Shangguan et al. — Nitrogen on root and gas exchange

Bot. Bull. Acad. Sin. (2004) 45: 49-54

Effect of nitrogen on root and shoot relations and gas exchange in winter wheat

Z.P. Shangguan^1,2,*, M.A. Shao^1,2, S.J. Ren², L.M. Zhang², and Q. Xue³

¹National Laboratory of Soil Erosion and Dryland Agriculture on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, Yangling, Shaanxi Province, 712100, P.R. China

²Northwest Sci-Tech University of Agriculture and Forestry, Yangling, Shaanxi Province, 712100, P.R. China

³Northwestern Agricultural Research Center, Montana State University, 4570 Montana 35, Kalispell, MT 59901, USA

(Received April 8, 2003; Accepted July 4, 2003)

Abstract. The seedling growth of two drought-resistant wheat varieties was studied under solution culture in a plant growth chamber. The results showed that the shoot dry weight and leaf gas exchange parameters increased with the increase of nitrogen supply, but decreased when nitrogen supply reached a certain level. The optimum nitrogen concentrations for shoot dry weight and gas exchange were different among the varieties. The root growth was negatively correlated with the increase of nitrogen supply. The distribution of root length in different layers was similar for the two varieties. The root length was the longest at the layer of 5-15 cm, the shortest below 15 cm, and in between at the layer of 0-5 cm. The water use efficiency (WUE) decreased with increasing ratio of root to shoot (R/S), while leaf photosynthetic rate tended to increase initially and then decrease. The increase in R/S was unfavorable to increase WUE, and the appropriate R/S for leaf photosynthetic rate was about 0.5.

Keywords: Coordinate growth; Nitrogen nutrition; Root and shoot relation; Winter wheat.

Introduction

Drought stress and nitrogen deficiency are major constraints to winter wheat production and yield stability in most rainfed regions (McDonald and Davies, 1996). An efficient use of limited nitrogen resources and better growth under limited nitrogen supply are desirable traits for crops in drought environments. Nitrogen deficiency induces modifications of many morphological and physiological processes (Ciompi et al., 1996; Shangguan et al., 2000a). Leaf nitrogen is mostly used for synthesis of components of the photosynthetic apparatus, and about 75% of leaf nitrogen is allocated to the chloroplasts (Shangguan et al., 2000b).

The nutrient supply and demand of root and shoot are inter-dependent due to their different functions and local environment (Passioura, 1983; Siddique et al., 1990; Li et al., 2001). The ratio of root to shoot (R/S) is an index that reflects growth and dry matter accumulation between root and shoot (Lioert et al., 1999). The R/S is affected both by genetic (Passioura, 1983; O'Toole and Bland, 1987) and environmental factors, such as water status (Miao et al., 1998; Grant, 1998; Hebert et al., 2001), nutrient availability (Liang, 1996; Marsh and Pierzynski, 1998; Maranov et al., 1998), and soil texture (Vos et al., 1998). Root growth is

closely related to physiological metabolism and dry matter accumulation in shoot (Siddique et al., 1990). An excessively low R/S indicates poor root growth, resulting in sufficient water and nutrients for shoot growth. An extremely high R/S may lead to root redundancy, which reduces shoot growth, yield, and water and nutrient use efficiencies (Zhang, 1995). Therefore, it is important to coordinate root and shoot relations and maximize dry matter accumulation and water and nutrient use efficiencies (Tomar et al., 1997; Kahn and Schroeder, 1999).

