Crop Science Grow Your Career with CSSA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 23 February 2005
Published in Crop Sci 45:740-747 (2005)
© 2005 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in Crop Science
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (6)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Subedi, K. D.
Right arrow Articles by Ma, B. L.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Subedi, K. D.
Right arrow Articles by Ma, B. L.
Agricola
Right arrow Articles by Subedi, K. D.
Right arrow Articles by Ma, B. L.
Related Collections
Right arrow Maize Management
Right arrow Maize
Right arrow Maize
Right arrow Plant Nutrition

Nitrogen Uptake and Partitioning in Stay-Green and Leafy Maize Hybrids

K. D. Subedi* and B. L. Ma

Eastern Cereal and Oilseed Research Centre (ECORC), Agriculture and Agri-Food Canada, Central Experimental Farm, K.W. Neatby Building, 960 Carling Avenue, Ottawa, ON, Canada, K1A 0C6

* Corresponding author (subedik{at}agr.gc.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrogen requirement, uptake, and remobilization patterns have been extensively studied for normal maize (Zea mays L.), but there is limited published work for the stay-green (SG) and Leafy hybrids. Under controlled greenhouse conditions, growth, N uptake and partitioning patterns of three contrasting maize hybrids (a conventional, ‘Pioneer 3905’, one bearing the SG trait ‘Pioneer 39F06 Bt’, and one with Leafy trait ‘Maizex LF850-RR’) were investigated. Individual plants grown in 6-L plastic pots were subjected to five different N fertilization regimes: (i) no N supply from seeding to V8, (ii) withholding N supply after V8, (iii) withholding N supply after silking, (iv) withholding N supply from 3 wk after silking to physiological maturity, and (v) continuous N supply from emergence to physiological maturity (control). Leaf chlorophyll content, dry matter, N uptake, and accumulation in different plant parts were measured. The Leafy hybrid had a greater number of leaves and total plant dry matter while kernel yield was similar to that of the other two hybrids. There were no differences in total N uptake and partitioning among the hybrids studied across all five N treatments. The SG hybrid (Pioneer 39F06 Bt) remained green until physiological maturity only when there was a continuous N supply in the growing medium. For all hybrids, N supply was more critical before silking than after silking as limiting N supply reduced ear size, kernel yield and N uptake. Restriction of N supply from seeding to V8 caused an irreparable reduction in ear size and kernel yield (30%). Withholding N supply fromV8 to maturity reduced kernel yield by 22% and N uptake by 53%. There was no yield reduction when N was restricted from silking, or 3 wk after silking to physiological maturity. The results indicate that stay-greenness in maize was exhibited only when there is an adequate supply of N in the growing medium and is not associated with greater N acquisition or remobilization than the conventional hybrids even with full N fertilization.

Abbreviations: DM, dry matter • NUE, nitrogen use efficiency • SG, stay-green


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NITROGEN MANAGEMENT is one of the extensively researched topics in agriculture. Nitrogen fertilizer is applied to maize shortly before planting and partly sidedressed at about V6 to V8 stages (Ritchie et al., 1993). Delayed sidedressing may lead to irreversible yield loss. Binder et al. (2000) reported that delaying N application till the V6 stage resulted in a near 12% reduction in maximum kernel yield. However, the study was based only on the conventional maize hybrids. There is an increased interest in understanding more fully the impact of varied N application during plant development when N is most critical for maximum yield, particularly in the Leafy and SG maize hybrids.

A number of annual cereals exhibit genetic variation for degree and rate of leaf senescence during grain filling (Thomas and Smart, 1993). Thomas and Smart (1993) characterized a stay-green trait (i.e., the phenotypes that exhibit delayed senescence) as having higher water and chlorophyll content in the leaves at maturity. There has been more research on the understanding of the phenomenon of SG trait in sorghum [Sorghum bicolor (L.) Moench] than in maize. In sorghum, Borrell et al. (2000) found that green leaf area at physiological maturity was an excellent indicator of the SG trait. The SG trait was viewed as a consequence of the balance between N-demand by the kernel and N-supply during the kernel filling (Borrell et al., 2001). Delay in senescence in sorghum is considered a valuable trait, as it improves genotype adaptation to post-flowering drought stress (Mahalakshmi and Bidinger, 2002). The stay-green mutation of the nuclear gene sid results in inhibition of chlorophyll degradation during leaf senescence in Lolium perenne L., thereby reducing N remobilization from senescing leaves (MacDuff et al., 2002).

