Effect of Photoperiod and Size at Flowering on Vegetative Growth and Seed Yield of Soybean1

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Abstract
In comparisons of soybean [Glycine max (L.) Merr.] ‘Ransom’ from growth chambers and field plots, seed yield per plant was generally lower in the chambergrown plants. To determine the limiting factors, we studied the effects of photoperiod and plant size at flowering on final seed yield in controlled environment rooms at 26/20 C (day/night) temperatures. To obtain plants of three size classes at time of flowering, a 37‐day period of photoinductive 8‐hour days was used at three ages: (22, 30, and 38 days after planting; three, six, and nine main stem trifoliolate stages respectively). Delaying the beginning of photoinduction for 8 days increased total seed weight 34%. When photoinduction was delayed 16 days there was an 84% increase in seed weight. The increased yield was primarily associated with an increase in the number of axillary nodes produced as photoinduction was delayed. Delayed photoinduction had little effect on the average numbers of pods per node, numbers of seeds per pod, or weight of individual seeds. When the plants were grown under 16‐hour daily periods of full illumination, after photoinduction was completed, final seed yield was significantly greater than under 12‐hour daily periods of full illumination, but vegetative growth was not significantly increased. It was concluded that by controlling flowering times in chamber‐grown plants, seed yields per plant and per unit area could match similar yields in field plants.

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