Elevated Temperature Delays Onset of Reproductive Growth and Reduces Seed Growth Rate of Soybean

The influence of temperature and carbon dioxide (CO2) on seed growth is important for both present and future climates. The objective was to examine soybean reproductive phenology and individual seed growth as affected by rising CO2 and increases in temperature. Soybean [Glycine max (L.) Merr.] 'Bragg' plants were grown to maturity in sunlit, controlled-environment chambers at two CO2 concentrations at diel, sinusoidal temperature treatments of 28/18, 32/22, 36/26, 40/30, and 44/34°C (daytime maximum/nighttime minimum). Initiating pods were tagged when 5-mm long and sampled periodically for measurements of seed growth attributes. There was no effect of CO2, 700 vs. 350 μmol mol-1, on reproductive phenology, single seed growth rate, or cotyledon cell size. Onset of seed growth was earliest at 28/18°C and was delayed up to 2 weeks as temperature increased to 40/30 and 44/34°C. Individual seed growth rate (ISGR) was most rapid at 28/18°C (6.4 mg seed-1 d-1) and decreased to 2.6 mg seed-1 d-1 at 40/30°C and 2.0 mg seed-1 d-1 at 44/34°C. Final seed size decreased as temperature increased. Above 36/26°C, mature seed were increasingly shriveled reaching 100% shriveling at 40/30°C. The number of cells per cotyledon was highest at 32/22°C and decreased at higher temperatures. High temperature decreased cell size (final mass per cell), decreased nuclear and/or plastid duplication during cell division, and resulted in fewer nuclei present in cotyledonary cells. Growth of soybean at supra-optimal temperature will delay post-anthesis reproductive processes, delay onset of seed growth, decrease ISGR, decrease seed size, and increase seed shriveling.