Abstract: 【Objective】The aim of the study is to determine the optimal amount of nitrogen application and to investigate the correlation between photosynthetic characteristics and yield of spring foxtail millet varieties through analyzing the performance of agronomic traits, photosynthetic characteristics and yield under different nitrogen levels.【Method】The experiment was carried out in a field-split plot design with three replicates with varieties as main plot and nitrogen fertilizer as secondary plot. Cultivars Changnong 35 and Jingu 21 were selected as the experimental materials, and the area of each plot was 15.0 m²(5 m×3 m) and the seedling density was 3×10⁵ plants/hm². The designed 6 levels of nitrogen were 0 kg·hm^-2 (N1), 45 kg·hm^-2 (N2), 90 kg·hm^-2 (N3), 135 kg·hm^-2 (N4), 180 kg·hm^-2 (N5) and 225 kg·hm^-2 (N6), respectively. Forty percent of the nitrogen was applied as base fertilizer, and 60% applied at jointing stage to booting stage. After plants heading, the SPAD (Soil and Plant Analyzer Development, SPAD) value of the top-three leaves was measured through SPAD-502 (Konica Minolta), and the Ci (intercellular CO₂ concentration, Ci), Pn (net photosynthesis rate, Pn) and E (transpiration rates, E) of the top-three leaves were measured using CIRAS-2 (PPSYSTEMS). 【Result】As the nitrogen levels increased, plant height, stem diameter and panicle length of spring foxtail millet exhibited a rising trend, and weight per panicle, grains weight per panicle and SPAD, E and Pn values of the flag leaf and the top-three leaves increased first and then decreased, however, 1000-grain weight had no significant difference in various nitrogen treatments. All of the above traits excepting 1000-grain weight reached to or tended to the highest value, meanwhile the yield tended to be stable at N level of 90 kg·hm^-2 for spring foxtail millet, which suggested that the N level of 90 kg·hm^-2 may be as the optimal amount of nitrogen application for spring foxtail millet. Furthermore, a regression equation between yield and various N levels was established and the maximum theoretical yield of spring millet was calculated. The results showed that there was no significant difference between the maximum theoretical yield and the observed one at N level of 90 kg·hm^-2 (P_{Changnong 35} = 0.5571, P_{Jingu 21}= 0.6632). Therefore, the N level of 90 kg·hm^-2 was confirmed as the optimal amount of nitrogen application for spring foxtail millet. The correlation analysis showed that the correlation coefficients between E, Pn of the flag leaf and yield and E, Pn of the top-three leaves and yield were 0.87, 0.86, 0.82, 0.83, respectively, and all reached a significant level.【Conclusion】 The optimal amount of nitrogen application was 90 kg·hm^-2 in South Central Shanxi ecological region and soil conditions, and E and Pn of the flag leaf and the top-three leaves were significantly positively correlated with the yield of spring foxtail millet.

Key words: spring foxtail millet, nitrogen application rate, photosynthetic characteristics, yield, correlation analysis