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GRAIN SORGHUM

Nitrate Reductase Activity of Diverse Grain Sorghum Genotypes and Its Relationship to Nitrogen Use Efficiency

^a Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583-0817 USA

jmaranville1{at}unl.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
High crop N use efficiency is desirable for reducing the cost and reliance on fertilizer N, and it may help reduce ground water pollution. The objectives of this study were to (i) characterize and compare the seasonal trends in nitrate reductase activity (NRA) of seven grain sorghum [Sorghum bicolor (L.) Moench] genotypes of tropical and U.S. origin, (ii) determine how NRA is related to total plant N, dry matter, grain yield, and N use efficiency, and (iii) examine how NRA and plant N accumulation affect N use efficiency for biomass (NUE₁) and grain production (NUE₂). The genotypes were evaluated in field trials in 1994 and 1996 at applied N levels of 0 and 100 N kg ha^-1 on a Sharpsburg silty clay loam soil (fine, smectitic, mesic Typic Argiudoll). Plant biomass, reduced N, NRA, NUE₁, and NUE₂ were determined at three growth stages (vegetative, anthesis, and physiological maturity). Genotypes did not differ for NRA at either N level; however, there was a definite tendency for the four tropical lines and two U.S. adapted lines to have greater NRA values at all growth stages than the one hybrid. Nitrate reductase activity decreased throughout the growing season, with the sharpest decline from anthesis to maturity. Nitrate reductase activity did not correlate with grain yield or shoot biomass, but did correlate with grain N concentration. Shoot and grain N concentration correlated with NUE₁, while grain and shoot N contents and shoot N concentration were correlated with NUE₂. Neither NUE₁ nor NUE₂ were significantly affected by the NRA level. Tropical lines had greater preanthesis N uptake accumulation (1.45 g plant^-1) and greater NUE₁ than the hybrid (0.90 g plant^-1) and U.S. adapted lines (0.73 g plant^-1). In terms of grain N use efficiency, the hybrid had a 14% greater value than U.S. adapted lines and a 17% greater value than the tropical lines. It appears that NRA is not a factor that limited N accumulation and use efficiency in grain sorghum.

Abbreviations: NRA, nitrate reductase activity • NUE₁, biomass nitrogen use efficiency • NUE₂, grain nitrogen use efficiency

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
NITROGEN is one of the major elements required for plant growth. It is a constituent of numerous important compounds found in living cells, including amino acids, proteins (enzymes), nucleic acids, and chlorophyll. Nitrate is the predominant source of N for crops in most agricultural soils (Feil et al., 1993). In order to be incorporated into amino acids, nucleic acids, and other compounds, NO^-₃ must be reduced to NH⁺₄. Nitrate reductase is the first enzyme in the process of NO^-₃ reduction to amino form (Kleinhofs and Warner, 1990). Nitrate reductase is a substrate-inducible enzyme and is thought to be the most limiting step in N assimilation (Hageman and Hucklesby, 1971; Beevers and Hageman, 1969; Kelly et al., 1995). For this reason, nitrate reductase activity (NRA) may be a selection criterion for grain yield and N assimilation potential (Hageman and Lambert, 1988; Sherrard et al., 1986).

The in situ rate of NO^-₃ reduction is controlled primarily by the rate of NO^-₃ uptake, rather than by alterations in NRA (Wilkinson and Crawford, 1993) or limitations in reducing power (Warner and Huffaker, 1989). Thus, NO^-₃ uptake appears to be of primary importance in N assimilation in NO^-₃–fed plants. Genetic variation in NRA is well documented in several species (Eck et al., 1975; Gallagher et al., 1983; Beevers and Hageman, 1969). Nitrate reductase activity is affected by factors such as environmental conditions (Srivastava, 1980) and plant developmental stages (Eck et al., 1975), as well as plant part, such as roots and tops (Fakorede and Mock, 1978). Furthermore, in vivo and in vitro assays usually give different results (Deckard and Busch, 1978). Variable results were found by several researchers (Croy and Hageman, 1970; Eilrich, 1968; Eck and Hageman, 1974) in their efforts to relate NRA to grain yield and N-related traits such as total reduced plant N, grain N content, grain N concentration, and N harvest index.

