Nitrate accumulation is one of the main factors of secondary soil salinization in protected horticulture in China. Previous studies have shown that a low red(R)/far-red (FR) ratio can improve the salt tolerance of plants under NaCl stress. However, the effect of a low R/FR ratio on plant growth under nitrate stress is not clear. In order to explore the effect of a low R/FR ratio on the adaptability of pakchoi under high nitrogen stress, the growth index, soluble protein content, soluble sugar content, nitrate content, nitrate reductase activity and Nia2 gene expression of pakchoi’s leaves were measured. The results showed that a high level of nitrogen (the addition of 80 mmol · L−1 NO3− (N80) and 160 mmol · L−1 NO3− (N160)) inhibited the growth of pakchoi and promoted the accumulation of osmoregulation substances and nitrate content, respectively. The reduction of the R/FR ratio under high nitrogen stress (L80) increased the fresh weight of the plants under it by 19.0%, reduced the nitrate content in the leaves by 22.7%, increased the NR activity by 29.9%, and made the Nia2 gene expression more significant, compared with N80. There was a similar mitigation effect of a low R/FR ratio under 160 mmol · L−1 excessive NO3− stress. Therefore, the reduction of the R/FR ratio can effectively control the nitrate content and improve the adaptability of pakchoi under high nitrogen stress. Thus, there is a practical application prospect for a low R/FR ratio for the production of pakchoi under a high level of nitrogen.
Nitrate accumulation is a major factor for the secondary salinization of greenhouse soil in China. Our previous study pointed out that a low ratio of red:far-red light (R:FR) can improve salt tolerance in pakchoi under excessive nitrate stress. However, the nitrogen metabolism mechanism is still unclear. To detect the effect of a low R:FR ratio on nitrogen metabolism of pakchoi under excessive nitrate stress, two extra additions of nitrogen of 80 mmol·L−1 NO3− (H80) and 160 mmol·L−1 NO3− (H160) with/without a low R:FR ratio (R:FR = 0.7) were set, and the growth index, chlorophyll content, key enzymes in nitrogen metabolism, nitrate and glutamic acid content and NRT gene expression level of pakchoi leaves were examined. The results indicated that a low ratio of R:FR could alleviate the reduction in growth and chlorophyll content in pakchoi under high-level nitrogen stress (H80 and H160). The activity of nitrate reductase (NR), glutamine synthetase (GS) and glutamine synthetase (GOGAT) decreased under H80 and H160 conditions, except of NR with the H80 treatment. The activity of glutamate dehydrogenase (GDH) increased under H80 treatment, but decreased under H160 treatment. However, the activity of GDH decreased further by reducing the ratio of R:FR. Excessive nitrate stress increased the nitrate content, and a low R:FR ratio could inhibit nitrate accumulation. However, the change in glutamic acid content was significantly increased under a low R:FR ratio without stress. Under the high-nitrogen level treatment (H160), the use of a low ratio of R:FR increased NRT gene expression. Therefore, a low R:FR ratio (R:FR = 0.7) could effectively promote the growth of pakchoi and improve its nitrogen metabolism, thus alleviating the stress effect of a high level of nitrogen in pakchoi.
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