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Published in Crop Sci 27:1197-1201 (1987)
© 1987 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Soybean Leaf Photosynthetic Response to Changing Sink Demand1

Michael J. Lauer and Richard Shibles2

To determine the extent to which photosynthesis of mature and immature soybean [Glycine max (L.) Merr.] leaves would respond to change in reproductive sink demand, plants were treated to enhance and diminish sink size. Thinning the stand at R3 induced greater pod set and an increase in pod growth rate of the remaining plants, whereas continuous partial depodding (leaving one pod to develop at each mainstem node) reduced pod growth rate by 65%. Thinning the stand resulted in maintenance of peak protein and photosynthetic rate in fully expanded (node 7) leaves of the remaining plants, whereas shortly after reaching maximum these activities declined by about 20% in controls. In expanding (node 10) leaves, thinning increased protein and photosynthetic rate about 20% compared with controls. Partial depodding had no effect on leaf photosynthetic rate, but did result in failure of protein to be mobilized to sinks. The response of fully expanded leaves as well as expanding leaves to increased sink size indicates that a source leaf can respond photosynthetically to changes in sink demand even after reaching full growth. That greater than normal sink size stimulated more rapid photosynthesis suggests that the soybean leaf possesses unrealized photosynthetic capacity. As a corollary, it may be concluded that factors determining pod retention are more limiting to yield in this species than is photosynthesis.

Key Words: Glycine max (L.) Merr. • Pod load • CO2-exchange • Soluble protein • Stomatal resistance

1 Journal Paper no. J-12498 of the Iowa Agric. and Home Economics Exp. Stn. Project no. 2275, a contributing project to North Central Regional Project, NC-14

2 Former research assistant and professor, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication January 5, 1987.




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