| Hand-held chlorophyll meter can be used to convenient, fast, non-destructive, timely determination of the relative chlorophyll content of potato plants, while its reading (SPAD value) also proved to be associated with the nitrogen content of plants, thus it is considered a potential tool for potato nitrogen diagnosis. However, the SPAD value of leaf varies by potato cultivars and the original SPAD value is difficult to assess the nitrogen status of different potato cultivars directly. Exploring a nitrogen diagnostic parameter suitable for different potato cultivars based on SPAD meter is necessary. Two-year field experiments were carried out to investigate the difiference among potato cultivars in terms of SPAD values of the third(L3), fourth(L4), fifth(L5), and eighth leaf(L8) from the apical apex of potato main stem at different growing stages. Based on the original SPAD values12kinds of sub-parameters were calculated to analyze whether they were affected by cultivars. The results showed as follows:1. The SPAD values of L3,L4,L5,L8decrease with the plant development and the sequence of different leaf position exhibits as L3> L4> L5> L8for all potato cultivars.2. Different from original SPAD value the sub-parameter SPADL4-L8/SPADL8is not affected by potato cultivars.3. The value of SPADL4-L8/SPADL8is sensitive to nitrogen fertilizer amount, with the increase of nitrogen amount the value of SPADL4-L8/SPADL8reduces gradually. Thus the value of SPADL4-L8/SPADL8can be used as universal parameter for potato nitrogen diagnosis.4. The model, Sc=-2×10-5D2+0.0033D-0.099, is built to calculate the threshold value corresponding SPADL4-L8/SPADL8from25to55days after potato emergence. Sc stands for the threshold value of SPADL4-L8/SPADL8; D stands for any day between25and55after emergence.(If the measured value of SPADL4-L8/SPADL8, denoted as Si, is higher than Sc at a specific day it needs top dressing of nitrogen fertilizer, and vice versa). |
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