories have been put forward as to the physiological traits that may contribute to genetic yield improvement. However, in reviewing the research literature, a number of incongruencies among the theories become apparent. The objective of this paper was to discuss some of the contradictory theories on the physiological traits associated with yield, especially as they address the role of assimilate supply to genetic yield improvement. The genetic material used, the means of parameter calculation, the phenological stage at which treatments were imposed can all impact the conclusions that may be drawn from a study. The conclusions from a study must thus be seen within the constraints of its experimental approach. Total dry matter (DM) accumulation appears to be a more important contributor to yield improvement than harvest index. The association between DM accumulation and yield is most apparent after the beginning of reproductive development, although the physiological traits associated with this increase is contentious: neither selection for maximum leaf carbon exchange rate (CER) nor maintenance of leaf CER during the seed filling period (SFP) has proved to consistently improve yield. Soybean cultivars have a complex feedback system which maintains a cultivar-specific source-sink ratio. Manipulation of the ratio of source supply to sink demand has shown that plants generally readjust to maintain their inherent source-sink ratio. A possible explanation for the lack of gain in yield through selection for greater assimilate supply alone may be because it was ineffective in increasing the source-sink ratio of the cultivar. The source-sink ratio may prove to be a more potent trait than assimilate supply alone to further soybean genetic yield improvement. (C) 2002 Elsevier Science B.V. All rights reserved.