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Response of Leaf Growth, Leaf Nitrogen, and Stomatal Conductance to Water Deficits during Vegetative Growth of Field-Grown Soybean¹

Changes in leaf area development, leaf N content, and stomatal conductance were examined in field-grown soybean [Glycine max (L) Merr.] in response to soil water deficits imposed during vegetative growth after canopy closure. The consequences of these changes for biomass accumulation were assessed in terms of the amount of photosynthetically active radiation (PAR) intercepted by the crops and the efficiency of its use to accumulate biomass. Mild water deficits decreased leaf expansion more than leaf production, while leaf senescence was similar to that in the well-watered control. However, the decrease in leaf area development was insufficient to have an impact on PAR interception, but biomass accumulation efficiency was decreased. The decrease in biomass accumulation efficiency was associated with lowered stomatal conductance, while specific leaf N content, a determinant of long-term photosynthetic capacity, was relatively unaffected. In contrast, severe water deficits decreased both the amount of PAR intercepted and its efficiency of use in biomass accumulation. Accelerated leaf senescence was much more significant in decreasing PAR interception than the decrease in leaf expansion and production. Specific leaf N content actually increased under severe water deficits but, given the extremely low stomatal conductances, had no effect on crop biomass accumulation.

Key Words: Glycine max (L.) Merr. • Solar radiation use efficiency • Specific leaf nitrogen content • Photosynthetically active radiation interception

² Crop physiologist, CSIRO Division of Tropical Crops and Pastures, Private Mail Bag 44, Winnellie, NT 5789, Australia; crop physiologist, USDA-ARS and adjunct professor, Agronomy Dep., Univ. of Florida; associate professor, Agronomy Dep., Univ. of Florida; and professor, Soil Sci. Dep., Univ. of Florida, Gainesville, FL 32611.

Received for publication January 27, 1986.