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Research Article
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Yielding Ability and Nitrogen Use Efficiency in Maize Inbred Lines and Their Crosses
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Manal M. Hefny
and
A.A. Aly
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ABSTRACT
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A field experiment was carried out to study the grain
yield and Nitrogen Use Efficiency (NUE) components of 16 inbred lines
of exotic yellow maize and their crosses. The experiment was cropped at
two nitrogen fertilizers; low (70 kg f-1) and high (140 kg
f-1), split-plot design was used. The results indicated that,
all measured traits were affected significantly by N levels, genotypes
and the interaction, except days to 50% tasseling and silking of inbred
lines which were not affected by N levels and the N x genotype interaction.
N deficiency caused delay in flowering time for male and female inflorescence,
accelerated leaf senescence, reduced total dry matter production, N-uptake
by plants, grain yield components and grain protein percentage. On the
other hand, nitrogen use efficiency for dry matter and grain production
and nitrogen harvest index were increased under limited soil N. Inbred
lines showed severe reduction for the above variables as compared to crosses.
The inbred lines 4, 9, 13 and 15 were distinguished for their superiority
in grain yield, nitrogen harvest index, harvest index, nitrogen use efficiency
for grain, N-uptake and protein percentage. Three lines, 13, 15 and 16
were the earliest in flowering and represented the highest stay green
percentage. While, the inbred lines 1, 8 and 14 were the most N-inefficient
for grain production and the lowest for grain yield. In relation to crosses,
high nitrogen harvest index, harvest index and nitrogen use efficiency
for grain were shown by the crosses (4x1), (8x7), (9x10), (9x12) and (13x15).
The crosses (4x1), (9x10), (13x15) and (13x16) surpassed the check and
recorded the highest grain yield. The highest stay green percentage was
revealed by the single cross Pioneer 3062 followed by the crosses (4x1),
(13x15) and (13x16). It is recommended to use the inbred lines; 4, 9,
13 and 15, as a N-efficient source for further studies, whereas using
the crosses; (4x1), (8x7), (9x10), (9x12) and (13x15) as N-efficient hybrids
for under N limited cultivation. Phenotypic correlation coefficients were
higher at low N compared with high application rate. High grain yield
was significantly associated with delayed leaf senescence, nitrogen harvest
index, harvest index, nitrogen use efficiency for grain and yield plant-1.
High nitrogen use efficiency for grain production correlated positively
with high yield, yield plant-1, NHI and HI. N-uptake was found
to be a function of growth rate at both levels of N fertilizer.
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INTRODUCTION
Maize is one of the three main cereal staples, which has to meet an increasing
demand for food and feed in the developing world (Cassman et al.,
2002). There are important agronomic, economic and ecological reasons
for which the primary models of cereal production appear to change for
more sustainable agriculture and/or more optimal input technology (El
Bassam et al., 1990). Such reasons make search for more efficient
cultivars adapted to less favorable nutrition an important breeding task.
D`Andrea et al. (2006) stated that, today, the largest investments
in maize breeding are made by the private sectors, where the whole selection
process (i.e., from early inbred line development to commercial hybrids)
takes place in the absence of N restriction. Consequently, the industry
does not find it profitable to develop genotypes for areas or markets
with low economic return, such as low soil fertility. This may lead to
loss of some adaptive traits to these environments.
Growth is a physiological trait associated with grain yield increases
in maize plants, moreover is a function of environmental factors (temperature
and solar radiation) and mineral nutrition, along with genotype and production
practices (Maman et al., 1999).
Worldwide, nitrogen, together with phosphorus is one of the macronutrients
that are most limiting to maize grain yield (D`Andrea et al., 2006).