Swank et al. (1982), working with maize (Zea mays L.), stated that the high carbohydrate content of grain (800 g kg^-1 carbohydrate vs. 15 g kg^-1 N, approximately) is an indication of a predominant role for photosynthesis in achieving maximum yields. However, they also recognized the importance of N fertilizer in enhancing maize yield. Thus, it seems reasonable to conclude that there is a very close link between C and N metabolism. Murata and Matsushima (1975), working with rice (Oryza sativa L.), have attributed two major roles to N: (i) the establishment of yield capacity and (ii) the establishment and maintenance of photosynthetic capacity. Tollenaar (1977) suggested that sink sizes in maize may limit crop yields.

Generally, nitrogen use efficiency (NUE) definitions fall into the broad categories of rate of gas exchange, biomass produced, or harvested product per unit of N absorbed or applied. Nitrogen use efficiency variations can be partitioned into differences in uptake efficiency and use efficiency (Moll et al., 1982; Pollmer et al., 1979). Nitrogen uptake efficiency is defined as total plant N content per unit of fertilizer N applied, whereas NUE is defined as grain yield per unit of N in the plant at maturity. Maranville et al. (1980), working with grain sorghum, defined NUE as (i) biomass production per unit total aboveground plant N (NE₁), (ii) as grain production per unit total aboveground plant N (NE₂) or (iii) the product of NE₂ and the ratio of grain N content to stover N content (NE₃). Efficiency of N uptake and use relative to the production of grain requires that processes associated with absorption, translocation, assimilation, and redistribution of N operate effectively (Moll et al., 1982). Differences among genotypes in N absorption and utilization have been demonstrated for several species by different investigators (Moll and Kamprath, 1977; Pollmer et al., 1979; Reed et al., 1980). Considerable evidence of genotypic differences in NUE has been reported for maize (Bruetsch and Estes, 1976; O'Sullivan et al., 1974) and for grain sorghum (Maranville et al., 1980), and nutrient efficiency definitions vary greatly (Clark, 1990). Currently, little is known about the relative importance of each process to genotypic differences in N use efficiency in grain sorghum. Our objectives were to (i) characterize and compare the seasonal trends in NRA of seven grain sorghum genotypes; (ii) determine how NRA is related to plant total N, total dry matter, yield, and NUE; and (iii) determine how NRA and plant N accumulation affect NUE.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Field experiments were conducted under dryland conditions in 1994 and 1996 at the University of Nebraska Agricultural Research and Development Center near Mead, NE. Seven grain sorghum genotypes (one U.S. hybrid, two U.S. adapted lines, and four tropical lines) were tested. The experimental site was different each year, although the soil at both sites was a Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudoll). The soil pH was 6.4 and 6.2, organic matter content was 2.1 and 2.2%, NO₃–N was 2.1 and 2.0 mg kg^-1, and Bray and Kurtz No. 1 P was 77 and 80 mg kg^-1 in 1994 and 1996, respectively. The experiments were conducted in a split-plot arrangement of treatments in a randomized complete block design with four replications. Whole-plot treatments consisted of two N levels of 0 and 100 kg N ha^-1 applied as granular NH₄NO₃ (34–0–0 N–P–K) in a surface band at 4 to 5 wk after planting. Whole-plot treatments were physically separated by 2 m to eliminate border effects of N fertilizer application. The seven genotypes were randomly assigned as the subplots in four rows 7.0 m long and 0.75 m apart.

