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Leaf Area and CO₂-exchange Rate as Determinants of the Rate of Vegetative Growth in Soybean Plants¹

The purpose of this experiment was to examine the interrelationships between CO₂-exchange rate (CER) and certain growth analysis variables in soybean [Glycine max (L.) Merr.] plants.

Soybean plants representing 16 cultivars were grown in a controlled environment chamber. At 22 and 29 days after planting, leaf area and dry weights of the leaves, supporting tissue, and roots were measured for each plant. From these data, the rate of vegetative growth (GR) was determined for each cultivar as well as leaf area growth rate (LAGR), specific leaf weight (SLW), and other growth analysis variables.

At 28 days after planting, the CER per unit leaf area of the terminal leaflet of the second trifoliolate leaf was measured. The resistance to CO2 diffusion was partitioned into boundary layer, stomatal, and mesophyll components. Gas exchange was measured independently for each leaf surface.

Growth rate was negatively related to CER and SLW and positively related to LAGR. Furthermore, CER was positively and negatively related to SLW and LAGR, respectively. As the leaves increased in SLW and CER across the cultivars, their rate of area expansion decreased. Since GR was positively correlated with LAGR and negatively correlated with CER, it appears that increases in CER did not compensate for decreases in LAGR; consequently, differences in GR among the cultivars were detected.

Key Words: Glycine max (L.) Merr • Specific leaf weight • Diffusion resistances • Net photosynthesis

² Former graduate assistant (now assistant professor, Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706) and associate professor, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907.

Received for publication April 7, 1976.