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Genotypic Differences in Soybean Affecting the Rates of Assimilate Transport from the Leaf¹

Six soybean lines [Glycine max (L.) Merr.] differing in growth rates and 15 lines differing in cross-sectional phloem areas in the petiole were selected for study. The objective was to identify and assess genetic differences that affect assimilate transfer. The ¹⁴C-assimilate transport from the fifth leaf of plants in the V7 to V8 growth stage was evaluated. The source-sink relationship for the fifth leaf was standarized among genotypes with respect to its leaf area (source) and axillary branch initials with developing leaves (sinks). Source-sink manipulations involved removal of either the branch initials or source leaves other than the fifth leaf (defoliation). A l-in-20 divergent selection among field-grown genotypes for petiole diameter scores created a 28% difference among genotypes for cross-sectional phloem areas in the petiole. Under the selected source-sink balance, genotypes had similar transport velocities and specific mass transfers but had different relative mass transfer rates. The transport differences were associated with comparisons having different phloem areas. Thus, no restriction to assimilate transfer could be associated with specific genotypes other than transport capacity. The initial response to decreasing source relative to sinks was the reduction of total sucrose in the petiole. Both transport velocity and rate were about one-third greater for defoliation than for control treatments. Genotypes can be selected which have larger phloem area but the existing vascular systems had a surprising capacity to meet increased requirements for assimilate utilization.

Key Words: Transport velocity • Mass transfer rate • Specific mass transfer • Soybeans

² Professor of genetics, Dep. of Genetics, North Carolina State Univ., Raleigh, NC 27650, and research associate (presently research agronomist), Irrigated Agric. Res. and Ext. Ctr., Washington State Univ., Prosser, WA 99350.

Received for publication April 11, 1983.