Low R/FR Ratio Affects Pakchoi’s Growth and Nitrate Content under Excess Nitrate Stress

Abstract

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.