The results on the accumulation patterns and redistribution of N-nitrate
in maize plants showed that although maize can absorb substantial quantities
of N following anthesis and mobilization of vegetative N accumulated before
anthesis provides the major source of N in the grain (Di Fonzo et al.,
1982). Grain maize NUE is defined as the grain yield per unit of nitrogen
available from the soil, including nitrogen fertilizer. The genetic variability
and genotype x nitrogen fertilization level interactions for NUE reflect
differences in responsiveness have been observed in many studies on maize
(Bertin and Gallais, 2000). Accordingly, they suggested that the limiting
steps in N-assimilation may be different when plants are grown under different
levels of N fertilizers. The post-anthesis N-uptake in grain filling can
be related to leaf senescence, because it prolongs the capacity of the
plant to absorb mineral N (Racjan and Tollenaar, 1999). The high cost
and energy-intensive production of nitrogenous fertilizers and the pollution
that resulted from their excessive use necessitate the identification
of alternative ways to lessen the dependence on high N inputs (Singh and
Arora, 2001). These objectives can be met through efficient farming techniques,
but also by using plant varieties that have better Nitrogen Use Efficiency
(NUE) (Gallais and Hirel, 2004). Lafitte et al. (1995) suggested
that a further progress for low N environments may achieved by selecting
N efficient genotypes. So the present study is designed with the aims
to: (I) identify new sources for nitrogen use efficiency from exotic materials
(ii) compare yielding ability and nitrogen use efficiency related-traits
in the parents and the resulting crosses and (iii) describe the relationships
between yield and nitrogen use efficiency for grain production and the
other tested variables.
MATERIALS AND METHODS
Twelve single-crosses were developed in the 2005 cropping season by crossing
sixteen yellow maize inbred lines in a hierarchical mating design (NCD
I) of Comstock and Robinson (1948). The inbred lines (Table
1) were introduced from National Plant Germplasm System, USA. The
inbred lines were classified into two sets, where each set comprise two
male inbred lines crossed to three female inbred lines. One check single
cross Pioneer 3062 was included for comparison. The 16 inbred lines and
their 12 crosses were sown on 1 May 2006 at the experimental farm of Suez
Canal University, Faculty of Agriculture, Ismailia, Egypt. The result
of physical and chemical characteristics of experiment`s site showed the
values of coarse sand, fine sand, silt and clay are found to be 89.9,
5.7, 2.7 and 1.7%, respectively.
Two levels of nitrogen fertilizer; 70 kg feddan-1 (LN) and
140 kg feddan-1 (HN) (1 ha = 2.4 feddan) were added as ammonium
nitrate. Nitrogen was added as single application 3 weeks after planting
for the LN treatment, whereas for HN treatment two equal doses were added,
3 weeks after planting and at the beginning of silking. The experiment
was laid out in a split-plot design with three replications. The N levels
allocated to the main plots, whereas genotypes (12 crosses and 16 parents)
distributed in sub-plots. Each genotype (cross and inbred line) was planted
in one row, 3 m long, 0.50 m apart and 0.25 m within row.
Table 1: |
List of maize inbred lines and their F1 crosses
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IL: Inbred Line |
Data were recorded on days to 50% tasseling and silking as 50% of the
plants in the plot presented their anthers and silks. Stay green (indication
of leaf senescence) was determined for each plot by visually assessing
the degree of green leaves at 3 wk before harvest. Total Dry matter (TDM)
(g m-2) was determined by multiplying total aboveground fresh
weight at maturity (stem + leaves) and percentage dry matter of a sub-sample
after drying at 70°C for 2 days, Grain yield (GY) (g m-2),
was determined by harvesting the ears from the sample area, shelled and
weighed. Harvest index (HI), was estimated as the proportion of grain
weight to the biological yield (above ground biomass including grains).
Yield per plant was determined by harvesting five individual plants,
hand-shelled and grain weighing and Kernel weight was taken on 100 grains,For
protein percentage determination, plant parts (grains and aboveground
biomass) were ground to pass through a 1 mm sieve for N percentage determination
using Kjeldahl procedure (AOAC, 1990), then N percentage was multiplied
by 6.25 (Oikeh et al., 1998) for protein percentage calculation.
The following parameters were calculated to estimate Nitrogen Use Efficiency
(NUE) (Maranville et al., 1980):
Nitrogen uptake (g N m-2) = TDMxN
concentration. |
Nitrogen use efficiency for biomass accumulation (NUEb) =
TDM (g)/g N uptake in the aboveground biomass, Nitrogen use efficiency
for grain yield (NUEg) = g grain/g N uptake in the aboveground
biomass.