Expression of the SG trait has been reported in maize (Tollenaar and Daynard, 1978; Ma and Dwyer, 1998; Rajcan and Tollenaar, 1999a, 1999b). However, there is a limited understanding of the physiological processes underlying this trait. Senescing during kernel filling is related to the quantity of light received by the leaves and N availability via remobilization to actively growing kernels of maize (Borras et al., 2003). Rajcan and Tollenaar (1999b) proposed that leaf senescence in a recent maize hybrid was delayed because of an improvement in the ratio of assimilate supply (i.e., source) to assimilate demand (i.e., sink) during kernel filling. They also found that total N uptake in aboveground portions were 10 and 18% greater in the SG hybrid than an older hybrid under low and high soil N conditions, respectively.

There are two sources of N for kernel development: absorbed N from soil and remobilized N from vegetative tissue (Ta and Weiland, 1992). Previous field studies show that the SG types had greater N uptakes than the conventional hybrids (Ma and Dwyer, 1998; Rajcan and Tollenaar, 1999b; Borrell et al., 2001). Borrell and Hammer (2000) found that N uptake from soil during grain filling averaged 116 kg in SG sorghum but only 82 kg ha–1 in non-SG hybrids grown in a terminal water deficit condition. Since the longevity and photosynthetic capacity of a leaf are related to its N status, it is important to understand the role of N in extending leaf greenness in SG maize. Nitrogen uptake and utilization varies considerably among conventional maize hybrids (Beauchamp et al., 1976, Chevalier and Schrader, 1977; Ma and Dwyer, 1998; Bertin and Gallais, 2000). McCullough et al. (1994) observed higher N uptake in a new hybrid than in an older (Pride 5) hybrid under limited N supply condition. However, the study was terminated at the V12 growth stage, and the total N uptake pattern and partitioning in different plant parts were not studied. In a field study, Ma and Dwyer (1998) observed that the SG hybrid (Pioneer 3902) had greater nitrogen use efficiency (NUE) than an early senescing hybrid (Pride 5). However, it is not clear whether or not under limited N supply in soil, the conventional hybrids senescent at which the SG hybrids still remain green. Similarly, it is not clear whether the higher NUE in SG hybrids is because they take up N for a longer period of time than the conventional types or whether remobilized N is more important for greater NUE. In this context, our hypothesis is that the SG trait is expressed in hybrids even when N is limited in the growing medium.

Leafy hybrids have recently gained popularity as silage maize (Roth, 2003). These hybrids have greater number of leaves and leaf area than their conventional counterparts (Shaver, 1983; Subedi and Ma, 2004). Because of the heavy foliage and higher biomass (Andrews et al., 2000; Subedi and Ma, 2004), the N requirement of Leafy hybrids may be greater than the conventional hybrids. However, in a field experiment, Costa et al. (2002) observed that Leafy genotypes did not require more N fertilization compared with conventional hybrids.

