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Growth Analysis of the Soybean Community¹

Growth analysis techniques were used to study components of dry matter accumulation in field soybean [Glycine max (L.) Merrill] communities at Lafayette, Ind. in 1968 and 1969. An analysis was made of dry matter accumulation in leaf, supporting, pod wall, and seed fractions, as well as the total aboveground portion of the crop. General approach with the 1968 data was to select mathematical functions which describe the dry weight and leaf area vs. time relationships and then to calculate, for each day during the growing season, instantaneous values of relative growth rate (RGR), crop growth rate (CGR), net assimilation rate (NAR), leaf area ratio (LAR), and the components of LAR. From the 1969 data, weekly mean RGR's and CGR's were calculated, using traditional growth analysis formulae.

The RGR of each individual plant fraction steadily decreased, at a decreasing rate, as the season progressed, At any given time, the most recently initiated plant fraction had the greatest RGR. The CGR of each fraction rose to a peak and then declined. Growth rate of the leaf component peaked first, followed in sequence by supporting, pod wall, and seed components. Apparently due to late planting in 1968, there was a greater overlap of vegetative and seed growth than in 1969 when planting was earlier.

Total aboveground RGR declined until early August, then rose to a secondary peak in mid-August. At the same time, there also occurred a peak in total aboveground CGR. The increases in RGR and CGR during August are attributable to a concurrent increase in NAR. The increase in NAR is interpreted as a response of the photosynthetic apparatus to an increased demand for assimilates. The increased demand for assimilates was due to rapid growth of the seed fraction.

Key Words: Dry matter accumulation • Yield components • Source-sink relations

² Assistant Professor, Professor, and Graduate Research Assistant, respectively, Department of Agronomy.

Received for publication February 13, 1970.