Post-flowering photoperiodic effect on reproductive efficiency and seed growth in soybean

Abstract

Photoperiod not only controls soybean ( Glycine max (L.) Merr.) flower induction, but also affects later stages of reproductive growth. Its effect on different yield-determining processes, however, is not well understood. The objective of this work was to determine the post-flowering photoperiodic effect on yield components, reproductive efficiency, and assimilate partitioning to developing seeds in the soybean cultivar Hood 75. Plants were induced to flower in short days. After flowering, three photoperiodic treatments were applied: (1) short days ( SD); (2) short days with interrupted night ( NI); and (3) a sequential combination of both ( NI-SD). Flower differentation, pod lenthening, and seed-filling periods were shortest in SD, intermediate in NI-SD, and longest in NI. The number of differentiated flowers, pods, and seeds were doubled by NI as compared to SD, but the percentages of setting and retained pods were not changed. The percentage of seeds that completed development was, however, increased by SD. More assimilates were partitioned into the seeds in SD, while NI increased the proportion partitioned into shoots. The nitrogen content of seeds was similar in NI and SD, while NI-SD seeds showed a slight increment in their N content. The N content of carpels, however, was high in NI, medium in NI-SD, and low in SD. Compared to NI, duration of the seed-filling period was decreased but the seed-growth rate and final seed size were increased by SD. Final yield was similar for plants in NI or SD, because seed number and seed growth rate compensated each other. Plants in NI-SD has a significantly higher yield, however, as a result of sequential increase in seed number caused by NI, followed by an increase in the rate of seed-filling caused by SD. The results support the existence of two sequential photoperiodic effects on soybean reproductive development: (1), the number of growing seeds is enhanced by NI and decreased by SD; and (2), the assimilate partitioning to seeds is enhanced by SD and decreased by NI. A non-destructive method for seed growth-rate calculation is also described.