Nitrogen nutrition has significant effects on root and shoot relations (Feng and Liu, 1996; Lioert et al., 1999). Nitrogen deficiency increased root surface area, increased consumption of assimilates, reduced the amount of nitrogen transported to shoot, decreased shoot growth, and resulted in an increased R/S ratio. However, extra nitrogen nutrition caused an excessive shoot growth, reduced the assimilate availability for root, and reduced the R/S ratio (Passioura, 1983). Therefore, the amount of nitrogen nutrition applied to plants must be optimal for root and shoot relations. Previous studies have shown that effective use of nitrogen fertilizer increased leaf photosynthesis, promoted root development, and extended space for root to extract water and nutrients in soil (Li et al., 1999). However, related research is relatively scarce, and then the effect of nitrogen nutrition on root growth needs to be enhanced. Objectives of this study were to (1) investigate the responses of shoot and root growth, leaf exchange, and WUE to nitrogen nutrition and (2) determine the relationships between gas exchange, WUE and R/S.

*Corresponding author. Tel: +86-29-7019107; Fax: +86-29-7012210; E-mail: shangguan@ms.iswc.ac.cn

Botanical Bulletin of Academia Sinica, Vol. 45, 2004

Materials and Methods

Plant Materials and Growth Conditions

The sterilized seeds of wheat (Triticum aestivum L.) varieties, Xiaoyan No.6 (drought resistant) and Xinong No. 1043 (drought resistant), were initially soaked in deionized water for 24 h, and then germinated in darkness at a constant temperature of 25°C on moist gauze for 48 h. Upon emergence, uniformly germinated seeds were seeded in sands. When the seedlings were about 2 cm tall, the healthy and uniform ones were transplanted into solution culture containers in a growth chamber.

The solution culture container was made of glass, 60 cm long, 10 cm wide, and 30 cm deep, with a volume of 2 L. There was a pipe to charge the used solution at the bottom of the container. The seedlings were transplanted into an aluminum frame wrapped with double nylon (120 items) net. The frame had screws to clutch on the edge of the container and were tied with a rubber band.

The environmental conditions in the plant growth chamber (Heraeus Votsch, Germany) were: a photosynthetic photon flux density of 130 µmol m^-2 s^-1 over plants; day/night temperature: 25°C/15°C; midday relative humidity: 65%; and a 12-h photoperiod. The growing conditions were controlled automatically by a computer program. The solution was ventilated by electrical pump with a flow rate of 150 ml·min^-1 from 8:00 am to 10:00 am every day. The seedlings were initially grown in distilled water for 4 days after being transplanted. Thereafter, the nitrogen nutrient solution was added. The solution was replaced every three days and adjusted to pH of 5.0 using KOH or HCl every day.

Treatments

There were six nitrogen treatments in each variety: 0 mmol·L^-1 NO^-₃, 3.75 mmol·L^-1 NO^-₃, 7.50 mmol·L^-1 NO^-₃, 11.25 mmol·L^-1 NO^-₃, 15.00 mmol·L^-1 NO^-₃, and 18.75 mmol·L^-1 NO^-₃ with six replications. All other ions in the solution were constant, except for SO₄^2- and Cl^-, which were used in equal portions to maintain charge balance. The basic nutrient solution was Hoagland solution. The compounds of the nutrient solutions were Ca(NO₃)₂·4H₂O : 0.5 mol·L^-1, KNO₃ : 0.5 mol·L^-1, NH₄N0₃ : 0.5 mol·L^-1, MgSO₄·7H₂O : 0.2 mol·L^-1, KH₂PO₄ : 0. 2 mol·L^-1, KCl : 0.5 mol·L^-1, CaCl₂ : 0.5 mol·L^-1, Fe-EDTA, and mineral elements (A-Z solution).

Leaf Gas Exchange and N Content Measurements

The gas exchange measurements were taken when seedlings had four leaves. A portable infrared CO₂ analyzer (Li-6400, Li-Cor, USA) was used to measure photosynthetic rate (Pn), transpiration rate (Tr), leaf conductance (gs), and intercellular CO₂ concentration (Ci). The youngest fully expanded leaf of three plants of each treatment was enclosed into the gas exchange chamber between 10 and 11.5 h for gas exchange measurements. Plant water use efficiency was determined as the ratio of Pn to Tr.

