Effects of low nitrogen stress on foxtail millet seedling characteristics and screening of low nitrogen tolerant varieties

Screening and cultivating low-nitrogen-tolerant crop varieties can be used as an effective measure for increasing the nitrogen-use efficiency of plants, reducing nitrogen fertilizer use, as well as limiting environmental pollution during cultivation. In the present study, 45 foxtail millet varieties were grown in different nitrogen conditions, low nitrogen (0.1 mmol·L^-1) and normal nitrogen (5 mmol·L^-1), using hydroponic methods. Nitrogen efficiency of plants was subsequently measured based on 22 indicators about growth, biomass, nitrogen content, nitrogen accumulation and physiology efficiency. The low nitrogen tolerance of the tested foxtail millet varieties was evaluated by using the comprehensive low nitrogen tolerance coefficient method and the membership function method based on principal component analysis. The results showed that foxtail millet seedlings exposed to low nitrogen stress demonstrated varying degrees of improvement compared to seedlings grown in normal nitrogen condition for root length, root-shoot ratio, number of root; and root, shoot, and plant nitrogen physiology efficiency. The remaining 17 indicators demonstrated different levels of reduction under low nitrogen stress. Forty-five foxtail millet varieties were subsequently divided into five categories (strong resistant, resistant, medium resistant, sensitive, and highly sensitive) for screening; five varieties with relatively strong low nitrogen tolerance were screened (varieties 11, 14, 17, 35, and 39). A GGE double plot was used to analyze the variety-low nitrogen tolerance related indexes. The predominant low nitrogen tolerance traits of low nitrogen tolerant varieties 39 and 14 were dry weight of root, fresh weight of root, and root length; while varieties 11, 35, and 17 were identified as having traits fresh weight of shoot, leaf number, leaf width, leaf length, nitrogen accumulation of plant, nitrogen accumulation of shoot, dry weight of plant, dry weight of shoot, nitrogen accumulation of root, root number, seedling height, and SPAD. Our results suggest that there are some differences in the low nitrogen tolerance mechanisms of different millet varieties. These findings provide a material basis for the breeding of foxtail millet varieties with low nitrogen resistance.