Effect of Temperature on Soybean Growth and Manganese Accumulation1

Abstract Manganese deficiency in soybeans (Glycine max (L.) Merr.) appears to be more severe in spring than in summer. This may result from a decrease in the Mn supply relative to growth or an increase in the functional requirement of Mn under cooler conditions. To examine the supply‐functional concentration relationship, experiments were conducted in growth chambers at various temperatures. Manganese deficient soybeans were exposed to various concentrations of Mn in nutrient solution and sampled periodically during vegetative growth. Both Mn accumulation and the growth parameters such as dry weight of plant parts, leaf area, and plant height increased exponentially with time. Since the growth parameters responded similarly, the results were expressed as the dry weight of the tops. The relative growth rate (RGR) and relative accumulation rate of Mn (RAR) were calculated. The RGR increased with both increasing temperature and Mn supply whereas the RAR increased primarily with increasing Mn supply. The RGR maximized between 10 and 20 ppb Mn in solution, indicating an adequate plant Mn concentration. At adequate Mn, the RGR increased from 0.08 to 0.12 g g−1 day−1 with increasing temperature and plant Mn increased with the Mn concentration in the nutrient solution. At inadequate Mn, the RGR increased with the Mn concentration in the nutrient solution and plant Mn equilibrated at a minimal concentration the functional requirement that decreased linearly from 7 to 4 µg/g with increasing temperature. Relative growth rate was then expressed as a function of plant Mn concentration and temperature. At higher temperatures, the functional requirement, of Mn was low but on increasing the top Mn concentration, the RGR plateaued at a high rate. On the other hand, at lower temperatures the functional requirement was greater and the RGR with adequate Mn was lower. At the functional requirement, the RGR was governed by the rate of Mn accumulation.