Formation of aromatic acids and growth inhibition of rice (Oryza sativa L.) plants in flooded soils with wheat straw added

The application of fresh wheat or barley straw often inhibits the growth of paddy rice plants (Oryza sativa L.) in the early stages. This inhibition is generally attributed to a deficiency of available N in soils with straw added, due to considerable N assimilation by the soil microbes. Some recent reports suggested a nutrient uptake inhibition in rice plants that was not caused by or related to nutrient deficiency in the soil due to assimilation with soil microbes. Thus, the mechanism of rice growth inhibition as a result of wheat straw application has yet to be clarified beyond dispute. Studies on the formation of inhibitors of rice plant growth and N behavior in flooded soils with wheat straw added were carried out as described below to clarify the mechanism. 1. Identification of aromatic acids as inhibitors of root elongation and N uptake by rice plants cultivated in soils with wheat straw added: Rice plants were cultivated with and without the application of wheat straw. The growth and N uptake of rice plants were reduced in the plots with wheat straw compared to the plots without straw. However, a larger amount of KCl-extractable N was present in the soil of the plots with straw application after 46 days of flooding, along with severe growth inhibition. In another experiment, plants and soils were sampled after 24 hours of sup15 NH4**+ application to the soil. Rice plants from soil with straw had accumulated less sup15 N seventeen days after straw application, and the soil at that time contained more NK4**+ than plants from soil without straw application. It was obvious that the decrease in N uptake by rice plants after wheat straw addition was reduced from N uptake inhibition and not from N deficiency due to assimilation by soil microbes. 2. Evaluation of the inhibitory effects of aromatic acids on rice plants' physiology: We tested the inhibitory effect of fractions and individual substances obtained from soil solutions on rice root elongation. Solutions collected from flooded soil incubated with wheat straw inhibited rice root elongation, particularly the fraction that could be extracted with diethylether under acidic conditions. The acidic fraction contained aliphatic acids and aromatic acids, such as benzoic acid (BA), phenylacetic acid (PAA), 2-phenylpropionic acid (2PPA), 3-phenylpropionic acid (3PPA), 3-phenylbutyric acid (3PBA), 4-phenylbutyric acid (4PBA), 2-hydroxybenzoic acid, and t-cinnamic acid. The results of a bioassay with aromatic acids indicated strong inhibition of rice seedling growth, particularly for 2PPA and 3PPA. Further examinations were performed using ' 'N-tracer in hydroponics to determine whether aromatic acids inhibit