Isamu Nouchi

Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water, rice biomass and temperature, and its modelling

To attempt to develop physicochemical and physiological modelling for methane transport from the rhizosphere to the atmosphere through rice plants, methane flux, methane concentration in the soil water, and the biomass of rice were measured in lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire growing season in 1992 at Tsukuba, Japan. The addition of exogenous organic matter (rice straw) or soil amendments with the presence or absence of vegetation were also examined for their influence on methane emissions. The total methane emission over the growing season varied from 3.2 g CH4 m-2 y-1 without the addition of rice straw to 49.7 g CH4 m-2 y-1 with rice straw and microbiological amendment. In the unvegetated plot with the addition of rice straw, there was much ebullition of gas bubbles, particularly in the summer. The annual methane emission due to the ebullition of gas bubbles,from the unvegetated plot with the addition of rice straw was estimated to be almost the same as that from the vegetated site with the addition of rice straw. In the early growth stage, the methane flux can be analyzed by the diffusion model (Flux=Methane concentration × Conductance of rice body) using parameters for methane concentration in the soil water as a difference in concentration between the atmosphere and the rhizosphere, and for the biomass of rice as a conductance of rice body. On the other hand, although the diffusion model was inapplicable to a large extent from the middle to late growth stage, methane flux could be estimated by air temperature and concentration in the soil water. Thus, methane transport from the rhizosphere to the atmosphere through rice plants consisted of two phases: one was an explainable small part by diffusion in rice body; the other was a large part strongly, governed by air temperature. The existence of gas bubbles in the soil may be related to the transition between the two phases

Measurements of the concentration in rainwater and of the Henry's law constant of hydrogen peroxide

A chemiluminescent method for measuring hydrogen peroxide was developed using hemin as a catalyst for luminol-based H2O2 oxidation, which gave a detection limit below 0.1 μg ∝−1 in the solution. It was shown that most atmospheric species did not give serious interference, and that negative interference of SO2; could be eliminated if pH of the collecting solution was above 10 whereas O3, gave significant positive interference. The Henrys law constant of H2O2 was experimentally determined to be 1.42 × 105 M atm−1 at 20°C at ambient concentration levels of H2O2. This method was also applied to the measurements of H2O2 concentration in rainwater in Tokyo, Japan, which was in the range of 5–1065μg ∝−1.

Effects of ozone on dry matter partitioning and yield of Japanese cultivars of rice (Oryza sativa L.)

Abstract Rice plants were exposed to ozone in field exposure chambers throughout most of the growing seasons of 3 years from 1987 through 1989. Cultivar ‘Koshi-hikari’ was used for the 1987 and 1989 experiments, and cv. ‘Nippon-bare’ was used for the 1988 experiment. Ozone exposure was performed for 7 h from 09:00 to 16:00 h each day, and ozone concentration in each of the five chambers was maintained at either 0.5, 1.0, 1.5, 2.0, or 2.75 times ambient ozone level. Throughout the growing seasons, plant samples were taken for measuring leaf area and dry weight of plant parts. At harvest, samples were taken to determine grain yield and yield components. The results of the ozone exposures indicated no consistent effect of ozone on leaf area growth, whereas total dry matter decreased with increased ozone concentration. The effect of ozone on total dry matter was particularly evident after heading. Ozone also affected dry matter partitioning. There was increased dry matter distribution to the leaf blades compared with the leaf sheaths, culms, or roots. Rice yield was significantly reduced by ozone. Although the yield was significantly different among the 3 years, there was no significant interaction between the ozone treatment and the year of the ozone exposures. The relationship between ozone concentration and yield loss of rice in this study was comparable to the relationship reported for rice cultivars in California, but was different from that reported for soybeans, which showed much greater ozone-induced yield loss than rice. Among the yield components, 1000 grain weight was significantly reduced by ozone. Harvest index was not affected by the ozone treatment. The above results indicated that rice yield is reduced by ozone in a realistic range of 20–100 nl 1−1. The results for yield components were compared with other reports. The increased dry matter distribution to leaf blade was discussed with regard to its implications for leaf area growth and lodging.

