Leaf Nitrogen Status as a Main Contributor to Yield Improvement of Soybean Cultivars

The N demand would increase with genetic improvement of soybean [Glycine max (L.) Merr] yield. Since yield improvement largely attributes to photosynthetic rate, which requires considerable N-derived enzymes to activate the photosynthetic metabolism, the variation of leaf N status over time and its association with nodulation are essential for understanding the mechanism of genetic gain of yield. In agronomic practice, this is also necessary for developing N management in combination with newly released cultivars to increase N use efficiency and yield. However, these issues have not been addressed in Northeast China, where is a major area of soybean production. A field experiment was conducted with 45 representative soybean cultivars, from maturity groups (MG) 00 to 0, released from 1950 to 2006 in Northeast China. The leaf N concentration and content at R5 had positive correlations with the year of release (P < 0.001) with 23.0 and 39.3% of rises over the period, respectively. Nitrogen content per unit area had been improved significantly (P < 0.01), resulting in a 1.08% increase per year. Both photosynthetic rate and C assimilation had significantly positive association with leaf N concentration or N content per unit area, which suggests that both indicators of leaf N status could be essential contributors to genetic yield improvement. The stable content of leaf N across different environments was enhanced over years, which could contribute to yield stability. Among soybean cultivars released across 56 yr, nodule number and fresh weight were progressively increased and a close and significant correlation between leaf N concentration and nodule fresh weight was observed. This indicates that modern cultivars have a better leaf status that poses superior nodulation ability than their predecessors.