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Environmental and Stress Physiology

Oyxgen and Temperature Effects on Soybean Seed Coat Respiration Rates

United States Department of Agriculture, Agricultural Research Service, University of Florida, Gainesville, FL 32611, Agronomy Physiology Laboratory, University of Florida, Gainesville, FL 32611

Soybean (Glycine max (L.) Merr) seed coat respiration rates in response to changing O₂ concentration and temperature were examined experimentally and with a mathematical analysis. The experimental observations showed seed coat respiration rates were sensitive to O₂ concentration below 0.25 micromole O₂ cm^–3. There was a steady decline in respiration rates from the saturating O₂ concentration down to about 0 to 0.03 micromole O₂ per cubic centimeter. Seed coat respiration rates were found to change linearly with temperature between 8 and 28°C. The explanation for these results was sought by examining the diffusion of O₂ into the vascular bundles of the soybean seed coat. Differential equations describing O₂ uptake in two distinct zones of the vascular bundle were solved. The outer zone was assumed to be O₂ saturated and respiration proceeded at a constant rate per unit volume. The inner zone was assumed to have respiration rates which were linearly dependent on O₂ concentration. The solution of this mathematical model showed considerable similarity with the experimental results. Respiration rates were predicted to saturate at about 0.31 micromole O₂ per cubic centimeter and to decrease curvilinearly below that concentration. While the mathematical model predicted an exponential response in respiration rate to temperature, it was found that the exponential response is difficult to distinguish from a linear response in the temperature range studied experimentally. Consequently, both the experimental and theoretical studies showed the importance of O₂ diffusion into soybean seed coat vascular bundles as a potential restriction on respiration rates. In particular, it was suggested that increases in the total length of the vascular bundles in the soybean seed coat was the major option for increasing the total respiratory capability.