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Source-Sink Relationships in Soybeans. 1. Effects of Source Manipulation during Vegetative Growth on Dry Matter Distribution

Sinks may supply signals stimulating assimilate production and translocation or are passive and simply utilize assimilates when available. These concepts were examined using the 13 to 27 day vegetative growth characteristics of six soybean genotypes which differed in growth rates and in distribution of dry matter between sinks (stems and roots) and source (leaves). Source reduced relative to sinks by removing zero, one, or two lateral leaflets of a trifoliolate leaf as they emerged during the 2-week growth period.

Net rate of assimilate production (NAR) and carbon dioxide exchange rates (CER) increased significantly when source was reduced relative to sinks. Reducing source by the one and two leaflet treatments decreased leaf starch levels in 5 hours by 7 and 19%, respectively; however, genotypes responded differently. Partial defoliation reduced leaf starch 42%. The distribution for this proportion of the photosynthate reflected source-sink manipulations. However, the ratio of dry matter distributed to sinks vs. source did not change when source was reduced by the one-leaflet treatment and increased only 3% by the two-leaflet treatment. Genotypes responded similarly. Increased NAR benefited source primarily through increased leaf area and specific leaf weights. Under continued illumination for 10 or more hours, CER decreased with a higher proportion of photosynthate retained in the leaf. Partial defoliation only reduced the magnitude of these effects. Thus, the dominant factor operating was the apparent control in the leaf for distribution of total photosynthate produced. The data supported the concept that vegetative sinks utilized assimilates when available rather than supplying signals which stimulate assimilate production and translocation.

Key Words: Net assimilate rate • Net carbon dioxide exchange rate • Starch levels

Received for publication March 18, 1981.