The effect of N fertilization on conventional maize hybrids has been extensively examined, but there is limited research on N use in Leafy and SG types. The objectives of this experiment were to: (i) assess whether or not Leafy, early senescing, and SG maize hybrids differ in critical periods of N uptake and partitioning; (ii) determine how long during the growing period maize is responsive to N supply; and (iii) monitor hybrid differences in SG characteristics within a given amount of N supply. An experiment under greenhouse condition with controlled N supply examined these objectives.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was conducted with a controlled environment and nutrition systems in a greenhouse. Plastic pot (22-cm diameter and 20 cm high) were filled with soil, peat moss, vermiculite and perlite mixture (3:1.5:1:1 v/v), which had a total N of 0.14%. Three maize seeds of uniform size were planted per pot on 11 June 2003. Plants were grown at 25/15°C day/night temperature regimes with a 16-h photoperiod. The temperatures in the greenhouse were maintained with automated air-conditioning system. The roof of the greenhouse was white washed to improve cooling during the hot summer days and to diffuse light uniformly. However, the day-time temperature in some hot days reached to as high as 31°C. Supplemental light was provided with fluorescent lamp to ensure adequate light (>1000 MJ m–2 d–1) during the cloudy periods. The relative humidity inside the greenhouse ranged between 65 to 90% during the growing season.

At three ligule stage (V3), seedlings were thinned to one plant per pot. Pots were manually watered with a measured volume of modified Hoagland nutrient solution (100 mg L–1 N) according to the particular treatment requirement (Table 1) twice a week initially to every other day at silking to physiological maturity. In the treatments restricting N supply, N-free nutrient solution containing all other nutrients similar to Hoagland solution was applied. Additional N was supplied with fertilizer NH4NO3 (34% N) at 3 g pot–1 V8 for N1, N2 and N5, at silking for N3 and 3 wk after silking for N4. Care was taken to avoid leaching and/or spillover of the applied nutrient solutions from the pot. Occasionally, nutrient solution for each treatment was supplemented with tap water, especially in hot and sunny days exceeding 25°C. Timing and amounts of total N applied in each treatment are given in Table 1.


View this table:
[in this window]
[in a new window]

  		Table 1. Timing, sources and amounts of nitrogen supplied in each treatment (N1, N supplied from V8 to maturity; N2, N supplied from seedling emergence to V8; N3, N supplied from seedling emergence to silking; N4, N supplied from seedling emergence to 3 wk after silking; and N5 (control), N supplied from seedling emergence to physiological maturity).

 
A 3 x 5 factorial experiment, arranged in a completely randomized design with four replications for a total of 60 pots, was conducted in a controlled environment greenhouse in Ottawa, Canada. The pots were further randomized within the greenhouse on a weekly basis to minimize any variations inside the greenhouse. The factors tested were three contrasting maize hybrids: Pioneer 3905 (a conventional hybrid with moderate SG), Pioneer 39F06 Bt (a near isoline of Pioneer 3905 with high SG property), and Maizex LF850 RR (a Leafy structure). All of the three hybrids have similar (≤2700) Crop Heat Unit (CHU; Brown and Bootsma, 1993) requirements. The N treatments were: N from V8 to physiological maturity (N1), N from seedling emergence to V8 (N2), N from seedling emergence to silking (N3), N from seedling emergence to 3 wk after silking (N4), and full N supply (continuously from emergence to physiological maturity, N5; control).

Phenological events (time taken to reach V8, silking and physiological maturity) were recorded. At V8 (before the imposition of N treatments), leaf greenness was measured on the eighth leaf with a chlorophyll meter (SPAD–502 Chlorophyll Meter, Minolta Camera Co. Ltd., Japan). The SPAD readings were then taken an average of three points (basal, mid and tip 1/3 portions) in the ear-leaf at silk stage, and at weekly intervals thereafter until physiological maturity. At physiological maturity, total number of leaves per plant and numbers of leaves below and above the ear and plant height were recorded. Ear length, ear diameter (after husk removal), and number of kernels per ear were recorded. Each plant was then separated into kernels, leaves, stalk, and root components. Husks were added to leaves while tassels and cobs were included with the stalk. Roots were separated from the growing medium and washed thoroughly with tap water. Samples were oven dried at 80°C for >72 h. Dry matter (DM) of each component was recorded and samples were ground to pass 1-mm screen. Nitrogen concentration in each sample was determined by automated dry combustion-gas chromatography with a Carlo-Erba T1500 Elemental Analyzer (Carlo Erba, Milan, Italy). Nitrogen content was calculated as the product of dry matter (g) and N concentration (%) in each plant part.

