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An Indicator of the R6.5 Stage of Development for Indeterminate Soybean

Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1991 Buford Circle, St. Paul, MN 55108

^* Corresponding author.

The interval between the soybean [Glycine max (L.) Merr.] reproductive stages R6 and R7 represents an important phase of soybean development that includes seed fill and the initiation of whole plant senescence. The time between stages R6 and R7 has been observed to be relatively long for soybean grown in northern maturity zones. The objective of this study was to identify an indicator that signaled a point midway between stages R6 and R7 (R6.5). Six soybean cultivars representing Maturity Groups 00, 0, and 1 were grown in the field at St. Paul, MN in 1983, 1984, and 1985. Changes in seed, pod, and leaf appearance were monitored during the R6 to R7 interval. The midpoint of the interval was observed to occur when all the normal pods on the four uppermost nodes of the main stem had their pod cavities completely filled, suture-to-suture, with seeds. At this stage, seeds had accumulated approximately half of their final dry weight. This stage also marked the initiation of whole plant senescence, or rapid leaf yellowing. We therefore recommend the pod cavities filled trait as an indicator of stage R6.5 for indeterminate soybean cultivars grown in northern maturity zones.

Received for publication November 10, 1987.