Genotypic Differences Of Nitrogen Use Efficiencies And Physiological Mechanism In Rice

N fertilization is yet the most important agricultural method for increasing grain yield while over use of N not only decrease the efficiency of nitrogen absorption and utilization but also causes so many environment problems. Researches have been done revealed that N use efficiency is varied in different rice genotypes. And it is an ideal approach for increasing N use efficiency to explore the potential and screen N efficient rice genotypes. In this research, a field experiment with 104 rice genotypes prevailing in the region of Yangtse rive as materials was carried out to study the characteristics and differences of rice genotypes'grain yield and N use efficiencies. Evaluation and classification of N use efficiencies were also done. Twelve rice genotypes (6 N-efficient and 6 N-low-efficient) belonging to late-maturing medium Japonica and early-maturing late Japonica were selected. The characteristics of rice matter production and accumulation, dynamics of N absorption and utilization, characteristics of root morphology and physiology, different indexes of leaf photosynthesis and etc were studied to provide feasible regulation approaches to increase N use efficiency in rice production. And the main results were as follows.1.Based on the whole growth duration and Dingying's Standard of classification of rice growth type, 104 rice genotypes in this research were classified into 5 growth types including early-maturing medium Japonica(EMMJ), medium-maturing medium Japonica(MMMJ), late-maturing medium Japonica(LMMJ), early-maturing late Japonica(EMLJ), medium-maturing late Japonica(MMLJ).All rice genotypes were grown under 4 N levels including 0 kg·666.7m-2, 10 kg·666.7m-2 (low), 15 kg·666.7m-2 (medium), 20 kg·666.7m-2 (high). With the increase of N level, the average grain yield of all rice genotypes increased while the differences among rice genotypes decreased with the coefficient of variation dropping to 16.87 at the N level of high from 20.34 at the N level of 0. Rice grain yield increased with the delaying of growth duration under 4 N levels which presented a tendency of MMLJ >EMLJ >LMMJ >MMMJ >EMMJ. With the increase of N level, grain yield of all growth types were increased except for MMLJ which presented a tendency of medium > high >low >0. And the genotypic differences of grain yields belonging to the same growth type were also existed under each N level.The average N use efficiency of all rice genotypes presented a tendency of medium > high >low. With the increase of N level, the differences among rice genotypes decreased with the coefficient of variation dropping to 21.28 at the N level of high from 28.75 at the N level of low. N use efficiency of all growth types increased with the delaying of growth duration under 3 N levels except for MMLJ and it presented a tendency of MMLJ >EMLJ >LMMJ >MMMJ >EMMJ at low N, a tendency of EMLJ > MMLJ >LMMJ >MMMJ >EMMJ at medium N and a tendency of EMLJ > LMMJ > MMLJ >MMMJ >EMMJ at high N. And the genotypic differences of N use efficiencies belonging to the same growth type were also existed under each N level.The maximal yield and its corresponding N use efficiency under 3 N levels were adopted as indexes to estimate and classify rice genotypes. Genotypes of each growth type can be classified into 9 types including high yield and high efficiency type, high yield and medium efficiency type, high yield and low efficiency type, medium yield and high efficiency type, medium yield and medium efficiency type, medium yield and low efficiency type, low yield and high efficiency type, low yield and medium efficiency type and low yield and low efficiency type through the statistical method of cluster analysis. Actually, several types could be absent in practice because of the quantity limit of genotypes. According to the method above, genotypes of EMMJ can be classified into 6 types, MMMJ 9 types, LMMJ 7 types, EMLJ 8 types, MMLJ 5 types. N efficient genotypes including 9 you 418, Wuyujing 3, Yangjing 9538 belonging to LMMJ and 86 you 8, Wujing 15, Siyou 422 belonging to EMLJ together with N inefficient genotypes including Nongken 57, Wunongzao, Zhendao 5 belonging to LMMJ and Zhengdao 196, Xiangjing 20-18, T1-56 belonging to EMLJ were selected for further experiment. 2.12 rice genotypes selected above were adopted as materials to investigate the differences of rice matter production and accumulation. The characteristics of rice matter production and accumulation and their correlations with N use efficiency revealed that, although there was no significant difference in number of tillers per unit ground area between two rice types with different N use efficiencies, the percentage of productive tillers of N efficient genotypes were obviously higher than those of N inefficient genotypes. Compared with N inefficient genotypes, N efficient genotypes had proper leaf area index (LAI), photosynthetic potential (PP), crop growth rate (CGR), and a superior matter accumulation before the critical stage of productive tillering, although their ratios of dry matter to the total accumulation in whole life were relatively low. During the period from the critical stage of productive tillering to heading, the unproductive tillers of N efficient genotypes were fewer than those of N inefficient genotypes. Therefore their LAI, PP, CGR and dry matter accumulation