Sorghum was planted 15 June 1994 and 19 May 1996, at a seeding rate of 120 000 to 125 000 seeds ha^-1. Final stands were approximately 100 000 plants ha^-1 each year. Weed control was achieved by preemergence herbicide application of a mixture of 7 L ha^-1 propachlor (2-chloro-N-isopropylactamilide), 3 L ha^-1 atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], 2.4 L ha^-1 glyphosate [N-(phosphonomethyl) glycine], and 2.4 L ha^-1 crop oil surfactant. Each season, one cultivation during the vegetative stage and hand hoeing were done to supplement herbicide action. The previous crop was grain sorghum without N fertilizer to help reduce soil N level. At each of three growth stages (vegetative, 10–11 leaves; anthesis, 50% of plants flowered; and physiological maturity), three plants were randomly sampled from the center two rows for analysis. At each sampling, the three plants were bulked and separated into leaves plus sheaths, stalks, and panicles and dried at 70°C in a forced-air oven for at least 72 h. Plant parts were then weighed and ground with a Wiley mill to pass a 2.0-mm screen. Panicles were threshed using a small head thresher. Seed subsamples were ground using a cyclone sample mill (Udy Co., Ft. Collins, Co). Kernel weight was obtained from 100 kernels. Total organic N concentration of each component was determined by the Kjeldahl method. Grain yield was estimated by harvesting one row of 6 m (4.5 m²).

Nitrogen content at the different growth stages was computed by multiplying N concentration by total aboveground plant dry matter. The determination of NUE followed the definitions suggested by Maranville et al. (1980). Biomass N use efficiency (NUE₁) and grain N use efficiency (NUE2) were computed as follows:

(1)

(2)

where TDM is total aboveground plant dry matter, Nt is total N in the plant, and GW is grain weight.

Nitrate reductase activity was measured on green leaves at the vegetative 10-leaf stage, at anthesis, and at maturity using the in vivo assay described by Klepper et al. (1971). About 0.25 g of leaf disks was collected from the top two fully expanded leaves of several randomly selected plants and placed in centrifuge tubes. The centrifuge tubes were put on ice (4°C) during transport to the lab. Leaf disks were transferred to 50-mL beakers containing 5 mL of solution containing 0.05 M KNO₃, 0.05 M KH₂PO₄, and 1% propanol surfactant (v/v) at pH of 7.5. Air was withdrawn from the leaves by vacuuming twice at 101 kPa (760 mm Hg) before incubation. Plant samples were allowed to incubate in the dark for 1 h at 30°C. Nitrite produced by the leaf disks and released into the solution was reacted with 2 mL of coupling reagent containing 1 mL of 0.02% (v/v) naphthylamine and 1 mL of 1% (w/v) sulfanilic acid. Absorbance was determined on a spectrophotometer at 540 nm. Results were expressed as µmol NO^-₂ cm^-2 h^-1, as a seasonal average (mean of all measurements throughout the season), or as a day unit, g N plant^-1 d^-1 (Eilrich, 1968), which can be converted to a seasonal input of reduced N (Croy and Hageman, 1970; Eilrich, 1968). The day unit was computed as (µmol N cm^-2 h^-1) x (total leaf area plant^-1) x (14 h d^-1) x (10^-6 mol µmol^-1) x (14 g mol^-1).

Data were subjected to analysis of variance using the statistical analysis system program (SAS Inst., 1989). Least significant difference (F-protected LSD) was used to compare individual genotypes. Significant differences were reported at the 0.05 level of probability. Genotypes were separated into three groups: (i) hybrid, (ii) U.S. adapted lines, and (iii) tropical lines and compared using single degree of freedom orthogonal contrasts. Pearson correlation coefficients were used to determine the relationship among measured parameters.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
The growing seasons in 1994 and 1996 were generally dry, with 134 and 170 mm less rainfall than the long-term average (742 mm), respectively. In 1994, May was very dry and June was extremely wet, followed by below-normal moisture the rest of the growing season. In 1996, soil conditions were excessively wet in May and dry later in the growing season. Mean monthly air temperature was greater than normal both years (average 6°C above normal) during the growing season. Solar radiation during active vegetative growth was, on average, 20.2 MJ m^-2 in 1994 and 19.7 MJ m^-2 in 1996.

No year x N level x genotype interaction was found, indicating that differences between the two years did not have an effect on measured biological variables; thus, data are presented as averages over years. The genotype x N level interaction was tested each year; consequently, for traits that had a significant genotype x N level interaction, results are presented by N level. In the absence of interaction, results are presented as genotype means over N levels.

