Adaptive Responses to Early Nitrogen Deficiency in Komatsuna Plants

Management of fertilizer application is essential for maintaining adequate food production while preventing the environmental impact caused by excessive fertilizer application. Therefore, it is crucial to understand how crops respond to early nitrogen deficiency. In the present study, we analyzed the responses to early nitrogen deficiency in komatsuna (Brassica rapa var. perviridis) plants, a popular leafy green vegetable in Japan. Komatsuna plants were grown in pot cultures under seven nitrogen levels (0, 30, 60, 90, 150, 210, and 270 mg N per pot), with nitrogen as ammonium nitrate. Shoots were harvested every 5 days from 14 to 39 days after sowing to measure dry weight, total nitrogen, and nitrate concentrations. The dry weight of komatsuna shoots increased significantly even after soil inorganic nitrogen was nearly exhausted. Under the 30 mg N treatment, nitrogen accumulation at 24 days accounted for 81% of the final accumulation at 39 days, whereas dry weight accounted for only 35% of the final value. This indicates about 65% of the final dry weight accumulated after nitrogen accumulation plateaued. To elucidate the molecular mechanisms enabling growth sustained by internal nitrogen, gene expression profiles in leaves were analyzed using RNA-seq in plants grown under marginal or ample nitrogen supply. The results indicated that transcriptional regulation of purine metabolism plays an important role in adaptation to early nitrogen deficiency. These findings contribute to the optimization of nitrogen fertilization by providing a foundation for developing markers to assess the need of top-dressing and for breeding crop varieties with high nitrogen-use efficiency.