Nitrate Uptake, Efflux, Reduction And Transformation, And Their Relationships To Petiole Nitrate Accumulation Over Leafy Vegetable Cultivars

Nitrate, a major nitrogen source for many plants, can be accumulated in large quantities in plants, particularly in leafy vegetables. Vegetables are the major source of human dietary intake of nitrate, accounting for more than 80% of the total nitrate-N intake for humans. High level nitrate accumulation is not only harmful to human health, but also unfavorable to plant nitrogen use efficiency. Nitrate accumulation in plant is affected by many internal and external factors, mainly including water, light, temperature, nitrogen supply, cultivars, nitrate reduction ability, and root uptake ability. Since the internal factors play the predominant role in affecting the nitrate accumulation, it is important to study the reasons of nitrate accumulation from the plant itself.Large variations in nitrate accumulation have been found among vegetable cultivars. These differences indicate the potential to select or breed for reduced nitrate accumulation in leafy vegetables. Therefore, to study the physiological characteristics of nitrate accumulation in vegetables can provide theoretical foundation for selecting vegetable cultivars with low nitrate accumulation. Generally, nitrate accumulation in plants is due to the imbalance in the capacities of nitrate uptake, reduction and assimilation. There are four fates of the nitrate in plants: directly reduction to nitrite in root cytoplasm; efflux back across the plasma membrane to the apoplasm; storage in vacuoles; transport from the xylem to leaves or stem for reduction and assimilation. Many studies have been mainly focused on one or two above processes, being prone to neglect other predominant regulation process. As for the study of nitrate uptake, the nitrate efflux process was often neglected.The present study by hydroponics and pot experiment was to investigate the process of nitrate uptake, efflux, reduction and transformation, and their relationships to petiole nitrate accumulation difference between two oilseed rape and spinach cultivars. The main conclusions are as follows:1. For two vegetable cultivars, petiole was their main organ for the shoot biomass, respectively, and the lateral roots were the main organ for the whole root biomass. However, petiole played the most important role in nitrate accumulation, and the next was main root. Moreover, the difference in petiole nitrate-N concentration could be used as an indicator for variation in shoot or whole plant nitrate-N concentrations among cultivars.2. The difference in petiole nitrate-N concentrations between vegetable cultivars increased with plant growth, and declined with the petiole nitrate-N concentration decrease. Biomass of the cultivar with greater nitrate-N concentration was more sensitive to the changes of petiole nitrate-N concentration. During the different growth stages, no significant differences existed in fresh weights of petiole, leaf blade, main root, shoot, and whole plant of the two oilseed rape cultivars except that in lateral roots. The cultivar with greater nitrate-N concentration had larger lateral root system, and it also had significantly greater lateral root to whole plant dry weight ratio and the root-to-shoot dry weight ratio than the cultivar with lower nitrate-N concentration. Larger lateral root system may be the basic resean for the cultivar to accumulate more nitrate-N concentration.3. Investigations on differences in the nitrate uptake capacity of the two oilseed rape cultivars by nitrogen depletion method, showed that the cultivar with greater petiole nitrate-N concentration had greater cumulative nitrate uptake and uptake rates compared to the cultivar with lower petiole nitrate-N concentration. This may be attributed to the cultivar with greater petiole nitrate-N concentration had higher maximum nitrate uptake rate (Vmax) and nitrate uptake affinity.4. Nitrate efflux varied diurnally, and the cumulative nitrate efflux and the differences between cultivars were greater in the morning than at other time. Thus, it was better to measure the nitrate efflux in the morning after being lightened for 1.5-2.0 h. The cultivar with greater petiole nitrate-N concentration had a lower cumulative nitrate efflux and efflux rate. However, the difference in nitrate efflux between cultivars was influenced by both the external and internal nitrate-N concentrations. When both the external and internal nitrate-N concentrations decreased rapidly, the cumulative nitrate efflux and nitrate efflux rate were even greater for the cultivar with greater petiole nitrate-N concentration, and so did for the decrease in petiole nitrate-N concentration.5. Further analysis showed that the cultivar with greater petiole nitrate-N concentration also had higher in vivo, in vitro and the ratio of in vivo / in vitro nitrate reductase activity (NRA), but their differences between cultivars were not significant. The averaged expressing degrees of the in vitro NRA were less than 30% for both cultivars. This indicated that the nitrate reductase activity could not completely explain the differences in nitrate accumulation among cultivars.6. When the external nitrogen was deprived, and the plant growth had not showed any symption of inhibition, the decrease of petiole nitrate-N concentration were greater for the cultivar with greater petiole nitrate-N concentration, and the accumulated nitrate use efficiency was also higher compared to the cultivar with lower petiole nitrate-N concentration. This suggests that the cultivar with greater petiole nitrate-N concentration could remobilize the accumulated nitrate to a higher level when the nitrogen supply was deficient.