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In organic paddy fields, one of the toughest weeds to control is Monochoria vaginalis (Burm. f.) Kunth. The application of fresh rice (Oryza sativa L.) bran to soil suppresses the germination of this weed. In stark contrast, the scarcely decomposable component of this material stimulates the germination. This review focuses on the elucidation of these conflicting traits. One of the chemical factors for the suppression is toxic substances such as organic acids. In addition, rice bran application enhances the piling of small soil particles by tubificids on the soil surface. This layer acts as a physical factor that slows germination, presumably because its fine grains prevent light from reaching the seed. The newly accumulated soil with small particle was sampled for the analysis of the settled soil volume in water (SSVW). This value corresponds to the mud volume in water per dry weight. The electrical conductivity (EC) of the soil solution and the SSVW of the soil surface are useful indicators to evaluate the chemical and physical factors that control germination, respectively. EC and SSVW have been traced in paddy fields where rice bran was applied. The chemical and physical factors were active during the earlier and latter halves of the month after transplanting, respectively. These factors worked in a complementary manner. Paddy soil is typically equipped with the ability to reduce oxygen (O2). This trait enhances the number of M. vaginalis plants because hypoxic conditions are preferable for germination. Anaerobic soil conditions are not required for the expression of O2-reducing power. A long-term application of rice bran might intensify this power because a scarcely decomposable component of rice bran contains a significant amount of O2-reducing substances. The presence of conflicting factors that suppress and enhance germination might destabilize the efficacy of the rice bran. [ABSTRACT FROM AUTHOR]

Rice bran application is practically used by Japanese organic rice farmers as a common measure in controlling Monochoria vaginalis, a known weed in organic rice production. To elucidate the inhibitory mechanisms of rice bran application on M. vaginalis germination, we focused on volatile fatty acids (VFAs) and aromatic carboxylic acids (ACAs) as organic acids produced through rice bran decomposition in the soil, which can possibly suppress M. vaginalis germination. Both solo solutions of 1 mM VFAs (propionic or isobutyric acid) and 0.2 mM ACAs (3-phenylpropionic or 4-phenylbutyric acid) exhibited M. vaginalis germination suppression, especially at pH 4.6, the lowest pH tested. In the 0.5–3 mM VFA solutions at pH 4.6, germination was strongly suppressed in the order of isobutyric > propionic > butyric > acetic acid. With the 0.05–0.2 mM ACA solutions at pH 4.6, the germination was strongly suppressed in the order of 3-phenylpropionic > 4-phenylbutyric > 2-phenylpropionic acid. Benzoic acid did not exhibit significant germination suppression. The solutions of 0.5 mM VFAs and 0.05 mM ACAs exhibited no suppression when applied separately; however, the certain mixture of a VFA and an ACA at the aforementioned concentration, for example, 0.5 mM isobutyric acid and 0.05 mM 3-phenylpropionic acid, exhibited significant germination suppression. These results showed the several combinations of VFAs and ACAs at even non-effective concentrations synergistically suppressed M. vaginalis germination. Abbreviations:2PPA: 2-phenylpropionic acid; 3PPA: 3-phenylpropionic acid; 4PBA: 4-phenylbutyric acid; ACA: aromatic carboxylic acid; EC: electrical conductivity; M. vaginalis: Monochoria vaginalis; VFA: volatile fatty acid [ABSTRACT FROM AUTHOR]

Organic paddy rice (Oryza sativa L.) cultivation can be negatively impacted because of the weeds growth like Monochoria vaginalis. Although the rice bran application is useful in combating weed, its performance is not consistent. In this study, laboratory experiments were conducted to elucidate the effects of rice bran on the suppression and promotion of germination of M. vaginalis. Suppression experiments involved incubating mixtures of flooded soil and rice bran. This was followed by a germination experiment in the collected soil solutions. A negative and significant correlation was observed between germination and the electric conductivity (EC) of the solution. None of the seeds germinated when the EC of the Gray Lowland soil was over 130 mS m−1. Conversely, the seeds in the Andosols' solution germinated even when EC was over 130 mS m−1. Thus, the solutions of the Andosols had different mechanisms of suppression than the Gray Lowland soils. To understand the promotion of germination, air-dried soil was prepared after the flooded incubation of rice bran and soil mixture, simulating a long-term application of rice bran. Germination was analyzed in a filtrate of the mixture of air-dried soil and distilled water. The addition of rice bran significantly promoted germination. There was a negative and significant correlation between germination and dissolved oxygen (DO) suggesting that hardly decomposable components like antioxidants in rice bran decreased O2 and stimulated germination because the seeds of M. vaginalis require very little O2 for germination. [ABSTRACT FROM AUTHOR]

Growth of the weed Monochoria vaginalis (Burm. f.) Kunth under the conditions of organic rice production is a serious problem. The reason for the growth of M. vaginalis being dominant, especially in organic rice production, is not fully understood. In this study, laboratory experiments were conducted to analyze soil and seed factors in relation to the promotion of germination. (1) After incubation of flooded soil with or without the addition of rice bran (0.3%, 0.6%, and 0.9% in an air-dried soil basis), soil solutions were recovered and seeds of M. vaginalis were incubated in the soil solutions. Germination in the soils solutions without and with 0.3% rice bran was greater than that in distilled water. However, germination was suppressed in the presence of 0.6% and 0.9% of rice bran. These findings indicate that the solution from the soils with rice bran has different effects that may either increase or decrease germination. (2) A mixture of air-dried soil and distilled water was filtered to obtain a soil solution. Seeds were incubated in the soil solution (same as above). Environmental and physiological factors affected germination: exposure of seeds to light was an environmental factor and high germination activity and shallow dormancy of seed were physiological. The recovered soil solution promoted germination when these factors were not optimized. (3) There was a negative and significant correlation between dissolved oxygen (DO) in the soil solution and germination, indicating that a low content of DO was a promotive factor for germination. (4) Based on an experiment using pH buffers, germination increased with decrease in pH of soil solution, as long as the pH ranged from 4.0 to 7.0. This finding indicates that pH is also a factor that promotes germination. [ABSTRACT FROM AUTHOR]