| Rice(Oryza sativa L.)is one of the important food crops in China,which plays a very important role in China's grain production for its high and stability yield.The improvement of crop yield is inseparable from the input of chemical fertilizer,rice production is no exception.Long-term and massive application of chemical fertilizer,especially nitrogen(N)fertilizer,not only lead to the waste of resources but also brings serious negative impacts to human living environment.Therefore,improving the use efficiency of N fertilizer not only concerns the immediate interests of farmers,but also concerns environmental quality.It is an important way to improve the N use efficiency to cultivate high N-efficient varieties,measure soil formula and apply fertilizer in combination with organic fertilizer,rational rotation,deep fertilizer application and combined use of N with other fertilizer.Nitrogen and potassium(K)are essential nutrients for the growth and development of rice,which also are the maximum absorption nutrients by root.The main rice growing areas in China are characterized with high temperature and rainfall,and the soil weathering degree is higher,which lead to the serious losses of soil N and K.On the other hand,the phenomenon of heavy application of N with light application of K is common in rice production for the relative lower price of N fertilizer.It is of great significance to explore the interaction effects of eN and K fertilizer on rice yield and N use efficiency.In this study,the effects of N-K combined application on rice yield and N use efficiency were studied by combining a three years located field experiment and a hydroponic culture experiment,and the growth and development mechanism,photosynthesis mechanism,and carbon and nitrogen metabolism mechanism of were elucidated.The main research results are as follows:(1)The application of N and K significantly improved the grain yield and N use efficiency of rice.Compared with no-N treatments,the grain yield was increased by 18.2%-21.6% after N application.The grain yield was increased by 9.7%-11.2% after K application compared with the treatments without K.The application of K fertilizer significantly promoted the absorption and utilization of N fertilizer.Compared with the non-K treatment,the N uptake,N agronomic use effieiency and N contribution rate was increased by 15.3%-27.1%,26.8%-45.2% and 16.2%-26.9%,respectively.The application of N fertilizer also promoted the absorption and utilization of K fertilizer.Compared with the treatment without N,the uptake of K,agronomic use efficiency of K and contribution rate of K were increased by 32.4% to 42.2%,52.3% to 110.1% and 20.9% to 75.8%,respectively.(2)Compared with the no-N treatment,the chlorophyll content,N content and magnesium(Mg)content in rice leaves of jointing stage were increased by 12.8%-13.1%,10.6%-11.1%,5.6%-14.3% and 7.5%-10.1% respectively after the supply of N,while the K content was decreased by 14.1%-24.8%.Compared with the no-K treatment,the chlorophyll content,N content and Mg content in leaves were reduced by 5.7%-6.5%,9.9%-10.9%,6.8%-7.2% and 6.8%-7.2% respectively after the supply of K,and the K content was increased by 11.9%-22.6%.The leaf area index,leaf dry mass after and the accumulation of total chlorophyll was increased by 60.5%-71.5%,32.8%-72.5% and 71.5%-79.9% respectively for the supply of N,while that was increased by 25.7%-28.2%,15.4%-40.9% and 13.7%-25.8% respectively for the supply of K.(3)Interaction of N and K significantly reduced leaf photosynthetic limitation,improved leaf photosynthesis rate and dry mass accumulation rate of the jointing stage.Compared with the treatment without N and K,the limitation of stomatal conductance,mesophyll conductance and physiological and biochemical conductance of leaves was decreased by 26.6%-79.9%,24.4%-54.1% and 44.1%-75.2% respectively after the combined use of N and K.The net photosynthetic rate,stomatal conductance,mesophyll conductance,the maximum electron transfer rate and the maximum carboxylation rate were increased by 32.2%,57.0%,25.1%,14.0% and 14.9% respectively.The total dry mass per plant,leaf dry mass and leaf area was increased by 120.5%,178.5% and 186.9%,respectively.Appropriate N/K ratio is an important prerequisite to ensure the higher photosyntheticcapacity of leaves.Under the conditions of this experiment,the N/K ratio of leaves suitable for leaf photosynthesis was 1.42-1.51,the N content of leaves was 2.93-3.23%,and the K content was 2.06%-2.14%.(4)The supply of N increased the total N content and photosynthetic N content in leaves of jointing stage,at the same time,the content of photosynthetic N was reduced and more N was distributed to stored N,which resulted in reducing utilization efficiency of photosynthetic N.Although K fertilizer reduced the total N content in leaves,it promoted the conversion of stored N to photosynthetic N,increased the content and relative content of photosynthetic N,and then improved the utilization efficiency of photosynthetic N in leaves.Compared with the treatments without N,the relative content of photosynthetic N in leaves after N application was decreased by 5.6% to 14.5%,while the relative content of stored N was increased by 8.7% to 33.8%.Compared with the no-K treatments,the relative content of photosynthetic N in the leaves was increased by 7.3%-17.7%,while the relative content of stored N was decreased by 8.7%-24.2%.Further analysis showed that the non-protein N of stored N were the main N reservoirs for the conversion of stored N to photosynthetic N.(5)Interaction of N-K significantly affected the carbon and N metabolism of rice.The application of N and K improved the photosynthetic capacity of leaves and promoted the production of carbohydrates.At the same time,the application of K significantly reduced the soluble sugar content in plants and promoted the transport of carbohydrates into the source organs.The application of N and K reduced the soluble sugar content in leaves by 8.0% and 14.7% respectively at the booting stage,and increased the soluble sugar content in seeds by 51.8% and 45.1% respectively at the harvest stage.The activities of glutamine synthase,glutamate synthase,glutamate dehydrogenase and nitrate reductase in the leaves and roots of rice were significantly increased after the supply of N and K,which promoted the synthesis of amino acids.The application of N also increased the total content of free amino acids in leaves,while the application of K promoted the metabolism of amino acids and significantly reduced the free amino acids content in leaves. |
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