Grain Yield
There was no N level x genotype interaction for grain yield and the results for that trait are presented averaged over N levels. Applied N increased grain yield (Table 1) . Hybrid HH 640 produced greater yield (4767 Mg ha^-1) than the U.S. (2958 Mg ha^-1) and tropical lines (3031 Mg ha^-1). Tropical and U.S. adapted lines had similar grain yield with CK 60, VG 146 and S 34 producing the highest yields. Kernel weight was not affected by N application. The hybrid HH 640 had greater kernel weight than U.S. adapted and tropical lines, and the number of kernels per unit area was greater with N fertilizer application (data not shown). The hybrid had greater kernel number per unit area (14583 kernels m^-2) than tropical (14495 kernels m^-2) and U.S. lines (9870 kernels m^-2). However, Malisor 84-7, S 34 and VG 146 were similar to the hybrid for the number of kernels per unit area. The hybrid was greater than U.S. lines and less than tropical lines for number of kernels per panicle. There was no N effect on kernels per panicle. The hybrid had greater grain yield because of its greater kernel weight and high kernel number per unit area. Tropical lines generally had reduced grain yield due to their limited kernel weight. Masi and Maranville (1998) reported similar results.

Nitrogen Accumulation
Tropical lines had greater vegetative N uptake and accumulation than the hybrid and U.S. adapted lines. Total plant N increased during the growing season for all groups of genotypes (Fig. 1) . No genotype x N level interaction was found for shoot N concentration, total shoot N content, grain N concentration, total grain content, or NRA, and therefore those results are presented as averages across N levels. Applied N increased shoot N concentration and total shoot N content at anthesis and physiological maturity (Table 2) . Genotypes differed at all stages for both traits. Genotype group differences for shoot N concentration were detected only at maturity with U.S. lines having a greater value (12 g kg^-1) than HH 640 (10.2 g kg^-1) and the tropical lines (8.8 g kg^-1). Shoot N concentration decreased during the growing season likely due to rapid growth rate which exceeded the N uptake and its subsequent reduction. Mean total shoot N content of genotype groups differed at anthesis and maturity, with tropical lines having a greater value than HH 640 and U.S. adapted lines. Since tropical lines had lower values for shoot N concentration than the hybrid and U.S. lines did, the difference in total shoot N content was due to the total shoot biomass produced. This observation points out the ability of tropical lines to accumulate greater biomass at lower leaf N concentration, a characteristic highly desirable in low-N environments such as farm land in developing countries where N fertilizers are hard to obtain and use by farmers. Masi and Maranville (1998) found similar tropical sorghum lines to have high NUE₁ and very large root systems.

View larger version (20K): [in this window] [in a new window]   Fig. 1 Total plant N for three groups of grain sorghum genotypes at different growth stages. Bars represent LSD (0.05) at given growth stage for Mead, NE (1994 and 1996)

Nitrogen Use Efficiency
The genotype x N level interaction was not significant for either NUE₁ or NUE₂ and the results are presented as averages across N levels (Table 3) . Tropical lines as a group had greater biomass N use efficiency (NUE₁) at anthesis and maturity than the hybrid and U.S. lines; however, their grain N use efficiency (NUE₂) was less than that of the hybrid and similar to the U.S. lines. At the vegetative stage, neither N level nor genotype group were different for NUE₁, but the genotypes within groups differed, ranging from 39.9 to 48.4 g g^-1. At both the anthesis and maturity stages, applied N levels decreased NUE₁ and NUE₂ values, similar to the findings of Maranville et al. (1980). At maturity, tropical lines had greater NUE₁ values (94.8 g g^-1) than U.S. adapted lines (81.7 g g^-1) and HH 640 (82.8 g g^-1).

Shoot biomass was positively associated with NUE₁ (Table 4) . Other N-related traits negatively correlated with NUE₁ were shoot N concentration and grain N concentration . Total grain N content was positively associated with NUE₂, while shoot N concentration and shoot N content were negatively correlated with NUE₂. Neither NUE₁ nor NUE₂ were correlated with NRA.

