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Analysis of Growth Within Distinct Strata of the Soybean Community¹

Growth analysis techniques were used to study components of dry matter accumulation within distinct strata of a field soybean [Glycline max (L.) Merrill] community at Lafayette, Indiana in 1968. An analysis was made of leaf area development and dry matter accmnulation in leaf, supporting, pod wall, and seed components of lower, middle, and upper main-stem segments, and also the branches. Mathematical functions were selected to describe leaf area and dry weight vs. time relationships. Crop growth rate (CGR) and relative growth rate (RGR) were calculated for each plant fraction at each day of the growing season.

The lower main-stem segment produced the most leaf area. However, due to abscission of lower leaves, the middle main-stem segment had the most leaf area by the time of rapid seed development.

Approximately 36% of the total dry matter in lower and middle main-stem segments and 47% of that in branch and upper main,stem segments was in the seed.

In each segment, the leaf component was first to reach peak CGR. This was followed in sequence by supporting, pod wall, and seed components. The time interval between vegetative and seed development decreased toward the top of the plant, resulting in a greater overlap of vegetative and seed growth.

Seed RGR did not vary due to position on the plant. Supply of assimilates, therefore, apparently limited seed growth to no greater extent at lower nodes than at upper nodes. In view of the decreased potential for photosynthesis toward the base of the soybean canopy, this suggests the occurrence of downward translocation of assimilates during pod filling.

Seed growth rate appeared to be controlled primarily by regulatory mechanisms within the seed, rather than by external availability of assimilates.

Key Words: Dry matter accumulation • Translocation

² Assistant Professor of Agronomy.

Received for publication October 19, 1970.

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