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Effect of the Reproductive Sink on ¹⁴C-Assimilate Partitioning between Starch and Water-Soluble Compounds in Soybean Leaves¹

The objective of this study was to determine if changes in reproductive sink demand influence the partitioning of carbon within the source leaf. Temporary (Late R2-R4) and continuous (Late R2-R8) light and CO₂ enrichment treatments were used increase the pod number and weight on field.grown soybeans (Glycine max L. Merr. ‘Hodgson 78’). During the last week of the temporary treatment, and at 1, 2, and 4 weeks thereafter, plants were pulse-labeled with ¹⁴4CO₂ at the tenth trifoliolate leaf. Leaf discs were removed at 0.5, 2, 4, 8 and 24 h after labeling and subsequently extracted for determination of ¹⁴C in the starch, water-soluble compounds (WSC), and residual fractions. Temporary and continuous CO₂ and light enrichment increased the reproductive sink load of field.grown soybeans at all sampling dates. On the average, total number of pods was increased 35% by light enrichment, and 38% by CO₂ enrichment. During this same period, light enrichment res~resulted in at least a 38% increase in total pod wall and seed dry weight while CO₂ enrichment resulted in a 28% increase. Light and CO₂ enrichment increased plant size during the sampling period. Leaf dry weight was increased 30% by light enrichment, and 39% by CO₂ enrichment. Stem plus petiole dry weight was increased 26% by light enrichment and 51% by CO₂ enrichment. Hence, light enrichment had a greater effect on reproductive than on vegetative growth, while CO₂ enrichment had a greater effect on vegetative than on reproductive growth. On the first sampling date, 11 August, significantly less label was incorporated and retained as starch in the light-enriched plants. No differences were observed in the levels of ¹⁴C-labeled WSC or starch for the light-enriched plants on 18 and 25 August, and during the entire experiment for the CO₂-enriched plants. On 7 September, 4 weeks after removal of the temporary treatment, significantly less label was retained as WSC, starch, and residual material in the light-enriched plants. The hypothesis that increased sink demand may influence assimilate partitioning is supported by these data.

Key Words: Glycine max L. Merr. • Total nonstructural carbohydrates • Water-soluble compounds • Light enrichment • CO₂ enrichment • Starch

² Research assistant (currently senior research biologist, Monsanto Agric. Products Co., St. Louis. MO./63167) and professor of agronomy and plant genetics, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication May 14, 1984.