Photoperiodic Control of Seed Filling for Soybeans1

Soybean plants [Glycine max (L.) Merr. ‘Ransom’] were grown in the controlled‐environment rooms of the phytotron at N.C. State Univ. to investigate the effects on floral and pod development of the morphological stage of plant development upon initial exposure to inductive short days, number of consecutive short days, and temperature. All plants were initially grown under long‐day photoperiods and at day/night temperatures of 18/14, 22/18, 26/92, and 30/26 C. In Experiment I, groups of plants were transferred to an inductive short‐day photoperiod at three stages of development: expansion of paired primary leaves, unfolding of third trifoliolate leaf, and unfolding of sixth trifoliolate leaf. Plants from each group were returned to the long‐day photoperiod upon appearance of the first open flower, when pods were becoming visible, or retained under the short‐day photoperiod until termination of the experiment. Measurements of vegetative and reproductive growth were made of all plants at 63 days after planting. In Experiment II, groups of plants were transferred to short‐day photoperiods either upon expansion of paired primary leaves or unfolding of sixth trifoliolate leaf. Plants from each group were returned to long‐day photoperiods after 0, 10, 15, 95, 30, and 50 consecutive short‐days. Measurements of vegetative and reproductive growth were made of all plants, regardless of when returned to long‐day photoperiod, at 50 days after initial exposure to short‐day photoperiod. Pod production efficiency (number of pods set/total number of flowers initiated) was greater for plants initially exposed to short days upon expansion of paired primary leaves than for plants exposed at a later morphological stage. However, the numbers of both flowers and pods/plant were increased by delay of exposure to inductive photoperiods due to increased numbers of main stem and axillary branch nodes. As thermoperiod was decreased, maximum pod production efficiency was achieved with fewer consecutive short‐days. An increased number of short‐day photoperiods beyond that required for efficient pod set was disruptive to vegetative growth. Dry weight of vegetative tissues and area of leaves generally was decreased, and pod weight was increased correspondingly, by increased numbers of consecutive short‐days. These data support a conclusion that the rate of seed‐fill for soybeans is responsive to photoperiodic conditions.