Changes of nitrogen content in leaf and its correlations with net photosynthetic rate of soybean cultivars released in different years

In order to understand the changes of leaf nitrogen content and its relationship with the net photosynthetic rate(Pn) during genetic improvement of soybean cultivars,leaf nitrogen content and photosynthetic characteristics of 21 soybean cultivars released from 1923 to 2005 in Jilin Province were studied.The results showed that the leaf nitrogen content had highly significant or significant positive correlation with released years in R2 and R6 stages,but it had no significant negative correlation in R4 stage.The growth rate of nitrogen content per leaf area was significantly higher than the net photosynthetic rate in R2 and R6 stages,resulting in highly significant or significant negative correlation between photosynthetic nitrogen efficiency and released years.But the nitrogen content per leaf area of modern cultivars was lower than the old cultivars in R4 stage,the net photosynthetic rate was still higher than the old cultivars,resulting in a significant positive correlation between photosynthetic nitrogen efficiency and released years.The chlorophyll content and the net photosynthetic rate would be high,if the nitrogen content in leaf was high,but the photosynthetic nitrogen efficiency would be low.In R4 stage,the photosynthetic nitrogen efficiency could be as a high-yielding cultivars bred indicator.