The dependence of methane transport in rice plants on the root zone temperature

Large diurnal and seasonal variations in methane flux from rice paddies have been found in many studies. Although these variations are considered to result from changes in methane formation rates in the soil and the transport capacity (e.g. biomass, physiological activities, and so on) of rice plants, the real reasons for such variations are as yet unclear. This study was conducted to clarify the effects of temperature on the rate of methane transport from the root zone to the atmosphere using hydroponically grown rice plants. Methane emission rates from the top of the rice plants whose roots were soaked in a solution with a high methane concentration were measured using a flow-through chamber method with the top or root of the rice plants being kept at various temperatures. The methane emission rates and methane concentrations in solution were analyzed using a diffusion model which assumes that the methane emission from a rice paddy is driven by molecular diffusion through rice plants by a concentration gradient. In the experiment where the temperature around the root was changed, the conductance for methane diffusion was typically 2.0-2.2 times larger when the solution temperature was changed from 15 to 30 °C. When the air temperature surrounding the top of the rice plant was changed, the change in conductance was much less. In addition, from measurements of methane flux and methane concentration in soil water in a lysimeter rice paddy during the 2 growing seasons of rice, it was found that the conductance for methane transport was correlated with the soil temperature at 5 cm depth. These results suggest that the temperature around the root greatly affects the methane transport process in rice plants, and that the process of passing through the root is important in determining the rate of methane transport through rice plants.

Apoplastic ascorbate contributes to the differential ozone sensitivity in two varieties of winter wheat under fully open-air field conditions

We studied leaf apoplastic ascorbates in relation to ozone (O(3)) sensitivity in two winter wheat (Triticum aestivum L.) varieties: Yangfumai 2 (Y2) and Yangmai 16 (Y16). The plants were exposed to elevated O(3) concentration 27% higher than the ambient O(3) concentration in a fully open-air field from tillering stage until final maturity. The less sensitive variety Y16 had higher concentration of reduced ascorbate in the apoplast and leaf tissue by 33.5% and 12.0%, respectively, than those in the more sensitive variety Y2, whereas no varietal difference was detected in the decline of reduced ascorbate concentration in response to elevated O(3). No effects of O(3) or variety were detected in either oxidized ascorbate or the redox state of ascorbate in the apoplast and leaf tissue. The lower ascorbate concentrations in both apoplast and leaf tissue should have contributed to the higher O(3) sensitivity in variety Y2.

Acceleration of 13C-labelled photosynthate partitioning from leaves to panicles in rice plants exposed to chronic ozone at the reproductive stage

To investigate the effects of low (0.05 micromol/mol) and relatively low (0.10 micromol/mol) concentrations of ozone on photoassimilate partitioning, rice plants grown in a water culture were fed with (13)C-labelled carbon dioxide at the reproductive stage in an assimilation chamber with constant concentration of (12)CO(2) and (13)CO(2). Rice plants were exposed to ozone 4 weeks before and 3 weeks after (13)CO(2) feeding. The dry weight of whole plants decreased with increasing ozone concentration, whereas net photosynthetic rate (apparent CO(2) uptake per unit leaf area) was unaffected, compared with the control, at the time of (13)CO(2) feeding. Dry matter distribution into leaf sheaths and culms was reduced more than that into leaf blades by ozone exposure. Although panicle dry weight per plant was reduced by ozone, the percentage of panicle dry weight to the whole plant tended to increase considerably. Exposure to ozone accelerated translocation of (13)C from source leaves to other plant parts. Partitioning of (13)C to panicles and roots was higher under ozone treatment than in the control. Respiratory losses of fixed (13)C from plants tended to decrease under treatment with ozone. The increase in photoassimilate partitioning in panicles can be considered to be an acclimation response of rice plants to complete reproductive stage under the restricted biomass production caused by ozone.

Mechanism of Methane Transport by Rice Plants

To clarify the mechanism of CH4 transport through rice plants, CH4 formation in flooded soil and emission through rice plants were examined by pot culture (soil and hydroponic culture) experiments. Methane concentration in surface water above soil was extremely low as compared to that in soil water. Methane concentration in vegetated soil was about one-third of that in unvegetated pots. The emission rate was 20 times higher in vegetated than in unvegetated pots. In addition, there was a linear relationship between CH4 emission rate from rice plants and CH4 concentration in the culture solution with a high CH4 concentration. Methane transport capacity of rice plants depended mainly on plant size. These results indicate that rice plants have a large capacity for CH4 transport and that rice plants play a primary role for CH4 flux from paddy fields.