were lower than those of N inefficient genotypes. After the stage of heading, the leaf area, photosynthetic potential of N efficient genotypes increased fast and their crop growth rates accelerated resulting from better population quality. N efficient genotypes presented obvious superiority in dry matter accumulation.3. At the three growth stages including critical stage of productive tillering, heading, and maturing, the amount of N accumulation of N efficient rice genotypes was obviously higher than that of N inefficient genotypes while at the stage of elongating, there was no significant difference in N accumulation between the two rice genotypes. The amount of N accumulation of N efficient genotypes was significantly higher than that of N inefficient genotypes during all growth phases except the phase from critical stage of productive tillering to elongating, at which the amount of N accumulation of N efficient genotypes was significantly lower than that of N inefficient genotypes. The percentage in N accumulation of N efficient genotypes was higher than that of N inefficient genotypes during the growth phases from elongating to heading and from heading to maturing while it showed the reversed trend during the phases from transplanting to critical stage of productive tillering and from the critical stage of productive tillering to elongating. The amount and the efficiency of N translocation before heading were obviously higher in N efficient genotypes than those in N inefficient genotypes. On the contrary, the contribution rate of transferred N to the total N of rice grain at maturity was significantly lower in N efficient genotypes than that in N inefficient genotypes. For N efficient genotypes, the amount of N accumulation before the critical stage of productive tillering was modest. And during the phase from the critical stage of productive tillering to heading, their N accumulation of usefulness were large while the N accumulation of uselessness were few. Therefore, till the stage of rice heading, the amount of N accumulation of N efficient genotypes was obviously higher than that of N inefficient genotypes. And the amount and the efficiency of N translocation before heading of N efficient genotypes were also higher than that of N inefficient genotypes. Because of the strong ability of N accumulation of N efficient genotypes after heading, their contribution rate of transferred N to the total N of rice grain at maturity was relatively lower than that of N inefficient genotypes before heading.4. At four growth stages including the critical stage of productive tillering, elongating, heading, and maturing, the indexes of root morphology and physiology including the root dry weight, root volume, total absorbing surface area of root, active absorbing surface area of root, ratio of active absorbing surface area to total absorbing surface area of N efficient genotypes were obviously higher than those of N inefficient genotypes. At the critical stage of productive tillering and the stage of elongating, the ratios of root to shoot of N efficient genotypes were significantly higher than those of N inefficient genotypes while the trend was contrary at the stages of heading and maturing. Before the stage of maturing, the root oxidation ability ofα-NA of N efficient genotypes were superior to those of N inefficiency genotypes while at the stage of maturing the root oxidation ability ofα-NA of N efficient hybrid rice was appreciably lower than that in some N efficient genotypes. For N efficient genotypes, their root morphology is good and root activity is vigorous which ensures the efficient absorption and utilization of N all their life. Meanwhile the proper ratio of root to shoot and the harmonious growth of root and shoot can also improve the efficiency of N absorption and utilization.5. At five stages after full heading, the photosynthetic indexes of flag leaf including chlorophyll content, net photosynthetic rate, stomata conductance, intercellular carbon dioxide concentration and transpiration rate of N efficient genotypes were obviously higher than those of N inefficient rice type. During the period of grain filling, the photosynthetic function duration and chlorophyll fluorescence parameters of N efficient genotypes were superior to N in efficient genotypes. For N efficient genotypes, they had better characteristic of photosynthesis and longer photosynthetic function duration. Meanwhile steady PSⅡof N efficient genotypes were favorable for effective photochemical quantum yield and strong light protection.6. After the stage of full heading, the enzyme activities of SOD and CAT decreased with the senescence of leaf and the decrease rate of N efficient genotypes were lower than those of N inefficient genotypes. With the senescence of rice leaf, the enzyme activities of POD increased first and then decreased, while it increase appreciably again at the late period of leaf senescence. MDA content of rice leaf increased continuously after the stage of full heading. At five stages after full heading, the enzyme activities of SOD, CAT and POD of N efficient genotypes were significantly higher than those of N inefficient genotypes while the reversed trend was shown in MDA contents of rice leaf. For N efficient genotypes, the process of flag leaf senescence was slow and their leaf photosynthetic function durations were longer than those ofN inefficient genotypes.