Effects of nitrate supply on plant growth, nitrate accumulation, metabolic nitrate concentration and nitrate reductase activity in three leafy vegetables
Introduction
More and more nitrogen fertilizers are applied in fields, since N fertilizer plays a significant role both in crop yield and quality [1], [2], [3]. Nitrate is often the major source of N available to higher plants [4], especially to vegetables. Nitrate uptake and distribution in crops is of major importance with respect to both environmental concerns and the quality of crop products. Nitrate, not taken up by a crop, may potentially contribute to ground and surface water pollution through nitrate leaching and soil erosion [2], [3]. On the other hand, nitrate taken up by plants causes high nitrate accumulation in plants, especially in most vegetables. Because edible parts contain very high concentrations of nitrate that has been implicated in the occurrence of methaemoglobinemia and possibly in gastric cancer as well as other diseases [5], [6], [7], nitrate accumulation in plants is a major concern, and is known to be a problem in most crops [8], [9], [10].
Although most higher plants are capable of reducing NO3− in both roots and shoots [4], nitrate is reduced more efficiently in leaves than in roots because of the readily available reductants, energy and carbon skeletons produced by photosynthesis, which is dependent on plant species [11], [12]. This is also true for most leafy vegetables. Nitrate taken up by a plant is either reduced or stored in the vacuoles or transported in the xylem transpiration stream to the leaf for reduction, and most is stored in the vacuole until released for reduction in the cytosol [10]. In addition, NR exists in the cytosol, therefore, nitrate in cytosol is called the nitrate metabolic pool (MP), and nitrate in the vacuole is called the nitrate storage pool (SP) [13], [14], [15]. Since NR is assumed to be the rate-limiting step for nitrate assimilation [16], [17], and the NR is an inducible enzyme, there is a close relationship between NR activity (NRA) and nitrate concentration in plants [18]. Furthermore, nitrate induces the expression of both the uptake and reduction systems [19]. Thus it can be seen that nitrate accumulation might be regulated by many factors, such as plant growth, endogenous nitrate, and nitrate uptake and reduction.
Most of the above studies were carried out either under two to three nitrate levels, or with seedlings, or in hydroponics, or with short-term nitrate induction, while the present work was conducted at a gradient of five nitrate supply rates (from nitrate deficiency to surplus), with flourishing plants at harvest time (vegetative stage), and with soil culture, such conditions being closer to natural environment and agricultural production. Three leafy vegetables, rape (Brassica campestris L.), Chinese cabbage (Brassica chinensis var. Oleifera Makino et Nenoto) and spinach (Spinacia oleracea L.), were planted in plastic pots with tilth layer soil. The purpose was to investigate plant growth, nitrate accumulation, the allocation of nitrate throughout the plant, the compartmental distribution in leaves, and the relationship between leaf NRA and nitrate accumulation at different nitrate supply rates under natural environment. We also analyzed the relationship between NRA and nitrate distribution in MP and SP in leaf blades at the five different nitrate supply rates for the three leafy vegetables. All the analyses were performed at the level of tissue and whole plants.
Section snippets
Plant material and growth conditions
The experiment was conducted in No. 1 Crop Experimental Base of Northwestern Science-Technology University of Agriculture and Forestry (NWSUAF), Yangling, Shaanxi Province, China. Three leafy vegetables—rape (Brassica campestris L.), Chinese cabbage (Brassica chinensis var. Oleifera Makino et Nenoto) and spinach (Spinacia oleracea L.)—were sown in 25 cm × 30 cm (diameter × depth) plastic pots filled with 5 kg field soil. Soils were taken from a fallow field of the college of agronomy of NWSUAF,
Effect of nitrate supply on plant growth
Nitrate supply exerted significant effects on plant growth (Table 1). Within lower supply (N1–N3), plant biomass increased with the increase of nitrate supply, while plant biomass decreased above N3, and a large reduction occurred at N5. The fresh weight of the three leafy vegetables showed a similar trend. In general, the maximum fresh yield occurred at N3, the minimum occurred at N1. However, there was a discrepancy of biomass at different N treatments among the three vegetables. The concrete
Discussion
In this study, we investigated the effect of nitrate supply on plant growth and nitrate distribution and NRA in the three leafy vegetables. To make explicit the significant differences of available nitrate content in soil among the five nitrate supply rates, the nitrate concentrations in soil were determined twice, 4 weeks after sowing and before sampling. The results showed that there were significant differences of nitrate content in soil among the five nitrate supplies, and the higher
Acknowledgements
We thank Qiong Zhao (an English teacher in Lanzhou University) for help with the English. Comments from Professor J.H. Weil and the anonymous reviewers have helped to improve the manuscript. The study was supported by the National Natural Science Foundation of China (30370843, 90102015, 39970429 and 30170161) and the National Key Basic Research Special Funds (G1999011707).
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