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Effects of Chilling on Photosynthetic Capacity, and Leaf Carbohydrate and Nitrogen Status of Soybean¹

When soybean [Glycine max (L.) Merr.] is exposed to cool temperatures (5 to 10°C) during late reproductive development, photosynthetic rates and duration are decreased. This limits the amount of available assimilate, shortens the seed-fill period, and potentially decreases yield. Greenhouse experiments were conducted to describe the photosynthetic, leaf carbohydrate, and leaf N responses of three soybean cultivars (Cobb, Davis, Tracy) exposed to cool temperature treatments (5 to 7°C) at the flowering (R2) or full seed (R6) stage of development. Plant photosyntheticapacity (PPC) and leaf photosynthetic capacity (LPC), expressed as a percentage of control, were measured before treatments began (Day 1) and each day thereafter through Day 5. Cooling the entire plant one night dropped PPC nearly 100% on the following day (Day 2) and the drop in LPC ranged from 60 to 80% at R2 and from 80 to 100%. at R6. Relatively low PPC and LPC rates were maintained until the day following one night of recovery at 19°C. Cooling the entire plant had a greater effect on photosynthesis than cooling only the roots. The decrease in photosynthesis was associated with a chill-induced water stress that was moderate to severe when the entire plant was cooled, and slight to moderate when only the roots were cooled. At R2, all cultivars recovered more than 90% PPC by Day 5, whereas at R6, Cobb and Tracy recovered about 80% PPC and Davis about 70% PPC. The latter responses imply genotypic differences in chilling stress responses. Plant recovery from chilling did not appear to be associated with carbohydrate level. The greater photosynthesis recovery of plants cooled at R2 than at R6 appeared to be related to a conservation of leaf N at R2, and a decrease in leaf N at R6.

Key Words: Glycine max (L.) Merr. • Cool temperature • Photosynthetic duration • Chill-induced water stress

² Former graduate research assistant and professors, respectively, Dep. of Agronomy, Univ. of Georgia, Athens, GA 30602.

Received for publication April 16, 1986.