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Response of Soybean Fruit Respiration to Changes in Whole Plant Light and CO₂ Environment¹

A nondestructive indicator of instantaneous seed growth rate in soybean [Glycine max (L.) Merr.] and other legumes would useful in studies of the physiology of seed growth. The responsiveness of soybean fruit respiration rate to whole plant environmental treatments known to affect photosynthate supply and seed growth was evaluated. Dark respiration (CO₂ efflux) rates of individual, intact soybean fruits at the mid-pod-filling stage were measured while manipulating photosynthetically active radiation (PAR) or ambient CO₂ concentration in growth chambers. The PAR levels employed were 450, 110, or 30 µmol m^–2 s^–1 and CO₂ concentrations were 300 or 150 µL L^–1. Fruit respiration rate.began to decline within 24 h after plants were subjected to either decreased PAR or CO₂ concentration. After 2 to 3 days the response was complete and fruit respiration rate remained constant at a level determined by the treatment for an additional 4 days, the longest period evaluated. The respiratory response was rapidly reversible when either PAR or CO₂ concentration were returned to control levels. As PAR was lowered from 450 to 110 or 30 µmol m^–2 s^–1, seed growth rate declined from 20.3 to 11.0 or 8.7 mg day^–1 fruit^–1, respectively. Fruit respiration and seed growth were apparently responding to changes in photosynthate supply. The results indicate that in situ fruit respiration rate is responsive to changes in the physiological status of the plant and may be useful as an indicator of instantaneous seed growth rate in some situations.

Key Words: Glycine max • CO₂ efflux • Photosynthesis • Seed growth

² Former graduate research assistant and associate professor of agronomy, Purdue Univ., West Lafayette, IN 47907.

Received for publication November 14, 1983.