| Summary: | 博士 === 國立中興大學 === 農藝學系所 === 104 === Taiwan is located in the subtropic area, which is generally the lowest latitude region where Japonica rice is cultivated. The cultivation environment here for the crops, which are two cropping seasons per year, is very unique. Thus, to understand the changes of morphogenesis of rice plants and establish a reasonable canopy structure, in order to promote the light use efficiency and ultimately increase the photosynthetic product, will significantly promote the crop yield and quality. A field experiment was carried out to determine the effects of nitrogen fertilizer application rates (60, 120 and 180 kg ha-1) on stem growth characteristics and dynamic changes of canopy structures of two rice cultivars TNG71 and TK9 grown in the first and second crop seasons during 2007 and 2008. We analyzed the relationships between canopy structures with light energy production and distribution, crop yield, and crop quality, to provide a theoretical basis for the development of modern cultivation methods as well as the breeding of ideal type of Taiwanese rice plants.
The two years data across four cropping seasons indicated that the growth period of rice plants varies depending on the cultivar, the doses of nitrogen fertilizer, the crop season, and the transplanting dates. The first crop season produced more leaves than the second season; TNG71 had 15 to 16 leaves in the first season and 14 leaves in the second season, while TK9 had 16 to 17 leaves in the first season and 14 to 15 in the second one. The length, width, and color of leave blades, plant height, number of tillers, the length of elongation internodes, the leaf sheath length of the top five leaves, the height of leaf position and panicle position, and the degree of rachilla exsertion of rice plants all significantly increased as the amount of nitrogen fertilizer used increase. The analysis of culm growth indicated that the lengths of upper internodes and leaf sheath were longer than lower nodes. However, elongated internode coverage rate by relative leaf sheath decreased significantly with the increase in nitrogen levels owing to the growth of internode response to nitrogen was more sensitive than leaf sheaths. The coverage of elongate internodes by leaf sheaths was mainly affected by the crop season, cultivar, and nitrogen application rates. Leaf sheath coverage rates in second crop season was higher than the first crop season attribute to the total leaf sheath length was longer in the second crop season, and reversely result in shorter panicle exsertion. By establishing the growth dynamics and relationships among each morphological characteristic, the leaf blade length of upper leaves would be a good estimator of lower internode length. This study indicated that responses of stem growth rate were significantly affected by nitrogen levels and subsequently would change of canopy structure and lodging resistance.
To reveal the relationship of growth and distribution of aboveground organs of rice plant to canopy structure, our study indicated that the canopy structure varied from cultivars, the amount of nitrogen fertilizer used, the cropping seasons, and different growth stages. The mean tilt angle (MTA) of rice plant leaves changed by the amount of nitrogen fertilizer used and growth stage. High amount of nitrogen fertilizer aided to regulate the leaf tilt angle prior to canopy closure and adjust the light receiving posture of canopy. The leaf area index (LAI) showed a curve distribution during the reproductive phase, reaching maximum at the heading stage, and gradually decreasing with the plant ages. There were positive linear correlations between LAI and plant height, LAI and the number of tillers. Multiple linear regression (MLR) models were established from plant morphological characteristics for the estimation of LAI which can reach 84.16% of variation of explanation. The more nitrogen fertilizer was used before plant heading, the faster the rate of light interception (LI) increased. In addition, the days it took to reach the maximum LI was also shorter, which help the plants receive higher LI during the entire growth period. LI kept constant after heading stage and would not decrease due to LAI reduction. MLR model which was based on plant morphological characteristics provide more than 90% of variation explanation for LI. The amount of nitrogen fertilizer used was increased and accomplish by increase in LAI and result in larger mutual shading rate, which in turn reduces the canopy light interception efficiency (LIE). On the other hand, when LAI was low, or when there was less mutual shading between leaves, LI was an important factor of increasing LIE. This study provided complete characteristics and changes of canopy structure, and therefore the relationship model between morphological characteristics and canopy structures can be applied to estimate and monitor the growth of rice plant.
Different cultivars and growth conditions created different canopy structures, which further affect the sun radiation interception, dry matter production and accumulation. The study indicated that dry matter accumulation, intercepted photosynthetically active radiation (PARi), radiation use efficiency (RUE), and radiation conversion efficiency (RCE) were all susceptible to the interactions between cultivars, the quantity of nitrogen fertilizer used, and the cropping seasons . The higher the usage of nitrogen fertilizer, the higher dry matter accumulation, PARi, RUE, and RCE. However, the biggest limitation of nitrogen fertilizer application during the second season was the low RUE. RCE was an indicator combining PARi and RUE. The PARi was only between 41.5 to 61.8% during the whole growth period which was mainly due to the low PARi of pre-reproductive phase, and thus the poor RCE. We recommended to emphasize the nitrogen fertilizer management during the pre-reproductive phase and to select cultivars with dynamic morphology for more radiation interception. RCE of rice plants during the whole reproductive phase was closely related to the crop yield (r=0.916). RUE was the most important factor that affects the grain yield in this study. Thus, on the basis of maintaining high RUE, promoting PARi of plant canopy will be a vital approach for increasing rice grain yield in the future. In addition, a MLR model combined with RUE and RCE which determined by rice plant morphological characteristics can be used to estimate the performance of RUE and RCE.
Milky-white kernels are caused by lacking of photosynthetic supply during the dough stage. The factors of their formation are the most widespread and complicated. The study found that the incidence of milky-white kernels (IMWK) varied with the rice cultivar, the amount of nitrogen fertilizer, and the weather of different planting seasons. The temperature for some parts of the crop season in this study had surpassed the boundary limitation reported by relevant literatures. In the future, the issue of high temperature causing milky-white kernels should be noted to the cultivation of rice in Taiwan. The more nitrogen fertilizer was used, the more significant IMWK. By analyzing the plant traits, canopy structure, and yield components, the study summarized the cultivation strategy in order to lower IMWK in the two cropping seasons in Taiwan. First, dark leaf colors of canopy structure during the heading stage of the first season should be avoided; Second, the large LAI, complex canopy structures such as LI and leaf area density, the large number of panicles and the number spikelets per unit area while the ripening stage were all the factors which would increase IMWK. Third, large LAI and large number of spikelets per unit area tended to increase IMWK during the ripening stage of the second season. Yet, the higher the LI and leaf area density during the ripening phase, the lower the IMWK. In addition to the study, we established a correlation model between unit grain photosynthetic product distribution amount (DMG) and IMWK, and found that there is a high coefficient of determination (R2) in early to middle dough stage of the first season and middle dough stage of the second season, but no strongly connected in late dough stage. Furthermore, the IMWK of TNG71 and TK9 can be effectively measured in 15 to 19 days and 10 to 14 days respectively after the heading stage. Combining the measurement of LI and RUE as well as the calculation of fertile rice kernels per unit area, IMWK can be estimated with rice plant traits and daylight monitoring, which can help achieve the goal of producing high-quality Taiwanese rice.
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