The experimental data were subjected to analysis of variance using the general linear model (SAS Inst., 1996). Treatment mean differences were separated by the least significant difference (LSD0.05) test if F tests were significant (P < 0.05).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Response of Hybrids
There were no significant hybrid x nitrogen interactions for any of the variables measured except SPAD readings at V8 and 4 wk after silking (Table 2). The two near isoline hybrids (Pioneer 3905 and Pioneer 39F06 Bt) were similar in all of the phenological and morphological parameters measured (Table 3). On the other hand, the Leafy hybrid (Maizex LF850 RR) was different compared with the non-Leafy hybrids in all parameters except for ear length and kernel DM. The Leafy hybrid required 2 to 3 more days to reach V8 and silking than the other two hybrids, but all hybrids matured at the same time. There were four additional leaves in the Leafy hybrid than in the other two hybrids, and 50% of the total leaves were above the ear compared with only 30% in the conventional hybrids. The Leafy hybrid was taller, had greater ear diameter, and set more kernels per plant than the other two hybrids. Despite greater number of kernels, mean kernel weight of the Leafy hybrid was less than that of the two conventional hybrids resulting in no differences in kernel yield among the three hybrids.


View this table:
[in this window]
[in a new window]

  		Table 2. Probabilities for the main effects and interactions between hybrid and nitrogen treatment for various morphological parameters, yield components, nitrogen concentrations and contents in different plant parts and leaf chlorophyll content (SPAD readings) at different growth stages of maize.

 

View this table:
[in this window]
[in a new window]

  		Table 3. Phenological stages, morphological parameters, yield components and harvest index of conventional (Pioneer 3905), stay green (Pioneer 39F06 Bt) and Leafy (Maizex LF 850 RR) maize hybrids grown under five N treatments averaged across five nitrogen treatments.

 
The phenology and morphology of the pot-grown plants of all three hybrids were similar to field grown plants. Total DM of the control plants (N5) were similar to the DM of individual plants reported in field experiments (Weiland and Ta, 1992; Ta and Weiland, 1992; Subedi and Ma, 2004). The dry matter allocation to different plant parts is shown in Table 4. Except for kernel DM, the Leafy hybrid had significantly higher DM in root, stalk, leaf and the entire plant than the other hybrids, which were similar. Because of greater overall biomass relative to kernel weight, the Leafy hybrid had smaller harvest index, which agrees with the findings reported under field conditions by Andrews et al. (2000) and Subedi and Ma (2004).


View this table:
[in this window]
[in a new window]

  		Table 4. Total dry matter (DM, g plant–1), nitrogen concentration (NC, %) and N content (g) in different plant parts or in the whole plant of conventional (Pioneer 3905), stay green (Pioneer 39F06 Bt) and Leafy (Maizex LF 850 RR) maize hybrids, averaged over five nitrogen treatments.

 
Nitrogen Response
Nitrogen treatment had no effect on time taken to reach V8 and silking, but there was a significant effect on time to physiological maturity. All hybrids, treated by withholding N after V8 (N2) matured significantly earlier (112 d) than other N treatments (115 d). There was no effect of N treatment on the number of leaves, plant height, and ear height (Table 2). However, significant effects of N were observed on ear length, ear diameter, and number of kernels per ear (Fig. 1) . When N was supplied only after V8 (N1), the ear diameter and the number of kernels per ear were significantly reduced in all hybrids. Although the number of kernels in maize has been shown to be a function of physiological conditions of the crop at critical period bracketing silking (Andrade et al., 1999), the restriction of N during the ear differentiation stage (i.e., before V8) caused an irreversible reduction of ear size and kernel number and reduced kernel yield by 30%. This clearly indicates that N supply at early growth stages before V8 is critical for determining ear size. Early season N stress effects on ear size have also been reported for field-grown maize grown without starter N application (Binder et al., 2000; Scharf et al., 2002).