Nitrogen harvest index (NHI) was estimated according to Koutroubas and
Ntanos (2003) = Ng/Nt
Nt |
: |
Total aboveground N and |
Ng |
: |
Grain N. |
The statistical analyses were performed according to Steel and Torrie
(1980) using GenStat Software Package, Release 4.24 to estimate the significance
effect of N levels, genotypes and their interactions. LSD values were
calculated and used to compare treatment means.
RESULTS AND DISCUSSION
Flowering Dynamics, Stay Green, Biomass Production and N-Uptake
The analysis of variance (Table 2 and 3)
indicated that days to 50% tasseling and silking showed significant differences
among inbred lines. In case of crosses, days to 50% silking responded
significantly with respect to applied nitrogen, crosses and the interactions,
whereas days to 50% tasseling were influenced significantly by crosses
and nitrogen levels. The overall means for the previous traits are presented
in Table 4-9. The limited nitrogen fertilizer did increase
the days required to reach flowering phase for inbreds and crosses. Under
N stress, plants were earlier in days to 50% silking than days to 50%
tasseling. This in turn resulted in an increase in Anthesis-Silking Interval
(ASI) (5.93 vs 4.82 days for inbreds and crosses, respectively).
Table 2: |
Mean squares of 16 yellow maize inbred lines cropped
at two levels of nitrogen (70 and 140 kg N f-1) |
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*, Significant at 0.05 levels of probability, MS: Mean
squares |
Table 3: |
Mean squares of 13 yellow maize single crosses cropped
at two levels of nitrogen (70 and 140 kg N f-1) |
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*: Significant at 0.05 levels of probability, MS: Mean
squares |
Table 4: |
Performance of 16 yellow maize inbred lines cropped
at two levels of nitrogen fertilizer (70 and 140 kg N f-1*)
for flowering, stay green, total dry matter and N-uptake traits |
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LN: Low Nitrogen, HN: High Nitrogen and X: Mean |
Table 5: |
Performance of 16 yellow maize inbred lines cropped
at two levels of nitrogen fertilizer (70 and 140 kg N f-1*)
for grain yield and its components |
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LN: Low Nitrogen, HN: High Nitrogen and X–:
Mean |
The inbred lines 16 and 13 were among the earliest for anthesis and silking
and the inbreds 1 and 3 were the most delayed. The crosses (13x16) and
(13x15) followed by the cross (9x11) were the earliest for anthesis and
silking. On the other hand the crosses SC Pioneer 3062 and 4x3 were the
most delayed. Regarding crosses, a difference of 8 (for anthesis) and
12 days (for silking) was recorded in time to flowering between the early
and late groups. For inbreds, there was 11 days difference between the
same groups. A set of 12 inbred lines were evaluated under field conditions
at two N levels, 0 and 400 kg N ha-1 by D`Andrea et al.
(2006), the results recorded an increase in thermal time required to anthesis
and silking at reduced level of N. Also, the delay in anthesis was less
than in silking which resulted in increasing in ASI but this was not common
to all inbreds. The same authors registered 7-10 days differences in flowering
time between the early and late flowering groups. Lafitte and Edmeades
(1995) confirmed that, increases in ASI is raised when maize plants are
subjected to various stresses such as drought or N deficiency.
Table 6: |
Performance of 16 yellow maize inbred lines cropped
at two levels of nitrogen fertilizer (70 and 140 kg N f -1)
for NUE traits |
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LN: Low Nitrogen, HN: High Nitrogen and X–:
Mean |
Table 7: |
Performance of 13 yellow maize crosses cropped at two
levels of nitrogen fertilizer (70 and 140 kg N f-1) for
flowering, stay green, total dry matter and N uptake traits |
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LN: Low Nitrogen, HN: High Nitrogen and X–:
Mean |
Stay green trait was influenced significantly by applied nitrogen treatments,
genotypes (lines and crosses) and their interaction. Similar results were
obtained by many authors when maize plants were cultivated under a range
of soil N fertilizers. Gungula et al. (2005) found significant
differences between N rates (0 to 120 kg N ha-1) and the tested
varieties for leaf senescence percentage which an indicator of the effect
of soil N on greenness. The reduction in N availability encouraged leaf
senescence as shown in Table 4 and 7.
