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Variation in Leaflet Orientation among Soybean Cultivars¹

The degree of leaflet orientation was measured for five soybean (Glycine max L., Merr.) cultivars on an hourly basis from 0620 (0700 EST) until 1820 (1900 EST) hours Local Solar Time during the 10 node stage of development (V10) and from 0520 (0700 EDT) until 1820 (2000 EDT) hours during the beginning pod stage (R3). Measurements were taken on 4 separate days during each growth stage. Our primary objective was to determine if differences exist among soybean cultivars in degree of leaflet orientation.

There was significant variation among cultivars in the degree of orientation of the center and the side leaflets of trifoliolates at different times of the day.during both growth stages. The orientation of the center leaflets of a trifoliolate differed from that of the side leaflets. The vertical inclination of the center leaflets tended to change over time while the horizontal inclination remained relatively constant. On the other hand, the horizontal inclination of the side leaflets tended to change over time while their vertical orientation remained relatively constant. However, due to their location relative to each other, the different directions of movement of the center vs. side leaflets seem necessary in order for all three leaflets to attain similar exposures to incoming radiation.

Maximum variation in leaf movement was noted from 0820 until 1620 hours during both V10 and R3 stages of development. Higher angles of inclination were obtained and maintained during R3 than V10.

The K concentration in pulvini was significantly higher (p < 0.01) during the tense state (higher angles of leaflet orientation) than during the relaxed state. It appears that the K concentration in the pulvinus might have some role in regulating the bending and straightening movements of the pulvini, perhaps by inducing changes in turgor pressure.

Key Words: V10 growth stage • R3 growth stage • Leaf angle • Leaflet vertical inclination • Leaflet horizontal inclination • Pottasium flux • Pulvinus • Glycine max L.

² Former graduate student, Dep. of Plant and Soil Science, Univ. of Tennessee (now soybean breeder, Delta and Pine Land Company, P.O. Box 1118, Wilson, N. C. 27893) and associate professor, Dep. of Plant and Soil Science, Univ. of Tennessee, Knoxville.

Received for publication October 13, 1981.

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