| Nitrogen element is a limiting factor for rice (Oryza sativa L.) growth, and nitrogen application is an important way to increase rice yield per unit area. Excessive nitrogen application to rice crop causes environmental pollution, increases the cost of rice farming, reduces grain yield of rice as well. Because of that, a lot of researchers both at home and abroad have devoted themselves to the study of improving the nitrogen utilization efficiency. Real-time Nutrient Management (RTNM) and Site-specific Nutrient Management (SSNM) are two major paddy field management techniques that can guarantee grain yield and improve the nitrogen utilization efficiency of rice meantime. Leaf nitrogen status diagnosed with a chlorophyll meter (SPAD) is an important means of RTNM and SSNM for rice production.In this research, a paddy field experiment was carried out at six nitrogen application levels and three different fertilization programs in order to study the characteristics of SPAD value of different position leaves under different nitrogen application levels and its application in nitrogen nutrition diagnosis in rice. The main conclusions emerging from the study are as follows:Through the study of the dynamic characteristics of SPAD reading during growth stage of rice and the spatial distribution of SPAD value of four leaves from the top to fourth leaf position on main stem of rice, a SPAD value was measured by using the chlorophyll meter at tillering stage, booting stage and filling stage of rice. The results showed that the greenness of rice leaf was regularly changed as "black-yellow" alternation patterns through the whole growth period of rice, and the sensitivity of the fourth leaf from the top to N was highest, giving it as the most ideal indicator of rice leaf position when using SPAD value to diagnose N status. Furthermore, the difference of SPAD values between the adjacent leaf and nitrogen application rates showed that value of SPADl4 - SPADL3 (SPADL4-L3) was highly related to the nitrogen content of leaf and plant. With the increasing nitrogen content, the difference between SPAD value of the fourth leaf and the third leaf decreased, regardless of fertilizer application condition and development stage. Thus, SPADL4-L3 value could be used as a suitable indicator for rice plant nutrition status at whole stage.The relationship between chlorophyll meter readings (SPAD values) and weight-based (Nw) and area-based (Na) leaf N concentration were also analyzed in this research. The results showed that there were significant (P< 0.05) or highly significant (P< 0.01) linear relationships between SPAD values and leaf N concentration both weight-based (Nw) and area-based (Na) at each growth stage. The relationship between nitrogen concentration and SPAD was stronger when nitrogen was expressed on a leaf weight rather than on a leaf area based N.Furthermore, the study of the relationship between SPAD values and Nw of different position leaves showed that the third leaf from the top (L3) was the suitable leaf when using SPAD value to diagnose N status at tillering stage of rice, because of its SPAD values correlated with Nw better than other leaves. Both at booting stage and heading stage, the fourth leaf from the top (L4) was the most ideal indicator when using SPAD value to diagnose N status. The reason was L4 exhibited active changes with variation of plant nitrogen status, and the relationship between SPAD values and Nw of L4 was the most relevant.According to the above research results, the range of criteria of SPAD values at each stage was determined primarily, and the relationship between SPAD values and the amount of top dressing was established meantime, which could be used as a important reference for precision fertilization of rice production. |
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