Takuhito Nozoe

Root-shoot interaction as a limiting factor of biomass productivity in new tropical rice lines

Abstract The objective of the present study was to identify the limiting factors in biomass productivity of new tropical rice lines (panicle weight type) by analyzing the relationship between root and shoot activity. Five field experiments using three new lines, IR65598-112-2, IR65600-42-5-2, and IR68544-29-2-1-3-1-2, and one of the highest-yielding indica rice varieties (panicle number type), IR72, were conducted at the International Rice Research Institute from 1997 to 2002. Specific absorption rate of N (SARN , mg N g-1 root dry weight d-1), specific stem sap exudation rate (sap exudation rate, g exudate g-1 root dry weight 12 h-1) and specific root respiratory rate (root respiratory rate, μmol O2 g-1 root dry weight h-1) were determined as indices of root activity. Relative growth rate (RGR, mg g-1 d-1) was used as an index of shoot activity. Compared with IR72, the new lines showed lower RGR and SARN values during the early growth stages. In contrast, during the late growth stages, these activities were higher in the new lines than in IR72. The SARB and sap exudation rate showed a linear correlation with RGR at successive growth stages, regardless of the genotypes and growth conditions. These findings indicated that active root-shoot interaction was maintained throughout the growth period in high-yielding tropical rice, including the new lines and IR72. Therefore, it was considered that SARN and the sap exudation rate are useful indices of root activity regulating RGR in the new lines. However, the root respiratory rate could not be used for selection, because the relationship between RGR and the root respiratory rate changed with the growth conditions. Our findings support the hypothesis that root and shoot activities during maturation are important factors affecting the productivity of the new lines, which have not yet been able to attain the targeted yield.

Characterization of Root Systems with Respect to Morphological Traits and Nitrogen-Absorbing Ability in the New Plant Type of Tropical Rice Lines

ABSTRACT The objective of this study was to characterize the root systems of the new plant type (NPT) of tropical rice (Oryza sative (L)) lines compared with the root system of IR72, one of the elite modern varieties. Two field experiments were conducted to observe plant growth under canopy conditions in the 1997 and 1998 wet seasons. In addition, a pot experiment was conducted in the 1998 dry season to observe plant growth under isolated conditions. The root systems of the NPT lines were characterized by roots heavier than those of IR72. The larger root dry weight (DW) is attributed partly to a genetically determined high root/shoot ratio because, irrespective of growth conditions, the ratio was larger in the NPT lines than in IR72. An increase in root DW was accompanied by an increase in the length of the whole root system, including fine roots. However, despite the differences between the NPT lines and IR72 in root DW and root length, there were no significant differences in the amount of nitrogen (N) absorbed throughout the growth period. Therefore, the specific absorption rate of N per root DW (SARN, mg N g−1 root DW d−1) was lower in the NPT lines than in IR72. We concluded that, although the root systems of the NPT lines grew prolifically, they were not necessarily more efficient in terms of N absorption compared with the root system of IR72.

Effects of the addition of Fe-containing slag fertilizers on the changes in Eh in paddy soil

Abstract Flooded soil with the addition of Fe-containing slags (1% to soil) was incubated under laboratory conditions. Five commercial slags were used in this study. The slag samples were numbered from I to V in ascending order of acid-soluble Ca content. The pH of the soil solution increased with the increase in the Ca content of the slags. All the Eh (oxidation-reduction potential) in the presence of the slags were lower than that of the control during the initial 8 d period of incubation. The Eh value in the presence of slag V which contained the largest amount of Ca was the lowest throughout the incubation period. These findings suggest that the major factor of Eh decrease among others including the increase in pH, dissolution of Fe(III), and production of Fe2+ was the increase in pH. In the case of slags II, III, and IV, the pH values were below 7.0during the early stage of incubation. Although the pH values increased with the increase in the Ca content, all the Eh values in the presence of slags II,...