Root Measurement

Fresh weight, dry weight, length, and diameter of root in each treatment were measured in different layers. The root samples were dyed. The image of roots was scanned by a numerical scanner to analyze the root perimeter (P) and surface area (A). Then P and A were used to calculate the root length (L) and diameter (d), using CIAT-root system analysis software (CID Inc., USA).

The shoot and root samples were oven-dried at 90°C for 30 min, then at 70°C for at least 48 h, and the dry matter weight was determined.

Leaf Nitrogen Content

For determining leaf nitrogen concentration, 200 mg of powdered material was analyzed with a modified Kjeidahl analysis using concentrated sulphuric and salicylic acid and Na₂SO₄, K₂SO₄ and Se in a ratio of 62:1:1 (w/w) as a catalyst (Page et al., 1982). The N-concentration of the digests was determined on a Skalar continuous flow analyser.

Statistical Analysis

Standard error, variance, regression and correlation coefficients, and significant differences among regression coefficients were calculated by standard methods with the DAPS - statistical package (Feng and Tang, 1997).

Results

Leaf Nitrogen Content

The leaf nitrogen content increased as nitrogen concentration increased in solution for both varieties (Table 1). The leaf nitrogen content increased more rapidly when the nitrogen concentration increased from 0 to 3.75 mmol·L^-1. When nitrogen concentration was over 3.75 mmol·L^-1, leaf nitrogen content increased slowly as nitrogen concentration increased.

Leaf Gas Exchange

The response of leaf photosynthetic rate (Pn) to nitrogen concentration was parabolic. The Pn increased first and then decreased as nitrogen concentration increased (Figure 1A). Comparing the two varieties, the Pn of Xiaoyan No. 6 was higher than that of Xinong No.1043 when the nitrogen concentration was lower than 10 mmol·L^-1. However, the Pn of Xiaoyan No. 6 was lower than that of Xinong No. 1043 when nitrogen concentration was over 12.5 mmol·L^-1 because the Pn of Xiaoyan No. 6 decreased more rapidly than that of Xinong No. 1043.

The response of leaf transpiration (Tr) to nitrogen concentration was similar to Pn (Figure 1B). For both varieties, the Tr reached the maximum at nitrogen concentration of 7.5 mmol·L^-1, and then decreased as nitrogen concentration increased. The Tr of Xiaoyan No. 6 was higher than that of Xinong No. 1043 except at nitrogen concentration of 18.75 mmol·L^-1. Compared to Pn, Tr was more sensitive to nitrogen treatment.

Shangguan et al. — Nitrogen on root and gas exchange

Shoot and Root Dry Weight

The plant shoot dry weight increased with increasing nitrogen supply, reaching the maximum when the nitrogen concentration was 11.25 mmol·L^-1. Thereafter, shoot dry weight decreased as nitrogen supply increased. At most nitrogen concentrations, shoot dry weight of Xiaoyan No. 6 was higher than that of Xinong No. 1043 (Table 2).

The root dry weight decreased as nitrogen concentration increased for both varieties, indicating lower nitrogen concentration was more favorable to the accumulation of root growth than to shoot growth. There was no significant difference in root dry weight between the two varieties (Table 2).

The variation of root weight at different depths of Xinong No. 1043 was similar to that of Xiaoyan No. 6, and the root growth of Xinong No. 1043 was more sensitive to nitrogen than that of Xiaoyan No. 6. With the increase of nitrogen supply, the root dry weight below the 10 cm layer was reduced. For the root dry weight at 0-10 cm, root dry weight increased at 3.75 mmol·L^-1, then decreased as nitrogen supply increased when nitrogen concentration exceeded 3.75 mmol·L^-1.