Figure 1
View larger version (17K):
[in this window]
[in a new window]

  		Fig. 1. Effect of different nitrogen treatments on ear diameter (cm), ear length (cm), and number of kernels per ear averaged over three hybrids of maize (Pioneer 3905, Pioneer 39F06 Bt, and Maizex LF850 RR). The nitrogen treatments are N1, N supplied from V8 to maturity; N2, N supplied from seedling emergence to V8; N3, N supplied from seedling emergence to silking; N4, N supplied from seedling emergence to 3 wk after silking; and N5 (control), N supplied from seedling emergence to physiological maturity. The bars with the same letter within each parameter are not statistically significant at P < 0.05.

 
When N was supplied only up to V8 (N1), ear size and kernel number was significantly lower than the control. There was some compensation with bigger size of individual kernels, but the mean kernel mass was reduced by 22%. After N restriction at V8, increased demand of N for kernel growth resulted in early leaf senescence for all hybrids including the SG hybrid. Interestingly, the restriction of N supply at silking (N3) had no significant reduction in yield components compared with N4 and N5, because remobilization of the reserve N in the plant and uptake of residual N from the growing medium might have supported the further growth after N restriction at silking.

Nitrogen treatment had significant effects on the DM of all the plant parts and the entire plant (Fig. 2) . Except for the leaf, all the components DM were significantly lower in N1 and N2 than the other N treatments. Leaf DM was significantly lower in the N2 compared with the N3 treatment, while all other treatments had similar DM.


Figure 2
View larger version (29K):
[in this window]
[in a new window]

  		Fig. 2. Partitioning of dry matter (g) in different parts of a maize plant as influenced by nitrogen treatments (N1, N supplied from V8 to maturity; N2, N supplied from seedling emergence to V8; N3, N supplied from seedling emergence to silking; N4, N supplied from seedling emergence to 3 wk after silking; and N5 (control), N supplied from seedling emergence to physiological maturity). The values are averaged over three hybrids, and the bars with the same letter within each plant part are not statistically significant at P < 0.05.

 
Nitrogen Uptake and Partitioning
There was no hybrid x N treatment interaction on the N concentration and content in different parts of maize plant (Table 2). Hybrids differed in N concentration in roots, leaves, and kernels (Table 4). The Leafy hybrid had significantly lower N concentrations in root and leaves than the SG and conventional hybrids. The SG hybrid had lower N concentration in kernels than its conventional counterpart (Pioneer 3893). The lower N concentrations in the roots and leaves of the Leafy hybrid were associated with significantly greater biomass in these components (Table 4). The marginal difference in N concentrations in various plant parts were eliminated when the whole plant N concentration was considered.

Bruns and Abel (2003) proposed that Bt hybrids may differ in protein synthesis compared with non-Bt hybrids because they found an increased N concentration in the whole plant at V5 of Bt hybrid with increased supply of N. They also reported that the concentration of {delta}-endotoxin increased with increasing N rates. We did not find a difference in total N uptake and kernel N content between the Bt (Pioneer 39F06) and non-Bt (Pioneer 3905) near isoline hybrids. Ma et al. (2005) also could not trace any difference in whole plant N uptake between the two near isoline hybrids (Pioneer 3905 and Pioneer 39F06 Bt) when grown in the field for three consecutive seasons.