Effects of methanogenesis and sulfate-reduction on acetogenetic oxidation of propionate and further decomposition of acetate in paddy soil

Abstract Under laboratory conditions, submerged paddy soil with the addition of propionate was incubated anaerobically. Propionate oxidation was suppressed by the addition of H2, and the specific inhibitor for methanogenesis (2-bromoethane-sulfonate; BES). The inhibition with H2 can be associated to the negative feedback of endergonic propionate oxidation with H2. The BES inhibition on propionate oxidation can be attributed to the uncoupling of propionate oxidation and methanogenesis. When sulfate was added, propionate oxidation was not suppressed by the addition of H2, but it was suppressed by the addition of the specific inhibitor for sulfate-reduction (molybdate). These findings suggest the occurrence of preferential coupling of propionate oxidation with sulfate reduction. During the oxidation of propionate after the addition of sulfate, acetate hardly accumulated in the absence of BES, but accumulated in the presence of BES. Propionate oxidation proceeded irrespective of the addition of BES. These fin...

Suppressive effect of rice bran incorporation in paddy soil on germination ofMonochoria vaginalisand its relationship with electric conductivity

Laboratory experiments were conducted to investigate (1) the effects of the addition of rice (Oryza sativa. L.) bran to paddy soil on the germination of Monochoria vaginalis (Burm. f.) Kunth, and (2) the relationship between the electric conductivity (EC) of the soil solution and germination. Soil samples were collected at 4 sites in Japan. After flooded soils with rice bran had been incubated for 7 d at 30°C, the soil solution was collected using a porous cup and the EC of the soil solution was measured. The amounts of rice bran added to the soil were 0%, 0.3%, 0.6% and 0.9% (weight(w)/w). In the soil solution obtained, seeds of M. vaginalis were incubated for 3 d at 30°C, and the germination percentage was then analyzed. The addition of rice bran suppressed germination, and the degree of suppression increased with increasing content of rice bran. Although the same amount of rice bran was applied to each soil, the degree of growth suppression by rice bran as well as the EC of the soil solution differed among the soils. In each soil, there was a positive correlation between the amount of rice bran and EC, and the degree of growth suppression significantly increased with an increase in EC. When EC was higher than 150 mS m−1, seeds of M. vaginalis hardly germinated. There was no significant correlation between the oxidation-reduction potential (Eh) of soil and seed germination, suggesting that EC is a more reliable and convenient indicator than Eh for evaluating the relationship between the addition of organic material and seed germination. In conclusion, the addition of rice bran to soil increases the EC of the soil solution, and EC is one of the factors that suppress the germination of M. vaginalis. The suppressive effect of rice bran on germination is different among soils. This fact is attributed to the difference in EC due to the addition of rice bran. Thus, it is expected that EC can be used as an indicator for determining how much rice bran to add.

Tolerance of Rice (Oryza sativa L.) and Echinochloa Weeds to Growth Suppression by Rice Straw Added to Paddy Soil in Relation to Iron Toxicity

Abstract The effects of addition of rice straw to submerged soil on the emergence and growth of rice (Oryza sativa L.) and two paddy weeds (Echinochloa oryzicola Vasing. and Echinochloa crus-galli (L.) Beauv. var. crus-galli) were investigated. Rice straw suppressed both the emergence and growth of transplanted plants depending on the amount of rice straw added (0%, 0.3%, 0.6% and 0.9% (w/w)) in the order of E. crus-galli > E. oryzicola > rice. The severe suppression of emergence and growth of E. crus-galli in the presence of 0.9% rice straw in hydroponic culture was thought to be due to high Fe content of the shoots. Since the difference in tolerance for the toxicity of rice straw is an important factor, the addition of organic materials into soil may help to suppress Echinochloa weeds selectively.