Root Length, Surface Area and Root /Shoot (R/S) Ratio

For both varieties, total root length was significantly reduced at higher nitrogen concentrations (Table 1). For Xinong No. 1043, total root length decreased when nitrogen concentration was over 11.25 mmol·L^-1. For Xiaoyan No. 6, root length decreased when nitrogen concentration was over 7.5 mmol·L^-1. For both varieties, total root length was the highest at the 5-15 cm depth layer, shortest below 15 cm, and intermediate at 0-5 cm.

Figure 1. The photosynthetic rate (A) and transpiration rate (B) of wheat leaves under different nitrogen treatments condition.

Botanical Bulletin of Academia Sinica, Vol. 45, 2004

For Xinong No. 1043, the average root diameter at 0-5 cm and below 20 cm was greater than that at 5-15 cm, especially at 10-15 cm in which the root diameter was the smallest. For Xiaoyan No. 6, the smallest root diameter was found between 5-15 cm. Based on the root length and diameter, the main uptake part of the root system in wheat seedlings was at the 5-15 cm depth since the root length was the highest and diameter was the smallest for this layer. For both varieties, the root surface area decreased as nitrogen concentration increased (Table 1).

The changes of R/S of the two varieties were similar under different nitrogen treatments. The R/S decreased gradually with the increase of nitrogen concentration, but it was increased when the nitrogen concentration exceeded 12.5 mmol·L^-1 (Table 2). Comparing the two varieties, R/S of Xinong No. 1043 was greater than that of Xiaoyan No. 6.

Discussion

The manipulation of crop physiological function to raise yield is always an important issue in agronomy, genetics, and ecology. The research on plant integrated response, root establishment, and rhythmical changes of shoot and root is still underway (McDonald and Davies, 1996; Li et al., 2001). In recent years, such research has focused on root and shoot interactions (Passioura, 1983; McDonald and Davies, 1996; Lioert et al., 1999; Hebert et al., 2001). Much has been conducted on root establishment, architecture and function, growth dynamics, metabolism, effects of genetics and the environment on root growth, and the relationship between root growth and yield (Passioura, 1983; O'Toole and Bland, 1987; Siddique et al., 1990; Li et al., 2001). Root growth appears related to genetic factors, soil conditions, temperature, and seed size (Lioert et al., 1999). The modern cultivars usually have a well-developed and deep root system to increase resistance to drought. However, in old cultivars, root growth is inhibited after the tillering stage, and the ultimate yield is reduced (Zhang and Shan, 1998). Cultivars with a greater R/S usually have a relatively greater water and nutrient uptake capacity, higher yield stability, and greater drought

resistance (Passioura, 1983). The R/S of early maturity varieties is smaller than that of late maturity. The root density increased in soil profile under irrigation, while the root penetrating capacity increased under drought conditions (Tomar et al., 1997). Liang (1996) found that increasing nitrogen supply increased R/S due to increase of root dry weight and the number of seminal roots. In the present study, nitrogen deficiency significantly decreased nutrient uptake by roots and nutrient supply to shoots. Therefore, root growth has a significant effect on shoot growth. The experimental results also indicated that root growth was significantly correlated with seedling growth.

Both shoot dry weight and gas exchange parameters increased first and then decreased as nitrogen supply increased. However, the optimal nitrogen concentration for shoot dry weight and gas exchange parameters was different. Nitrogen concentration had a negative effect on root growth (Table 2). Maranov et al. (1998) found that increasing nitrogen increased root growth but decreased shoot growth when plants grew under severe water stress because the limited water extracted by roots was mainly consumed by the root system itself and only a small amount of water was transported to shoot. In this study, both roots and shoots were grown under well-watered conditions, and a small quantity of nitrogen nutrition could meet the demand of root growth. However the demand of shoot growth would be met only when the nitrogen concentration was relatively high due to the position of the shoot as compared to the root. In dryland farming areas, crop production is mainly dependent on the highly efficient use of limited water and its full extraction by roots. The results of our study showed both root and shoot growth was significantly correlated with WUE, and an increase in both root and shoot could increase WUE. Because of the high inter-dependence between root and shoot, the stronger the root system, the better the foundation for the robust shoot growth and more efficient water use (Feng and Liu, 1996). However, it was also found that WUE decreased with increasing R/S (Figure 2A) while the response of leaf photosynthetic rate to R/S exhibited a parabolic trend (Figure 2B), indicating that the increase of R/S was unfavorable to improving WUE The optimum R/S for photosynthetic rate was about 0.5.

