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In north-east Thailand where rainfed paddy cultivation is commonplace, direct dry seeding is replacing transplanting to increase the frequency of successful plantings and to save labor. The present study shows the impact of this change on paddy vegetation from agricultural and ecological viewpoints. One hundred and seventy-nine paddy fields from a wide range of climatic, topography, soil and hydrological conditions were used in this study. Authors visited these fields every 3 weeks during the rainy seasons of 1996–1998 to collect data about water conditions, rice growth, progress of cultivation, paddy vegetation and roughness (unevenness) of the soil surface. Analysis of this data revealed that direct dry seeded fields are resource-poor and have a wider range of water conditions within a field than transplanted fields due to a rougher soil surface. This results in lower productivity of total vegetation for rice growing in direct dry seeded fields. However, weed productivity was not significantly different between the direct dry seeding and transplanted fields. Direct dry seeded fields have more species-rich vegetation and greater diversity than the transplanted fields. Previous planting methods for particular fields did not influence productivity and diversity of paddy vegetation. It is concluded that the use of direct dry seeding increases biomass production of rice and the diversity of the paddy vegetation. The present results also suggest that an analytical framework be used to integrate agricultural and ecological studies of paddy vegetation in order to harmonize agricultural productivity with biological diversity.  相似文献   
2.
The relationship between the behavior of clomeprop ([ RS ]-2-[2,4-dichloro- m -tolyloxy]propionanilide) and its residual phytotoxic activity in the soil was investigated in the laboratory with special emphasis on the concentration in the soil water. The phytotoxic activity of clomeprop on radish seedlings ( Raphanus sativus L. var. radicula cv. Akamaruhatsukadaikon), as the test plant, became greater with time after application but the inhibition was different between the two soils, which had different properties. The amount of 2-(2,4-dichloro-3-methylphenoxy)propionic acid (DMPA), a hydrolyzed and active metabolite of clomeprop, in the soil water and total soil increased with time, corresponding to the decrease in the amount of clomeprop under non-water leakage conditions. The residual phytotoxic activity of clomeprop in the soil was more highly correlated with the concentration of DMPA in the soil water than with the amount of DMPA in the total soil. In addition, a leaching column test was conducted with clomeprop and DMPA. The DMPA easily moved downward and the concentration in the soil water in the upper layer decreased with time after application. It is supposed that the downward movement of DMPA was one of the factors influencing the lasting effect of clomeprop in the field.  相似文献   
3.
Understanding the behavior of Jacopever Sebastes schlegeli to visual stimulation is important and requires determination of visual sensitivity through the use of effective stimulation. In the present study, we measured the visual acuity, visual accommodation and spectral sensitivity of Jacopever. Visual acuity was calculated from the maximum cone density in the retina. In five specimens, the total length ranged from 17.6 to 24.5 cm and the calculated minimum resolving angles and visual acuities were 9.5–10.7 min and 0.09–0.11, respectively. Measurement of lens movement for visual accommodation indicated a mean distance and direction of movement of 0.31 mm and 16.0° in an upward and forward direction, respectively. The direction of accommodation and visual axis measured from the retinal area of maximum cone density were almost identical. The calculated focal distance was 0.74-fold the total length in the nearest point to infinity. We also determined the spectral sensitivity electrophysiologically by recording S-potentials from 45 specimens of total length of 15.4–47.5 cm. The S-potentials were recorded from 300 horizontal cells in the retina. All responses showed a similar spectral pattern, consisting of hyperpolarization in response to various wavelengths of stimulating light; the maximum amplitude of the response was recorded at 522 nm.  相似文献   
4.
Economic growth in recent years has induced a labor shortage for agriculture in north-east Thailand. Labor-saving techniques, such as the replacement of transplanting with direct seeding, have become widespread. Direct seeding methods have advantages over transplanting regarding labor savings and reductions in production risks, but may cause weed problems. Therefore, rice yield reduction by competition with weeds becomes a potential problem in this region. In the present research, the extent of weed competition in rice production from the viewpoint of resource level and disturbance intensity was studied. A large number of paddy fields from a wide range of climatic, topographical, soil and hydrological conditions were analyzed. The survey was conducted every 3 weeks, and data on paddy cultivation, weeds and water conditions were recorded. Average yield of direct seeded rice was significantly lower than that of transplanted rice. The yield decreased with resource decreases in both direct seeded (DSF) and transplanted paddy fields (TF). Although the yield was not different under resource-rich conditions, the yield of direct seeded rice was lower than that of transplanted rice under resource-medium and -poor conditions. Competition intensity was not different among resource levels or between DSF and TF. Competition intensity in all fields was between 0.02 and 0.09 on average, indicating 2–9% yield losses caused by competition with weeds regardless of resource level and disturbance intensity. This suggests that low yield of direct seeded rice under resource-medium and -poor conditions was not mainly caused by competition with weeds, whereas yield losses by weeds were observed in all field types. However, from an economical viewpoint, weeding should be applied only to limited parts of this region with stable and relatively high rice yields.  相似文献   
5.
