Characterization of nitrate assimilation in Lactuca sativa L. under different nitrogen sources

Optimal fertilization management is critical for romaine lettuce ( Lactuca dolichophylla K.) cultivation, a major economical vegetable in China. Herein, we chose a romaine lettuce genotype ( Lactuca sativa L.) known for “year-round production” and as “high-yielding,” to examine the effect of varied nitrogen (N) fertilizer forms on nitrate (NO 3 − ) accumulation. We cloned NO 3 − reductase (NR) and nitrite (NO 2 − ) reductase (NiR) of romaine lettuce and analyzed their expressions at different growth stages in response to different N forms. Five N treatments with varying NO 3 − -N:NH 4 + -N ratios were examined. Our results showed that lettuce leaf NO 3 − content decreased significantly with the increase of NH 4 + -N in fertilizer. The lowest NO 3 − content was observed when NO 3 − -N:NH 4 + -N ratio was 0:100. NR and NiR activity and expression initially were upregulated by the increase in NO 3 − content, with the highest level at 70:30 NO 3 − -N:NH 4 + -N ratio, but quickly declined thereafter. We also found the temporal activation pattern of N transporters was different from that of NO 3 − reductase and NO 2 − reductase, suggesting there is a coordination between N translocation and N reduction in lettuce leaves to maintain proper N homeostasis. These findings provide insights into the role of NO 3 − -N:NH 4 + -N ratio in regulating NO 3 − assimilation, balancing N metabolism, and reducing NO 2 − toxicity in vegetable production.