Similar results were achieved by different authors; Racjan and Tollenaar
(1999) found that leaf longevity was enhanced by an increase in soil N
supply. In addition, reduced N availability accelerated post flowering
leaf senescence than at high N and maize inbred lines showed differences
in their magnitude of response (D`Andrea et al., 2006). Gungula
et al. (2005) declared that highest percentage of leaf senescence
at the lowest N-level (30 kg N ha-1), while the lowest leaf
senescent percentage was recorded at 120 kg N ha-1. In their
results, Borrell et al. (2001) established that roots of the stay
green sorghum maintain greater capacity to extract N from the soil compared
with the non-stay green hybrids during kernel filling. They assessed it
as a consequence of the balance between N-demand by the kernel and N-supply
during the kernel filling. The inbred lines showed leaf greenness reduction
more than their crosses (31.0 and 24.04%, respectively) when grown at
N deficiency conditions. The inbreds 4, 5, 6, 7, 9, 13 and 15 and the
crosses Pioneer 3062, 9x10, 4x1, 13x16 and 13x15 were characterized by
their highest greening percentage at physiological maturity and kernel
filling phase at reduced N soil. The previous result can be explained
based on Boràas et al. (2003) suggestion that delay in
senescing for the previous group of genotypes during kernel filling is
linked to the quantity of light received by the leaves and N availability
via remobilization to actively growing kernels of maize. In addition,
such genotypes maintain their green leaves longer than others and in turn
represent differences in photosynthetic capacity (Gungula et al.,
2005).
Table 8: |
Performance of 13 yellow maize crosses cropped at two
levels of nitrogen fertilizer (70 and 140 kg N f-1) for
flowering, stay green, total dry matter and N uptake traits for grain
yield and its components |
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LN: Low Nitrogen, HN: High Nitrogen and X–:
Mean |
Table 9: |
Performance of 13 yellow maize crosses lines cropped
at two levels of nitrogen fertilizer (70 and 140 kg N f-1)
for NUE traits |
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LN: Low Nitrogen, HN: High Nitrogen and X–:
Mean |
Results in Table 2 and 3 revealed the
significant effects of N fertilizer levels, genotypes and the interaction
on biomass production. The limited nutrition promoted reduction in biomass
production by 34.97 and 31.52% for inbreds and their crosses, respectively.
Greater reduction in biomass production and plant growth rate for maize
inbreds than for hybrids around silking period was affirmed (Uhart and
Andrade, 1995). Their result was related to the reduction in leaf expansion
and light interception efficiency as suggested by D`Andrea et al.
(2006). Our results are go in line with the idea supported by Tollenaar
et al. (2004) in that reduction in biomass production usually observed
in inbred lines is more related to the negative effects of inbreeding
on leaf expansion and light interception than to photosynthesis levels.
The consequence of N stress strengthens this tendency. The inbreds 13,
8, 9 and 2 showed the highest dry matter production at N stress planting,
where the production ranged from 26.75-61.42% above the average. The cross
Pioneer 3062, showed the highest dry matter production relative to the
crosses (13x16), (8x5) and (9x12) at 70 kg N f-1, with registered
6.79-67.22% increase over general mean (Table 4-9).
N-uptake was differed significantly by N levels, genotypes and their
interaction (Table 2 and 3). Nitrogen
accumulation increased from 4.22 to 9.99 and from 3.37 to 7.99 g N m-2
in crosses and their inbred lines respectively with application of 140
kg N feddan-1. Differences in N-uptake due to hybrid effects
were twofold higher than that of inbreds. The lines 8, 13, 9, 2, 6 and
4 and the crosses (4x2), (13x16), Pioneer 3062 and (8x6) exhibited the
highest N uptake at N limited nitrogen supply. It is clear that total
dry matter production at harvest is correlated and affected by N-uptake
at both levels of N (Fig. 1 and 2)
in hybrids and inbred lines. Singh and Arora (2001), confirmed the same
results for wheat genotypes cultivated at 40 and 120 kg N ha-1
and mentioned that rate of dry matter production may control nitrogen
uptake and growth is tightly controlled by nitrogen supply. Thus demand
factor is an important determinant of uptake rate.. In contrast, inbreds
11 and 14 and the hybrid (8x7) accumulated the least dry matter and had
the lowest N uptake.