Effects of chronic ozone exposure on growth, root respiration and nutrient uptake of rice plants

To clarify the response of growth and root functions to low concentrations of ozone (O(3)), rice plants (Oryza sativa L.) were exposed to O(3) at 0.0 (control), 0.05 and 0.10 ppm for 8 weeks from vegetative to early heading stages. Exposure to 0.05 ppm O(3) tended to slightly stimulate the dry weight of whole plants up to 5 weeks and then slightly decrease the dry weight of whole plants. However, these effects were statistically significant only at 6 weeks. Exposure to 0.10 ppm O(3) reduced the dry weight of whole plants by 50% at 5 and 6 weeks, and thereafter the reduction of the dry weight of whole plants was gradually alleviated. Those changes in dry weight can be accounted for by a decrease or increase in the relative growth rate (RGR). The changes in the RGR caused by 0.05 and 0.10 ppm O(3) could be mainly attributed to the effect of O(3) on the net assimilation rate. Root/shoot ratio was lowered by both 0.05 and 0.10 ppm O(3) throughout the exposure period. The root/shoot ratio which had severely decreased at 0.10 ppm O(3) in the first half period of exposure (1-4 weeks) became close to the control in the latter part of exposure (5-8 weeks). Time-course changes in NH(4)-N root uptake rate were similar to those in the root/shoot ratio especially for 0.10 ppm O(3). On the other hand, root respiration increased from the middle to later periods. Since it is to be supposed that plants grown under stressed conditions change the ratio of plant organ weight to achieve balance between the proportion of shoots to roots in the plant and their activity for maintaining plant growth, these changes in root/shoot ratio and nitrogen uptake rate under long-term exposure can be considered to be an adaptive response to maintain rice growth under O(3) stress.

Changes in Antioxidant Levels and Activities of Related Enzymes in Rice Leaves Exposed to Ozone

Abstract Changes in the levels of antioxidants such as ascorbate and glutathione and activities of related enzymes in rice leaves exposed to ozone at high or low concentrations were investigated. In rice plants (Oryza sativa L. cv. Koshihikari) exposed to ozone at a high concentration (0.50 × 10-6 m3 m-3 (0.50 ppm)) for 8 h, L-ascorbate was oxidized while the content of oxidized glutathione (GSSG) increased without the concomitant decrease of the content of reduced glutathione (GSH). In addition, the activities of ascorbate peroxidase (AP), dehydroascorbate reductase (DHA reductase), and Superoxide dismutase (SOD) declined, though the glutathione reductase (GR) activity remained unchanged. A high concentration of ozone was considered to destroy the protection system against active oxygen species, resulting in the injury of the cell structure and functions. On the other hand, in the rice leaves exposed for 5 weeks to ozone at low concentrations (0.05 × 10-6 and 0.10 × 10-6 m3 m-3), the amount of L-ascorbat...

Importance of physical plant properties on methane transport through several rice cultivars.

The transport rate of methane from a nutrient solution to the atmosphere via rice plants was controlled both by the methane concentration in the nutrient solution and by plant physiology. Measurements on 11 rice cultivars indicated that the conductance of the rice plant for methane increased as the plants developed. The conductance varied from 0.3±0.2 to 1.2±0.7 μmol min-1 mM-1 plant-1 during the tillering stage, and from 0.6±0.3 to 2.5±0. 2 μmol min-1 mM-1 plant-1 during the reproduction stage. Based on measurements of all plants, the conductance positively correlated with parameters of plant size. Conductance correlated best with stem inter-cellar volume at the tillering stage, and with root volume at the reproduction stage. For both stages taken together, the correlation between conductance and root volume was the most significant. The conductance could be approximated by a multiple linear regression using root volume and the length of root bundle. Higher conductance was found to generally associate with the plants with larger root volume, heavier root fresh weight.