Amount of N remobilized from storage tissues is important in grain N use efficiency; it varies among genotypes and seems to be under genetic control (Moll et al., 1982; Dhugga and Waines, 1989; Eilrich and Hageman, 1973). Our results confirm this observation, in that N remobilized from vegetative parts during grain filling was highly and significantly correlated with NUE₂ ( ; data not shown).

Nitrate Reductase Activity
Because grain yield is dependent on N status of the crop, significant positive correlations between NRA and grain yield have been reported by Eck et al. (1975) and Eilrich and Hageman (1973). We found no significant correlation between NRA and grain yield (Table 4). However, NRA is not the only factor determining grain yield. Thus, close relationships between NRA and grain yield are not always found.

Nitrate reductase activity in the leaves results in reduced N for the grain sorghum plant, and the enzyme activity is related to the accumulation of reduced N by the plant. Our results do not support the generally accepted relationship. The N-related traits shoot N concentration , shoot N content , grain N concentration , and grain N content did not correlate significantly with NRA (Table 4). Apparently, NRA was not the only factor influencing reduced N accumulation and partitioning in plant.

Nitrate reductase activity was not significantly different between N levels (Table 5) , which is in contrast to findings of Eilrich and Hageman (1973), Croy and Hageman (1970), and Eilrich (1968) with wheat (Triticum aestivum L.) and of Jung et al. (1972) with maize. Since NO^-₃ reductase is an inducible enzyme, the lack of response to applied N was not fully understood. Genotypes did not differ for NRA at vegetative and physiological maturity, but at anthesis M35-1 had significantly less NRA than other tropical genotypes. Genotype groups did not differ for NRA at the three sampling dates. Deckard and Busch (1978), Gallagher et al. (1983), and Rao et al. (1977) reported significant variability among genotypes of the same species for NRA. Kelly et al. (1995) did not find significant NRA differences among genotypes of wheat. From the vegetative to the maturity stage, NRA decreased, with the greatest decline from anthesis to maturity. Nitrate reductase activity did not correlate with grain yield, shoot biomass nor to any N-related trait in this study (Table 4). These results differ from those of Hageman and Lambert (1988), Sherrard et al. (1986), and Eck et al. (1975), who found that NRA correlated with either yield or yield components. However, the relationships between NRA and grain yield and N-related traits (shoot N content, grain N content, grain N concentration, and N harvest index) have been highly controversial. This controversy is partly due to the fact that yield is a very complex trait resulting from multiple processes and their interaction and the environment. There was a markedly high level of NRA at vegetative and anthesis growth stages for all genotypes when evaluated at the whole-canopy level. The active accumulation of reduced N during these stages has been pointed out by other researchers as an important factor that is associated with high yields (Swank et al., 1982; Eck and Hageman, 1974). The sustainability of green leaf area toward maturity may also be an important characteristic in providing reduced N and carbohydrates for the grain. When expressing NRA in terms of estimated seasonal N input (g N plant^-1 season^-1), a decreasing seasonal pattern still occurred except for the tropical lines, which increased slightly from the vegetative stage to anthesis. This increase may be due to the increase in total leaf area production of the tropical lines during the same period (results not shown). The same seasonal pattern has been found by Eck et al. (1975), Eilrich and Hageman (1973), Simmons and Moss (1978), and Teare et al. (1974).

View larger version (21K): [in this window] [in a new window]   Fig. 2 Nitrate reductase activity (NRS) for three groups of grain sorghum genotypes expressed as a seasonal N input. Bars represent LSD (0.05) within growth stage from Mead, NE (1994 and 1996)

	Conclusion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Contribution of the Dep. of Agronomy as Paper no. 12383 of the Journal Series of the Nebraska Agric. Res. Div. Research supported in part by USAID Grant no. DAN1254-G-00-0021 through the International Sorghum and Pearl Millet Collaborative Support Program (INTSORMIL).

Received for publication September 21, 1998.

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