Figure 2. The relationship of R/S and WUE_I (A) and that of R/S and Pn (B) of wheat.

Shangguan et al. — Nitrogen on root and gas exchange

Wheat is one of the main crops planted extensively in semi-arid and semi-moist areas, and different cultivars have different adaptability to nitrogen and water. Wheat's adaptable mechanisms for drought at different nitrogen levels were probably different. Low nitrogen wheat adapted to drought through a drought resistant structure, high nitrogen wheat through osmotic adjustment (Xue and Chen, 1990). A close relationship also existed between wheat adaptability to water and features of root growth (Liang, 1996; Li et al., 2001). Non-drought resistant wheat cultivars' sub-roots mostly expanded shallowly, and the angle was big; however, drought resistant wheat cultivars expanded deeply, and the root angle was small, so different drought resistant cultivars have different root morphology (Duan et al., 1997). The drought resistant wheat cultivars had the character of root/shoot and a greater quantity of root at deep layers (Miao et al., 1998; Shangguan and Shao, 1999). The semi-arid area of the Loess Plateau had a deep soil profile, a loose soil structure, and a great storage capacity for soil water. The deep soil profiles not only allow plant roots to expand more deeply, they also reinforce the resistant ability of crops by using deeply storied water (Shangguan and Shao, 1999). A greater ratio of roots/shoots maintained the water balance in plants well. However, if the root system is deeply extended, the shallow storied water is limited, and then the significance of the great ratio of roots/shoots loses importance. Therefore, during agricultural production in a semi-arid area, choosing wheat cultivars of different resistance in different quantities of nitrogen fertilizer to benefit the root distribution and absorbing ability that can improve wheat photosynthetic response and productivity.

Acknowledgements. This work was funded by a state major basic research project (Project No: 2002CB111502) and the National Natural Science Foundation of China (Project No: 30270801).

Literature Cited

Ciompi, S., E. Gentill, L. Guidi, and G.F. Soldatini. 1996. The effect of nitrogen on leaf gas exchange and chlorophyll fluorescence parameters in sunflower. Plant Sci. 118: 177-184.

Duan, S.S., W.X. Gu, D.Y. Zhang, and F.M. Li. 1997. Relationship between root system characteristics and drought resistance of wheat populations in semiarid region. Chin. J. Appl. Ecol. 8: 134-138

Feng, G.G. and Y.Q. Tang. 1997. Data Analysis Platform. Chinese Agriculture Press, Beijing.

Feng, W.L. and C.M. Liu. 1996. Regulation of soil water on the growth and distribution of root system of crops. Res. Ecol. Agro. 3: 5-9.

Hebert, Y., E. Guingo, and O. Loudet. 2001. The response of root/shoot partitioning and root morphology to light reduction in maize genotypes. Crop Sci. 41: 363-371.

Grant, R.F. 1998. Simulation in ecosystem of root growth response to contrasting soil water and nitrogen. Ecol. Modeling 107: 237-264.

Li, L.H., S.Q. Li, J.H. Zai, and J.T. Shi. 2001. Review of the

relationship between wheat roots and water stress. Acta Bot. Boreali-Occidentalia Sin. 21: 1-7.

Li, Y.Z., F.X. Wang, and L.H. Liu. 1999. Use and management of soil water and nitrogen resource. I. Soil water and nitrogen conditions and root development. Plant Nutr. Fert. Sci. 5: 206-313.

