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Water Relations and Growth of Soybeans in Drying Soil¹

Soybean plants (Glycine max (L.) Merrill ‘Hill’) were grown in moist soil in ll4-1iter containers for 63 days in the greenhouse, and then subjected to a 9-day drying cycle to evaluate water relations and growth in a drying soil. On day 1 of the drying cycle, measurement of xylem pressure potential (P), leaf diffusive resistance (R_L), area, soil water potential (SWP), and root dry weight was initiated. Measurements were made at 0500 to 0630, 1000 to 1200, and 1400 to 1600 hours (CDT) on days 1, 5, 7, and 9. The earliest time period was before sunrise. Atmospheric demand was calculated for each sample period.

Xylem pressure potential at each time period of each day decreased linearly with, and was significantly related to (R² = 0.97, 0.92, and 0.93, respectively) decreasing base (presunrise) SWP of the drying soil for the duration of the experiment. The data indicated the possibility of estimating P of plants growing in a drying soil from base values of SWP. Extremes in atmospheric demand, as well as SWP, influenced levels of P of plants growing in wet soil. The largest decline in P occurred between the presunrise and 1000 to 1200 hour time periods of each day in both soil environments. Presunrise wilting of plants growing in the drying soil occurred at day 7, when the corresponding base (presunrise) P was about —18 bars. No establishment of a water potential gradient between upper- and mid-canopy leaves occurred in either soil environment. Decline in leaf enlargement corresponded to a base P of about —4.5 bars, and cessation of enlargement occurred when base P was between —10 and —12.9 bars.

As SWP and P declined, R_L increased. Vapor pressure deficit (VPD) was the most significant single determinant of R_L (R² = 0.56) at the 1400 to 1600 hour time period; however, the interaction between VPD and P influenced R_L more than either variable singly at this time period (R² = 0.67). The interactions between VPD and base and between VPD and base SWP both influenced R_L above the level of single variable effect at both the 1000 to 1200 and 1400 to 1600 hour time periods.

Key Words: Glycine max (L.) Merrill • Soil water potential • Base soil water potential • Xylem pressure potential • Base xylem pressure potential • Leaf enlargement • Leaf diffusive resistance • Pressure chamber • Atmospheric evaporative demand

² Research agronomists ARS, USDA, Stoneville, MS 38776; and associate professor of forestry, Univ. of Missouri, Columbia, MO 65201.

Received for publication June 21, 1976.