There were no significant differences among the hybrids for N content in the stalk, leaves, and kernels (Table 4). The Leafy hybrid accumulated more N in the roots than Pioneer 3905 because of significantly higher root DM. Irrespective of N treatment, there was no significant difference among hybrids for total N acquisition on a whole plant basis despite the hybrids differed in their stay-greenness at physiological maturity. This indicates that the SG hybrids do not require more N than the conventional hybrids to remain green at maturity. This result is in contrast to previous field studies (Ma and Dwyer, 1998; Rajcan and Tollennar, 1999b; Borrell and Hammer, 2000; Borrell et al., 2001) indicating that the SG hybrid had greater total N uptake, although the SG hybrids used in these studies were different than the one used in our study (Pioneer 39F06). However, in terms of kernel N content, Ma and Dwyer (1998) found no indications of greater N allocation to the kernels of the SG hybrid. The greater total N accumulation by SG hybrids in the field conditions was possibly due to the fact that they remained green after physiological maturity and would have taken N for a longer period of time than the early senescing ones, if they were harvested at different times or waited until all plants senesced completely. Within each N treatment, all hybrids had similar amounts of N uptake. This result also contrasted with other reports that N uptake and utilization varies with maize genotypes (Beauchamp et al., 1976; Chevalier and Scharder, 1977; Bertin and Gallais, 2000), possibly due to the fact that the number, productivity level and maturity range of hybrids used in this experiment was small. The findings of this experiment agree with those of Costa et al. (2002) indicating that Leafy hybrids do not require more N fertilizer compared with conventional hybrids.

Nitrogen treatment had a highly significant effect on N concentration in each component individually or the entire plant (Fig. 3) . In all plant parts, the N1 and N5 treatments resulted in significantly greater N concentration, while the N2 treatment resulted in the lowest N concentrations. In all cases, the N concentrations in the N3 and N4 treatments were similar. The N1 treatment recovered its N concentrations in different parts as it continued to accumulate N until maturity. It appeared that N uptake in N1 and N5 treatments continued after N was restricted at 3 wk after silking N4, resulting in higher N concentrations compared with the N4 treatment.


Figure 3
View larger version (30K):
[in this window]
[in a new window]

  		Fig. 3. Effect of nitrogen treatments on the nitrogen concentration (%) in different parts of a maize plant (g) averaged over three hybrids. The nitrogen treatments are N1, N supplied from V8 to maturity; N2, N supplied from seedling emergence to V8; N3, N supplied from seedling emergence to silking; N4, N supplied from seedling emergence to 3 wk after silking; and N5 (control), N supplied from seedling emergence to physiological maturity. The bars with the same letter within each plant part are not statistically significant at P < 0.05.

 
Nitrogen content in different parts of the maize plant was significantly affected by N treatment. In the entire plant, N restricted at V8 (N2) had the lowest N accumulation (53% of total N content in N5). Although the N1 treatment had comparable N concentrations to N5 (Fig. 3), total N uptake was lower by 27% than in N5 because of significantly lower biomass in kernels, roots, and stalk. The sink size (i.e., ear) in the initially N-deficient plants (N1) did not recover after continuous N supply from V8. This resulted in a significantly reduced kernel yield than in N5. As a result, N concentration in the kernel of N1 was high, but because of the lower total DM, there was a significant difference in total N uptake between N1 and N5.

There was no difference in kernel N content among N3, N4, and N5 treatments (Fig. 4) . On a whole plant basis, N accumulation in the N3 treatment was 27% lower than in the N5 treatment (2.41 vs. 3.31 g plant–1), mainly attributed to lower N contents in the roots, stalk, and leaves. Although there was no difference in kernel N content, the total N content in stalk and leaves were increased by 69 and 15%, respectively, between N3 to N5 treatments (i.e., change in N content from silking to physiological maturity). On a whole plant basis, about 17% more N was accumulated after silking (N3 vs. N5), but most of this N remained in the stalk (Fig. 4).


Figure 4
View larger version (30K):
[in this window]
[in a new window]

  		Fig. 4. Effect of nitrogen treatments on the total nitrogen uptake (g) in different parts of a maize plant averaged over three hybrids. The nitrogen treatments are N1, N supplied from V8 to maturity; N2, N supplied from seedling emergence to V8; N3, N supplied from seedling emergence to silking; N4, N supplied from seedling emergence to 3 wk after silking; and N5 (control), N supplied from seedling emergence to physiological maturity. The bars with the same letter within each plant part are not statistically significant at P < 0.05.