Liang, Y.L. 1996. Adjustment of physiological characters of root systems of wheat by soil moisture, nitrogen and phosphorus, Acta Phytoecol. Sin. 20: 255-262.

Lioert, F., C. Casanovas, and J. Penuelas. 1999. Seedling survival of Mediterranean shrub land species in relation to root: shoot ratio, seed size and water and nitrogen use. Functional Ecol. 13: 210-216.

Miao, G.Y., Y.T. Zhang, J. Yin, Y.S. Hou, and X.L. Pan. 1998. A study on the development of root system in winter wheat under unirrigated conditions in semi-arid Loess Plateau. Acta Agron. Sin. 15: 104-115.

Marsh, B.H. and G.M. Pierzynski. 1998. Root response to rates of banded nitrogen and phosphorus fertilizers. Develop. Plant Soil Sci. 82: 471-482.

Maranov, A., A. Samedovam, and T. Shirvany. 1998. Root-shoot relationships in plant adapation to nitrogen deficiency. Develop. Plant Soil Sci. 82: 147-154.

McDonald, A.J.S. and W.J. Davies. 1996. Keeping in touch: response of the whole plant to deficits in water and nitrogen supply. Adv. Bot. Res. 22: 229-300.

Kahn, B.A. and J.L. Schroeder. 1999. Root characteristics and seed yields of cowpeas grown with and without added nitrogen fertilizer. HortScience 34: 1238-1239.

O'Toole, J.C. and W.L. Bland. 1987. Genotypic variation in crop plant root systems. Adv. Agron. 41: 91-145.

Page, A.L., R.H. Miller, and D.R. Keeney. 1982. Methods of soil analysis. Part 2. Chemical and Microbiological Properties, 2nd Edn, American Society of Agronomy: Madison, WI.

Passioura, J.B. 1983. Root and drought resistance. Agric. Water Manage. 7: 265-280.

Shangguan, Z.P. and M.A. Shao. 1999. Physiological mechanism for improving crop water use in arid region. J. Hydraulic Engineering 10: 33-37.

Shangguan, Z.P., M.A. Shao, and J. Dyckmans. 2000a. Effects of nitrogen nutrition and water deficit on net photosynthetic rate and chlorophyll fluorescence in winter wheat. J. Plant Physiol. 156: 46-51.

Shangguan, Z.P., M.A. Shao, and J. Dyckmans. 2000b. Nitrogen nutrition and water stress effects on leaf photosynthetic gas exchange and water use efficiency in winter wheat. Environ. Exp. Bot. 44: 141-149.

Siddique, K.H.M., R.K. Belford, and D. Tennant. 1990. Root:shoot ratios of old and modern, tall and semi-dwarf wheat in a Mediterranean environment. Plant Soil. 121: 89-98.

Zhang, Z.B., and L. Shan. 1998. Comparison study on water use efficiency of wheat flag leaf. Chinese Sci. Bull. 43: 1205-1210.

Zhang, D.Y. 1995. Analysis of growth redundancy of crop root system in semi-arid area. Acta Botanica Boreali-Occidentalia Sinaca 15: 110-114.

Tomar, H.P.S., K.S. Dadhwal, and H.P. Singh. 1997. Root characteristics and moisture-use pattern of spring sunflower as influenced by irrigation, nitrogen and phosphorus. Ind. J.

Botanical Bulletin of Academia Sinica, Vol. 45, 2004

Agron. 42: 515-519.

Vos J., P.E.L. van der Putten, M.H. Hussein, A.M. van Dam, and P.A. Leffelaar. 1998. Field observation on nitrogen catch crops: II. Root length and root length distribution in relation to species and nitrogen supply. Plant Soil. 201:

149-155.

Xue, Q.W. and P.Y. Chen. 1990. Effects of nitrogen nutrition on water status and photosynthesis in wheat under soil drought. Acta Phytophysiologica Sin. 16: 49-56.