Grain Yield Determinants
As shown in Table 2 and 3, Nitrogen
levels, genotypes and N levelsxgenotypes were significant for grain yield,
yield plant-1, 100-grain weight and HI of hybrids and inbred
lines. Mean squares due to N levels were highest than those due to genotypes
for both groups when estimated for all traits except HI of hybrids. The
significant effect of genotypexN level interactions on the measured traits
confirms that genotypes perform differently under different N application.
The inbred lines group showed severe reduction in grain yield, yield plant-1,
100-grain weight and HI (48.70, 46.06, 16.75 and 21.21%, respectively)
when planted under N limited input conditions compare with hybrid group
(34.50, 34.50, 9.64 and 4.17%, respectively). Present results support
the data achieved by Betrán et al. (2003) in that inbred
lines are relatively more sensitive to low soil nutrition compare with
hybrids and grain yield for maize inbred lines grown at reduced N recorded
65% reduction of that under high N. Similarly, Gallais and Hirel (2004)
recorded 38, 32 and 9% reduction in yield, kernel number plant-1
and kernel weight. Plant grain yield was strongly reduced by N deficiency
relative to grain weight, D`Andrea et al. (2006) established that
yield plant-1 is severely reduced by low N application and
such response is mainly related to variation in kernel number per plant,
whereas kernel weight is less susceptible to N stress and grain yield
is less responsive to its variation (Gallais and Hirel, 2004; D`Andrea
et al., 2006).
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Fig. 1: |
Relation between N-Uptake and DM for 16 maize inbred
lines cropped at (a): 70 and (b): 140 kg N feddan-1 |
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Fig. 2: |
Relation between N-Uptake and DM for 13 maize single
crosses cropped at (a): 70 and (b): 140 kg N feddan-1 |
The inbred lines 4, 13, 9, 7, 5 and 15 produced the highest grain yield,
yield plant-1, HI and grain weight under low N input and the
range in grain yield reduction varied between 28-46%. Although the lines
10 and 11 recorded low yield at stress condition, both were not sensitive
to low N (25.60 and 31.44%, respectively) (Table 5).
On the other hand, the best hybrids for the previous traits were (4x1),
(13x16), (13x15), (9x10) and (9x12), which surpassed the check SC pioneer
3062 (Table 8). HI is the proportion of grain yield and
biological yield, accordingly, inbreds with high HI has the capacity to
accumulate total amount of aboveground dry matter and translocate it to
the growing grains. So the inbred lines with the highest HI are those
with high yielding ability. D`Andrea et al. (2006) reported two
cases of HI reduction in maize inbred lines, the slight decrease which
attribute to the effect of N deficiency on the relationship between plant
growth rate change and kernel number per plant. Beside the strong reduction
which includes inherent biomass partitioning pattern of the genotype around
silking and N availability controlling. It is suggested that there is
reduced efficiency for converting dry matter at anthesis to reproductive
sinks under N stress (Abbate et al., 1995), this in turn
is represented in the effect of N stress on the reduced HI in some maize
hybrids.
Nitrogen Use Efficiency, Grain Protein Percentage and Nitrogen Harvest
Index
There were statistically significant differences (Table
2 and 3) between N levels, genotypes and their interaction
for NUEb, NUEg, grain protein percentage and the
proportion of total plant N in the grains at maturity (NHI). Under soil
limited fertility, protein percentage reduced in grains by 28.5 and 21.21%
for inbred lines and crosses, respectively. The results showed that, N
stress caused 12.19% reduction and 6.25% increase in NHI for inbred lines
and hybrids, respectively.
Regarding NUEb and NUEg, N stress increased NUE
for plant biomass and grain yield by 47.29 and 12% vs 51.30 and 42.02%,
for inbred lines and crosses, respectively. It is apparent that NUEb
for inbred lines and their crosses was nearly close, but a wide difference
exists among both groups for NUEg. The ranges in NUE for dry
matter and grain yield for inbred lines were too low compared with their
crosses. This may be due to (Table 4-9) the effect of
inbreeding depression on these traits, added to that, inbred lines represented
more efficiency for biomass production than grain yield production. Only
five inbred lines 4, 13, 15 and 9 and 7 had the highest NUEg and NHI.