Effects of the addition of Fe(0) on the reduction of Fe(III) in submerged rice soil

Abstract The present study aimed at determining firstly whether Fe(0) is oxidized to Fe(II) in submerged rice soil, what is the acceptor of electrons released from Fe(0), and finally how the Eh values changed in the presence of Fe(0). Under laboratory conditions, flooded soil with the addition of Fe(0) was incubated anaerobically. Although the addition of Fe(0) may increase the amount of Fe(II), it is difficult to determine whether Fe(II) is derived from Fe(III) or Fe(0). For the current experiments, therefore, P-poor soil with the addition of ferric phosphate (Fe(III)PO4) and P-rich soil were used. If the electrons from Fe(0) flows to Fe(III)PO4 and/or the complex of Fe(III) oxides and phosphate in soil, Fe(III) should be reduced to soluble Fe(II). This reduction increases the Bray-P content because of the production of Fe(II)3(PO4)2 that is more soluble than Fe(III)PO4. When P-poor soil with the addition of Fe(III)PO4 and Fe(0) was incubated, the amounts of both Fe(II) and P in soil increased. This finding indicates that Fe(0) was oxidized to Fe(II) and, subsequently, that the electrons released from Fe(0) flowed to Fe(III)PO4. When the P-rich soil with Fe(0) was incubated, the amount of Fe(II) increased, unlike the amount of P. These findings show that Fe(0) was oxidized to Fe(II). However, the electrons released from Fe(0) did not flow to the complex of Fe(III) oxides and phosphate in soil. On the basis of these results, it can be concluded that the electrons from Fe(0) flows more easily to Fe(III)PO4 than to the complex of Fe(III) oxides and phosphate in soil. In the presence of Fe(0) and Fe(III)PO4 , the changes in the Eh values were investigated. If Fe(0) alone contributes to the maintenance of the redox equilibria, then the Eh values should increase with the change of Fe(0) to Fe(II) because Fe(0) and Fe(II) are the reduced and oxidized forms, respectively. In the current study, however, the addition of Fe(0) did not increase the Eh values. The presence of Fe(0) decreased the Eh values and increased the pH and Fe(II) content during the incubation. These findings suggest that Fe(0) and Fe(III) oxides in soil contribute to the maintenance of the redox equilibria in soil.

Mechanisms of decomposition of acetic acid by methane-producing bacteria in submerged paddy soil

Abstract During most of the rice season in Japan, paddy fields are usually flooded with irrigation water. Although fresh organic materials such as rice straw are important elements for rice cultivation, their incorporation into flooded paddy fields sometimes inhibits the growth of rice plant. Especially in cool areas, the accumulation of acetic acid (AA), which is generated as intermediate product of the decomposition of organic matter, is considered to be responsible for this inhibition (Takijima 1963). The AA produced in submerged paddy soil is decomposed by anaerobic bacteria such as methane-producing bacteria (MPBs) (Hori et al. 1990). To control the accumulation of AA in paddy soil, therefore, the role of MPBs in the decomposition of AA needs to be analyzed.

Effect of EDTA on reduction process in submerged paddy soil with rice straw

Abstract Submerged paddy soil with powdered rice straw was incubated anaerobically. In the presence of 10 p.M of EDTA, the amount of Fe(II) significantly increased during the early stage of incubation. This finding suggests that EDTA extracted some part of Fe(III), which was utilized then by Fe(III)-reducing bacteria. These increase in the Fe(II) content was accompanied both by the suppression of sulfate-reduction and by the enhancement of acetic acid (AA}-decomposition. The addition of EDTA did not enhance the CH4 -production during the early stage of incubation. These findings suggest that the reduction of the EDTA-extractable Fe (III) resulted in electron deficiency and that the electron flow was mainly diverted from sulfate reduction to Fe(III)-reduction in the early stage of incubation. As a result of this electron deficiency, Fe(III)reducing bacteria may utilize AA as an electron donor. The addition of EDT A enhanced both the production and the decomposition of AA in the late stage of incubation as ...

Effect of Ni-EDTA on production of methane and decomposition of volatile fatty acids in paddy soil

Abstract In flooded soil, volatile fatty acids (VFAs) which are intermediate products of organic matter decomposition are toxic to rice plant. The VFAs consist mainly of acetic acid (CH3COOH), and methane (CH4)-producing bacteria (MPBs) decompose them into CH4 and carbon dioxide (CO2).