【作者】 黄彩变；

【导师】 王朝辉；

【作者基本信息】 西北农林科技大学 ， 植物营养学， 2009， 博士

【摘要】 硝态氮是植物的主要氮源,可在体内大量累积,以叶类蔬菜更为严重。人体摄入的硝态氮约有80%以上来源于蔬菜,因此,蔬菜累积过多的硝态氮既影响植物氮素有效利用,也不利于人体健康。植物体内硝态氮累积是多种内源和外源因子共同作用的结果,主要包括水分、光照、温度、氮素供应、基因型、硝态氮的还原及吸收等。由于外源因子终究要通过内源因子起作用,因此,从植物本身出发来研究硝态氮累积才是解决问题的关键。不同蔬菜品种间的硝态氮含量存在显著差异,这表明选育出低硝态氮累积品种是完全有可能的。因此,查明品种间硝态氮累积差异原因,可为低硝态氮累积品种的选育提供理论依据。植物对硝态氮的吸收和还原同化不平衡是造成其在体内累积的主要原因。硝态氮进入植物根系后,主要有4种去向:在根系细胞质直接被还原;细胞质中的硝态氮通过质膜又流入质外体,即根系溢泌;进入液泡被贮存起来;转运至茎、叶等器官,被还原同化。目前,大量研究集中于上述一个或两个过程和硝态氮累积的关系。特别是在研究硝态氮吸收时,往往忽略根系硝态氮溢泌。本研究在以往研究工作的基础上,选出两个硝态氮累积差异较大的油菜和菠菜品种,采用土培和水培相结合的方法,从根系硝态氮吸收和溢泌,及其在植物体内的转移、还原、累积等过程进一步揭示叶类蔬菜品种间硝态氮累积差异的原因。取得的主要结论有:1.对两个蔬菜品种苗期的研究发现,叶片是其地上部分生物量的决定器官,侧根是整个根系的主要构成部分。硝态氮累积的情况却不同,叶柄是主要累积器官,其次是主根。品种间叶柄硝态氮含量和地上部分及整株的含量相关性更好。2.蔬菜品种间叶柄硝态氮含量差异随植株生长呈先增加而后减小的趋势,和地上部分干重、鲜重及水分含量的关系也因生长时期而异。比较两个油菜品种不同时期的生物量发现,品种间叶柄、叶片、主根、地上部分及整株的鲜重差异均不显著,但高硝态氮累积品种的侧根鲜重却显著高于低硝态氮累积品种,侧根所占整株干重的比例及根冠比均显著高于低累积品种。侧根发达可能是品种硝态氮累积量高的一个基本原因。3.采用氮素耗竭法,比较了两个油菜品种硝态氮吸收能力差异,结果表明叶柄硝态氮含量高的品种对硝态氮的累积吸收量和吸收速率显著高于低硝态氮累积品种。这可能与高累积品种的最大吸收速率(Vmax)显著高于低累积品种有关,高累积品种对硝态氮的亲和力也较高。4.根系硝态氮溢泌日变化较大,早上累积溢泌量最高,且品种间差异最大。要比较品种间硝态氮溢泌差异,测定应在早上光照1-2 h后进行。测定两个油菜品种的根系溢泌发现,叶柄硝态氮含量高的品种其根系硝态氮溢泌量和溢泌速率较低。品种间硝态氮溢泌差异却受外界氮素供应及植株体内硝态氮含量影响。当营养液硝态氮浓度迅速下降时,叶柄硝态氮含量高的品种其叶柄的硝态氮含量下降更多,根系硝态氮溢泌量也较高。5.进一步比较叶片的硝酸还原酶活性发现,叶柄硝态氮含量高的品种其叶片的内源、外源及内源/外源硝酸还原酶活性也较高,但品种间的差异却不显著。酶活性发挥程度都较低,平均不足30%。硝酸还原酶活性差异难以解释品种间硝态氮累积差异。6.当外界氮素缺乏,两个油菜品种的生长尚未受到明显抑制,高硝态氮含量品种叶柄的硝态氮下降幅度最大,其累积硝态氮的再利用效率也较高。说明硝态氮累积量高的品种在外界供氮不足时,能较大程度地再利用累积的硝态氮。更多还原

【Abstract】 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.更多还原