Soybean Fruit Development and Establishment at the Node Level Under Combined Photoperiod and Radiation Environments.

Long days during soybean post-flowering increase seed number. Yield formation processes are generally described in terms of resource availability. Thus, the photoperiodic effect on seed number has been previously associated to a prolonged radiation offer because long days prolong the crop’s cycle. However, evidences of important intra-nodal processes independent of assimilate availability suggest that photoperiodic effects at the node level might also contribute to yield formation. We describe the dynamics of flowering, pod development, growth and establishment at the node level and identify putative mechanisms responsible for increasing pods per node in response to long days. Two field experiments were conducted with a factorial combination of radiation levels and photoperiod extension regimes imposed from flowering to maturity. Long days increased pods per node on the main stems, by increasing pods on lateral racemes (usually dominated positions) at some main stem nodes. More flowers opened on lateral racemes because the flowering period was extended. Flowering was prolonged under long days because active seed filling was delayed on primary racemes (dominating positions). Long days delayed the development of individual flowers into seed filling pods, delaying the initiation of pod elongation without changing pod elongation rate. The embryo development matched the external pod development independently of the pod’s chronological age. In conclusion, our results suggest that long days during post-flowering enhance pods per node through a relief of the competition between pods of different hierarchy within the node. As long days postpone the elongation and active growth of the dominating pods, flowering extends and more pods establish at usually dominated positions. This proposed physiological mechanism could be introduced as a favourable trait in soybean breeding programs.