The phytotoxic activity of soil-applied thenylchlor (2-chloro- N -[3-methoxy-2-thenyl]-2',6'-dimethylacetanilide) on the growth of rice seedlings and its behavior in two different types of soil, Ryugasaki soil and Tsukuba soil, after application was investigated with emphasis on the concentration in the soil water. The greatest inhibition of thenylchlor on the growth of rice seedlings was found immediately after application to both the Ryugasaki and Tsukuba soils. The phytotoxic activity decreased with time in both soils. However, the rate of decrease in phytotoxic activity was slower in the Ryugasaki soil than in the Tsukuba soil. The concentration of thenylchlor in soil water, the amount adsorbed on the soil solid, and the amount in the total soil reduced with time after application to both soils. The amount of thenylchlor adsorbed on the soil solid phase was more persistent than that in the soil water in both soils and the concentration in the soil water was higher in the Ryugasaki soil than in the Tsukuba soil at any given time. The residual phytotoxic activity of thenylchlor on the growth of the rice seedlings in the soil was highly correlated with its concentration in the soil water but not with the amount in the total soil. These results suggested that the residual phytotoxic activity of thenylchlor in the soil is determined by its concentration in the soil water after application.  相似文献   
6.
Rainfed paddy cultivation predominates in north-east Thailand. Direct dry seeding is replacing transplanting rice cultivation to increase the frequency of successful plantings and to save labor. The present study clarified differences in weed vegetation of direct dry-seeded and transplanted paddy fields under rich, medium and poor water conditions from agricultural and ecological viewpoints. A large number of paddy fields from a wide range of climatic, topographic, soil and hydrological conditions were used. A survey was conducted every three weeks, and data regarding paddy cultivation, weeds and water conditions were collected. Expansion of direct dry seeding may increase species with in-between water adaptability to hygrophytes and mesophytes, and decrease hydrophytes and mesophytes. This characteristic was remarkable under poor water conditions. Rice cover was similar in either planting method under rich water conditions. However, cover of direct-seeded rice was significantly smaller than that of transplanted rice under medium and poor water conditions. Species diversity was higher in direct dry-seeded paddy fields than in transplanted paddy fields under medium and poor water conditions. Impact of direct dry seeding in the fields with rich water conditions on the vegetation is small in the context of maintaining rice production by transplanting and species diversity. However, direct dry seeding is mainly adopted in fields with poor water conditions. Impact of the changes in cultivating methods is large on rice productivity, species composition and species diversity under such conditions. Therefore, how to best use resource-poor fields may be the key to maintaining rice production and regional-level species diversity.  相似文献   
7.
The effects of NaCl-induced stress on physiochemical factors such as inorganic cations, proline, malondialdehyde (MDA) and polyamines were investigated in the gramineous weed, Echinochloa crus-galli Beauv. var. formosensis Ohwi ( E. crus-galli ) and rice ( Oryza sativa L. cv. Nipponbare). Growth inhibition at the 2nd leaf stage under salt stress was more severe in rice than in E. crus-galli . Water content in the 2nd leaves was also more severely decreased in rice, indicating that E. crus-galli was more salt-tolerant. After NaCl treatment, Na+ accumulated in the 2nd leaves of both plant species but not in their roots. Proline accumulation in the 2nd leaves was significantly higher in salt stressed E. crus-galli than in rice, suggesting the significance of proline production in the salt tolerance of this weed. Content of MDA of the rice increased more greatly with NaCl treatment than that in E. crus-galli . NaCl treatment affected polyamine metabolism of both plant species, but the response of each plant to salt stress was somewhat different, especially in the leaves. Leaf putrescine and spermidine contents were high in non-stressed plants in salt-sensitive rice, although rather lower in E. crus-galli in response to NaCl concentrations. These results indicate that an increase in proline and changes in polyamines relates to the salt tolerance of E. crus-galli .  相似文献   
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