Elsevier

Plant Science

Volume 167, Issue 3, September 2004, Pages 635-643
Plant Science

Effects of nitrate supply on plant growth, nitrate accumulation, metabolic nitrate concentration and nitrate reductase activity in three leafy vegetables

https://doi.org/10.1016/j.plantsci.2004.05.015Get rights and content

Abstract

Three leafy vegetables, rape (Brassica campestris L.), Chinese cabbage (Brassica chinensis var. Oleifera Makino et Nenoto) and spinach (Spinacia oleracea L.), were grown in plastic pots with 5 kg soil per pot at five nitrate supply rates, 0.00 (N1), 0.15 (N2), 0.30 (N3), 0.45 (N4), and 0.60 (N5) g N kg−1 soil to investigate the effects of nitrate supply on plant growth, nitrate accumulation and nitrate reductase activity (NRA) 9 weeks after sowing. The optimum yield appeared at N3, while above N4, a strong decrease in plant growth occurred. The nitrate concentration increased with nitrate supply in the whole plant and the different organs except in roots where nitrate concentration at N5 decreased compared with N4. The nitrate concentration in both the metabolic pool (MP) and the storage pool (SP) of the leaf blades increased with nitrate supply. From N1 to N2, NRA increased most rapidly. The highest NRA occurred at N4. However, nitrate reductase (NR) activities were not significantly different between N3, N4 and N5, which imply that there is a threshold of nitrate concentration in MP (NMP) to induce NRA. The parameters of NR for nitrate were measured by the in vivo method. The Km values we obtained were similar to the reported values by the in vitro method, which confirms the feasibility of the anaerobic method for determining NRA and NMP. Finally, the effects of the posttranslational regulation of NR were discussed.

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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