 
There was no significant difference in kernel N and total N contents between the N4 and N5 treatments. These two treatments differed in stalk N content, which was increased by 37.5% (0.32 vs. 0.44 g) from 3 wk after silking (N4) to maturity (N5). This indicates that uptake of N by maize plants was continuous even 3 wk after silking, but most of this N was concentrated in the stalk.

The percentage reduction of N content in different parts of the maize plant relative to the control treatment (N5) is shown in Fig. 5 . It was found that the overall reduction of N content was at a maximum in the N2 treatment followed by the N1 and N3 treatments. It is interesting to note that in the N2 and N3 treatments, the reduction in N content compared with the control treatment from stalk and root were much more severe than in the leaf and kernel, indicating that N was remobilized from the stalk and root reserves to meet the demand of the kernel when supply was restricted in the growing medium. The N concentrations and contents in the kernels of N3 and N4 treatments were similar to N5, but N in the stalk and leaf differed significantly among the N3, N4, and N5 treatments. This indicates that when there was a shortage of N in the growing medium, N, primarily from the root and stalks, was remobilized for kernel growth. Bertin and Gallais (2000) also observed that the reduction of N content in an unfertilized N treatment was greater in the stalk (39%) than in kernels (18%). Since there were no hybrid x N treatment interactions for N concentration or N uptake (Table 2), these findings do not support our hypothesis that the SG hybrid takes up more N or remobilize its previous uptake to maintain greenness. MacDuff et al. (2002) also found no difference in rates of dry matter production and partitioning between shoots and roots, and repartitioning of N from shoots to roots between the SG and normal plants of Lolium perenne.


Figure 5
View larger version (33K):
[in this window]
[in a new window]

  		Fig. 5. Percentage reduction in nitrogen content in various parts of a maize plant due to different nitrogen treatments, as compared to the control treatment (N5). The nitrogen treatments are N1, N supplied from V8 to maturity; N2, N supplied from seedling emergence to V8; N3, N supplied from seedling emergence to silking; N4, N supplied from seedling emergence to 3 wk after silking; and N5 (control), N supplied from seedling emergence to physiological maturity.

 
Nitrogen Supply and Stay-Greenness
Leaf greenness as measured by the SPAD revealed that irrespective of the N treatment, the maximum SPAD values were recorded at silk stage (Fig. 6) . For all N treatments, the Leafy hybrid had consistently lower SPAD readings at all growth stages except at silking. The other two hybrids had similar leaf chlorophyll content. There was a hybrid x N interaction for leaf chlorophyll content at the V8 growth stage. The Leafy hybrids had significantly lower SPAD values in the N1 treatment compared with the other treatments, but SG hybrid had similar values in all N treatments. This indicates that the SG hybrid maintained greenness in the absence of N at an early stage at which other hybrids did not. Nevertheless, after 4 wk of silking, SPAD readings in the N2 treatment of the SG hybrid were significantly lower than in conventional hybrid, and the ear leaf senesced earlier (Fig. 6). The lower amount of N available in the plant was remobilized to kernel growth, and the leaves senesced quicker even in the SG hybrid in the N2 treatment. When there was an adequate N supply (N5), the ear-leaf and all leaves above the ear maintained greenness until physiological maturity in the SG hybrid. This clearly indicates that the SG trait is expressed only when there is adequate N supply in the growing medium. In the Leafy hybrid, even in the N5, ear-leaf dried earlier while other top leaves were green possibly because the ear-leaf was emerged earlier as it was positioned at a lower node than in the other hybrids. The hypothesis tested here was that under limited N-supply, the SG hybrid exhibits its character to remain green has been rejected. This finding is also in agreement with Rajcan and Tollenaar (1999a) who found that leaf longevity was enhanced by an increase in soil N supply. Borrell et al. (2001) suggested that roots of the SG sorghum maintain greater capacity to extract N from the soil compared with the non-SG hybrids during kernel filling. This finding was not evident in our study. However, the current study clearly demonstrated that N is constantly taken up beyond 3 wk after silking by all maize types, but the role of N was more important before silking especially for yield component formation. Nevertheless, the growing conditions in the field may be different than the greenhouse because of less restricted root growth and proliferation, variations in light quality, variability in soil NO3–N releasing patterns and availability of soil moisture, all of which might change the responses such as N uptake and partitioning.


