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Dry Matter Partitioning as Influenced by Competition Between Soybean Isolines¹

Intergenotypic competition has been shown to bias estimates of seed yield among soybean [Glycine max (L.) Merr.] genotypes. It is unknown how early in plant development intergenotypic competitive effects occur and what plant parts are affected. The objectives of this research were to determine what plant characteristics were affected by intergenotypic competition and how early in plant development these effects could be detected. Two soybean isolines 'Clark' and Clark-e₂, which matures 14 days earlier than Clark, were grown in hill plots at two spacings, 0.38 and 0.76 m between hills in 1981 and 1982. Plots were grown at Lafayette, IN on a Chalmers soil (finesilty, mixed, mesic Typic Haplaquoll). Three areas of hills were planted, (i) all hills of Clark, (ii) all hills of Clark-e₂, and (iii) alternate hills of Clark and Clark-e₂. At the fourth trifoliolate stage and at 2-week intervals thereafter, three adjacent hills of Clark, of Clark-e₂, and three hills of each isoline in the area planted to alternate hills were harvested. Dry weight of stems, leaves, and pods plus seeds were recorded. Results were similar for the two spacings. Stem weight of Clark was significantly greater when bordered by Clark-e2 than when self-bordered from the time of initial seed development through maturity. Stem weight of Clark-e2 was unaffected by the phenotype of surrounding plants. Leaf weight of Clark was significantly greater when bordered by Clark-e2 than when self-bordered from the time of initial seed development until leaves began to yellow and drop. Like stem weight, leaf weight of Clark-e2 was unaffected by the phenotype of the surrounding hills. Competitive effects for pod weight (including seeds) were apparent throughout pod and seed development to maturity. Pod weight of Clark was enhanced and of Clark-e₂ was depressed when the two genotypes were grown in alternate hills.

Key Words: Glycine max (L.) Merr., • Growth analysis • Plant development • Hill plots

² Supervisory research geneticist, USDA-ARS and professor of agronomy, Purdue Univ., W. Lafayette, IN 47907.

Received for publication August 31, 1984.