The impact of short-term nitrogen starvation and replenishment on the nitrate metabolism of hydroponically grown spinach

Abstract

Nitrate is used as a source of supplemental nitrogen for plants, but it can also contribute to nitrate accumulation in green vegetables, which is unhealthy for humans. This study involves short nitrogen starvation and nitrogen replenishment treatments to minimize the nitrate content in spinach (Spinacea oleracea L.) in order to maximize nitrogen consumption and reduce the nitrate content. For this purpose, five different nitrogen application levels (N0 = 0 mmol·L−1, N4 = 4 mmol·L−1, N8 = 8 mmol·L−1, N12 = 12 mmol·L−1, N16 = 16 mmol·L−1) were used in hydroponic culture, in order to screen out the appropriate nitrogen application level of spinach and carry out short-term nitrogen starvation and replenishment treatment, the nitrate and nitrite contents, nitrate reductase and nitrite reductase activities, nitrate reductase gene and nitrite reductase gene were measured every 5 days, and plant growth was monitored during the experiment together with morphological adaptations at the root level. Our results showed that N8 level of the five nitrogen levels was the most effective for the growth of hydroponic spinach. In contrast to the N8 level, spinach roots and leaves thrive healthier after subsequent short-term nitrogen starvation and replenishment (N8*), optimum nitrate metabolism and key enzymes, and decreased expression levels of NR and NiR genes after nitrogen deprivation, however, their expression increased after nitrogen replenishment. It indicated that NR and NiR genes can be used as a reference gene to reduce the nitrate content in vegetables. In conclusion, our results provided a practical approach for optimum nitrate fertilization that will not only improve spinach growth but also reduce nitrate content harmful for human intake.