Figure 6
View larger version (23K):
[in this window]
[in a new window]

  		Fig. 6. SPAD readings at V8, silking, 2 wk after silking and on a weekly intervals thereafter until physiological maturity in conventional (Pioneer 3905), stay green (Pioneer 39F06 Bt), and Leafy (Maizex LF 850 RR) maize hybrids. The vertical bars are the LSD 0.05 values.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Three maize hybrids contrasting in leaf number and stay-greenness grown in a precisely controlled N supply system did not differ in N acquisition, partitioning, and remobilization to different plant parts at physiological maturity. The SG hybrid remained green at physiological maturity only when there was an unrestricted N supply, indicating that the SG is a trait that is exhibited only under adequate N available conditions. Although the limitation of N in the growing medium after V8 significantly reduced yield components, kernel yield, and N uptake, the restriction of N up to V8 caused greater yield reduction, reflected in reduced ear size and number of kernels. It was also observed that when N supply was unrestricted, all maize types took up N continuously beyond 3 wk after silking. However, the N taken up at this later stage was accumulated mainly in the stalk. Plants adequately supplied with N until silking were able to maintain kernel yield and kernel N concentrations similar to plants continuously supplied with N throughout, although total N uptake was significantly lower by 17%. This study adds new information on critically N responsive stages of maize, N uptake, and partitioning behavior of contrasting maize hybrids and N response to stay-greenness. This information can be used for the decisions concerning early N amendment if maize is grown without starter N fertilizer, and on the selections of hybrids for different purposes. Similarly, the results can be useful in need-based N application, thereby reducing potential N loss and nonpoint pollution.


   ACKNOWLEDGMENTS
 
The authors highly appreciate the excellent technical assistance provided by Ian Smith, L. Evenson and V. Deslauriers. ECORC Contribution No: 04-409.

Received for publication June 14, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


Related articles in Crop Science:

THIS ISSUE IN CROP SCIENCE

Crop Science 2005 45: vi. [Full Text]  



This article has been cited by other articles:


Home page
 J Exp BotHome page
B. Hirel, J. Le Gouis, B. Ney, and A. Gallais
The challenge of improving nitrogen use efficiency in crop plants: towards a more central role for genetic variability and quantitative genetics within integrated approaches
J. Exp. Bot., July 1, 2007; 58(9): 2369 - 2387.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
K. D. Subedi and B. L. Ma
Dry Matter and Nitrogen Partitioning Patterns in Bt and Non-Bt Near-Isoline Maize Hybrids
Crop Sci., May 31, 2007; 47(3): 1186 - 1192.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
K. D. Subedi, B. L. Ma, and D. L. Smith
Response of a Leafy and Non-Leafy Maize Hybrid to Population Densities and Fertilizer Nitrogen Levels
Crop Sci., July 25, 2006; 46(5): 1860 - 1869.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
B. L. Ma, K. D. Subedi, D. W. Stewart, and L. M. Dwyer
Dry Matter Accumulation and Silage Moisture Changes after Silking in Leafy and Dual-Purpose Corn Hybrids
Agron. J., June 5, 2006; 98(4): 922 - 929.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
K. D. Subedi and B. L. Ma
Ear Position, Leaf Area, and Contribution of Individual Leaves to Grain Yield in Conventional and Leafy Maize Hybrids
Crop Sci., September 23, 2005; 45(6): 2246 - 2257.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in Crop Science
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (6)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Subedi, K. D.
Right arrow Articles by Ma, B. L.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Subedi, K. D.
Right arrow Articles by Ma, B. L.
Agricola
Right arrow Articles by Subedi, K. D.
Right arrow Articles by Ma, B. L.
Related Collections
Right arrow Maize Management
Right arrow Maize
Right arrow Maize
Right arrow Plant Nutrition

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome