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Leaflet Orientation in Water–stressed Soybeans¹

Terminal leaflets of soybeans [Glycine max (L.) Merr.] were observed to become erect and then to invert during a period of soil drying. This study examined the interaction between this movement, abaxial and adaxial stomatal response, water potential, and surface reflectance of terminal leaflets as plant water status decreased. Observations were made on plants in a field with a Rhodic Paleustalf soil type and in which was located a weighing lysimeter.

Terminal leaflets first assumed an erect orientation and then an inverted position as total plant available water decreased below 40%. With more severe water stress a greater proportion of leaflets were affected and lateral leaflets also become inverted. The onset of terminal leaflet inversion occurred over a range of leaf water potential values of –1,400 to –1,725 kPa. Inversion of the leaflets exposed the abaxial surface which was more reflective and which maintained a higher level of diffusive conductance under water stress conditions than did the adaxial surface. Leaflet reorientation in water–stressed soybeans may be a mechanism by which water loss is reduced while still maintaining some level of productivity.

Key Words: Leaf water potential • Leaf diffusive conductance • Glycine max (L.) Merr. • Leaf reflectance • Adaxial vs. abaxial leaf surfaces

² Senior plant physiologist, Water Res. Commission, present address: CSIRO, Division of Irrigation Research, Private Mail Bag, GRIFFITH NSW 2680, Australia; and agricultural researcher, Soil and Irrigation Res. Inst., respectively.

Received for publication September 30, 1980.