Adequate range of boron nutrition is more restricted for root nodule development than for plant growth in young soybean plant

A pot experiment was conducted under growth chamber conditions to determine the lower and upper critical levels of boron (B) for plant growth, nodule development, and nodule acetylene reduction activity (ARA) in young soybean plants. Plants of a soybean cultivar, Tachinagaha, were grown in pots cont aining river sand to which a nutrient solution with different B levels was added and were inoculated with Bradyrhizobium japonicum A1017. At 8, 12, an d 16 d after sowing (DAS), among the plants supplied with the solution at 0 , 1.0, and 2.0 mg B L-1, plants with 1.0 mg B L-1 showed the highest values for dry shoot and fresh root weight, root length, total number of developi ng nodules and meristematic nodules (DMN), and ARA. At 20 DAS plants grown with 11 B levels (0-2.0 mg L-1) were compared. The B critical deficiency le vels for soybean dry shoot weight, fresh root weight, root length, DMN, num ber of complete nodules, and ARA were approximately 46, 35, 34, 57, 60, and 50 mg B kg(-1) dry matter, and the critical toxicity levels were approxima tely 114, 137, 134, 97, 104, and 89 mg B kg(-1) dry matter, respectively. T he optimum B levels for the growth characters were approximately 34 to 137 mg B kg(-1) dry matter. The optimum range of B levels for nodule formation and function was more restricted than that for the growth characters. Based on the results of treatments with various B concentrations, 0.4 mg B L-1 w as found to be the concentration most beneficial for all the growth charact ers including nodule formation at the early stage (20 DAS) of development o f soybean plants.