Objectives We conducted a study on the growth and nitrogen metabolism characteristics of cucumber seedlings subjected to low temperature stress and varying exogenous nitrate ( NO₃⁻-N) supply levels. Our objective was to elucidate the mechanisms cucumber employs to counteract abiotic stress and provide a theoretical foundation for the scientific management of nutrients in soilless cucumber cultivation.

Methods For our hydroponic experiment, we utilized cucumber seedlings of the ‘Xintaimici’ variety at the four-leaf-one-bud stage. Hoagland’s nutrient solution served as the base solution, and we prepared four treatment solutions with NO₃⁻-N concentrations of 0 (N0), 6 mmol/L (N6), 12 mmol/L (N12), and 24 mmol/L (N24). The cucumber seedlings were grown in these solutions and exposed to low temperature stress conditions (15°C during the day and 10°C at night) for four days. Then the seedlings were harvested for the analysis of relative chlorophyll content (SPAD), biomass, activities of nitrogen metabolism and antioxidant enzymes, and the expression of genes related to nitrate nitrogen metabolism.

Results Under low temperature, N24 treatment exhibited the highest SPAD values, soluble protein and proline content, the highest activities of nitrate reductase, glutamine synthetase, glutamate synthase and peroxidase, and the highest expressions of GS-1 and GS-2 both in roots and leaves. While N6 treatment exhibited the high dry weight and the activities of superoxide dismutase and catalase. In addition, N24 treatment was recorded the highest expression levels of nitrate nitrogen uptake and transport genes (NRT1s), nitrate reducing genes (NRs, NiR) and glutamate synthase genes (GOGATs) in root, which was consistent with the nitrate nitrogen content in root. While N6 treatment was recorded the highest expression levels of GOGAT-1-1, GOGAT-1-2, GOGAT-2-1 and GOGAT-2-2 in leaves. Under low temperature, N6, N12 and N24 treatments reduced the malondialdehyde contents in cucumber roots and leaves compared to N0. The principle component analysis results showed that N6 treatment achieved the top comprehensive scores in resisting low temperature stress.

Conclusions Under low temperature and low exogenous NO₃⁻-N supply conditions, cucumber seedlings upregulate the expression of NRT1.2, NRT1.3, NRT1.9 and NRT1.10 to enhance the transport of NO₃⁻-N from roots to leaves, increase the activities of enzymes related to nitrogen assimilation, and maintain higher dry matter accumulation, thereby enabling normal growth and adaptation to low temperatures. However, under low temperature and high NO₃⁻-N supply conditions, cucumber seedlings up regulate the expression of NRT1s (NRT1.5, NRT1.8) genes in roots and increase NO₃⁻-N uptake, but the induced high activities of nitrogen assimilating enzymes are not conducive to dry matter accumulation. Overall, lower exogenous NO₃⁻-N supply levels (6 mmol/L) are beneficial for cucumber seedlings in resisting low temperature stress.