Evaluation of root activity by bleeding sap from the basal stem in soybean plants under excessive soil water

The objective of this study was to examine the possibility of evaluating root activity by bleeding sap from the basal stem in soybean plants under excessive soil water since the bleeding rate is concerned with the root activity. The soybean variety of Enrei was grown in pots and field in 2002 and in 2003. Excessive soil water treatments were conducted for 12 to 18 days about one month after sowing. The dry matter was not reduced but the nitrogen content of leaf was reduced in the excessive soil water treatment at the 5% or 1% significant level. The bleeding rate per plant was reduced after 6 days of treatment, and the response of the bleeding rate was earlier than that of the nitrogen content of leaf. The nitrogen content of bleeding sap was reduced after 2 days of treatment, and kept at a lower level than that of control throughout the treatment. We could have a correct understanding of the nutrition absorption by the total nitrogen content of bleeding sap regardless of the amount of nitrogen fixed by root-nodule bacteria, despite the nitrogen fixation was reduced by excessive soil water treatment. The results suggest that bleeding rate per plant and the nitrogen content of bleeding sap are useful to evaluate root activity of soybean plants under excessive soil water.