Soybean Flowering Fallacy

Shawn P. Conley, Soybean and Wheat Extension Specialist, Department of Agronomy


  • There is an old rule-of-thumb that soybean does not flower until after the summer solstice — the longest day of the year occurring on June 21st in the Northern Hemisphere — yet many of us have seen soybean flower much earlier.
  • Early planted soybean experience shorter days before June 21st, so floral induction and the subsequent appearance of flowers may occur ahead of the summer solstice.
  • Soybean management decisions depend on proper identification of reproductive stage R1 (1st flower), which means relying on scouting to observe flowers, not calendar date.

Soybean is a ‘short-day’ plant.

Nearly all plant species depend on seasonal change in day/night length as a cue for initiating flowering so that it occurs at a seasonally optimum time. In both natural and cultivated systems, this ensures successful pollination, seed fill, and dispersal/harvest. In crop species, breeders can genetically develop culti- vars that have specific adaptation to latitudinal zones of north-south variance in day/night photoperiod. This is a major reason that different soybean maturity groups are grown at different latitudes (Mourtzinis and Conley, 2017).

There are two main types of plant spe- cies photoperiod dependency, known as ‘long-day’ and ‘short-day’. These historically assigned names are misleading; we now know plants actually measure the length of the night, not the day. For both types, there is a critical night length that varies between species and among wild ecotypes or crop cultivars adapted to different latitudes. For most short-day plants (like soybean), exposure to a few successive nights longer than the critical length will induce flowering. Long-day plants require the successive nights to be shorter than the critical night length to flower (Taiz et al., 2015).

There are two cyclic processes at play in the mechanism of photoperiod sensitivity: 1) the solar 24-hour cycle of day and night, and 2) a within-plant circadian rhythm. This circadian rhythm also keeps time, but because it does not precisely do so, it must be entrained to keep ‘plant time’ close to 24-hour solar time.

A protein found in plant leaves called phytochrome is responsible for photo- period detection. This protein is con- verted into an active form by sunlight and returns to an inactive form in the dark. Expression of certain genes in the plant are controlled by circadian rhythm, increasing expression at specific time intervals. When expression of these ‘clock’ genes occurs in the daylight, a signal that induces flowering (GmFT) is repressed, so flowering does not occur (Taiz et al., 2015; Cao et al., 2016). This model is consistent with the mechanism that controls flowering in rice, which is a model short-day plant. These mechanisms in soybean are less well understood than in rice, in part because there are multiple, redundant genes in soybean, which may allow some soybean cultivars to have reduced photoperiod sensitivity (Cao et al., 2016). The molecular mechanisms that control soybean photoperiod sensitivity and flowering induction are areas of ongoing research. The exact mechanism of perceiving night length and floral induction is complex, but the main idea is that longer nights, which is perceived by the leaves, induce flowering in soybean. Continue Reading

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