However for NUEb seven inbred lines (1, 2, 3, 6, 8, 12 and
14) showed the highest NUEb values accompanied with low NHI
(Table 6). As already confirmed by Gallais and Hirel
(2004), maize genotypes exhibiting low agronomic performance at low N-input,
i.e., those having low NUE, are those reacting more to nitrogen application.
Therefore, genotypexnitrogen appears to be essentially due to variation
in the adaptation of the plant to low N-input.
As indicated in Table 9, the crosses: (4x1), (9x10),
(8x7) (9x12), (13x15) and (9x11) combined high NHI and NUEg,
thus such crosses are characterized by efficient partitioning of dry matter
to grains and producing grains per unit of plant nitrogen at lower nitrogen
level. The crosses (8x5), (4x1), (9x10), (9x12), (9x11) were superior
and efficient in producing high dry matter per unit of applied N compared
to Pioneer 3062. The hybrids, (8x6) and (4x2) were characterized by their
N-inefficiency because of their poor NUEg and NHI. The previous
results are supported by studies carried out by many researchers. Webb
et al., (1998) related the increase in N accumulation of shoots
and reduction in NHI in all wheat genotypes with N fertilization to the
less efficient use of N. In contrast, Oikeh et al. (2007) recorded
20% reduction in NHI for maize cultivars grown at 0 compared to 120 kg
N ha-1. They also recorded a large NHI value of 0.63 for N-use
efficient maize cultivars. Koutroubas and Ntanos (2003) attributed the
high NUEg for Indica compared to Japonica cultivars of rice
to their high N translocation from the vegetative tissues to grains during
the post-anthesis period and thus increase NHI. They also added that,
increasing in NHI and HI favored high NUEg. Singh and Arora
(2001) confirmed a decline in NUE for grain and dry matter production
in wheat cultivars as the N rate was increased from 40 to 120 kg N ha-1.
Relationship Between Grain Yield, NUEg and the Other Measured
Variables
The phenotypic correlation coefficients (rp) measured between
grain yield and NUEg and the other tested traits for inbred
lines and crosses at both levels of N fertilizer. At 70 kg N, grain yield
of inbred lines correlated negatively with number of days to 50% tasseling
and silking. Protein percentage although correlated negatively with grain
yield, the value was small (Table 10). On the other
hand, grain yield recorded positive and significant values with yield
plant-1, stay green, NUEb, NHI, HI and grain weight.
The results are in accordance with those established by Betrán
et al. (2003) in that negative phenotypic correlation exist between
maize grain yield and male and female flowering dates in both inbreds
and their hybrids across a range of stress and non stress environments.
They affirmed that such earlier genotypes could escape the intense stress
at flowering. The same authors recorded negative and significant phenotypic
correlation values between senescence and maize grain yield for hybrids
(-0.38) and inbreds (-0.48). Grain yield recorded weak correlation with
N-uptake. Under optimal conditions of N fertilization, there were significant
relationship between yield and yield plant-1, N-uptake, NUEg
and HI. Regarding NUEg, a significant and positive relation
was recorded with NHI, HI, grain yield and stay green, the values were
higher at N stress compared with conditions of available N.
Table 10: |
Phenotypic correlation (rp) between grain
yield (g m-2), NUEg (g g N-1) and
the other tested variables at low and high nitrogen applications |
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*,**: Significant at 0.05 and 0.01, respectively, GY:
Grain Yield, NUEg: Nitrogen use efficiency for grain production |
Data for crosses showed positive and significant phenotypic correlation
values between yield; greenness, protein percentage and NHI. Whereas,
positive and significant relationship were recorded between NUEg
and NHI and HI. Grain yield did not associate with N-uptake compared with
values recorded for inbred lines. This may give the crosses the advantage
of the ability of N translocation from the stover to the grain. The correlation
values were higher at stress conditions, since at optimal level of N no
significant values were recorded. NUEg showed positive and
significant relationships with NHI and HI at both low and high N-input.
In similar way, Koutroubas and Ntanos (2003) recorded positive association
between NUEg, HI, NHI and explained this relationship on the
basis of dry matter and N translocation from the vegetative parts to the
grains which raise the HI and NHI in turn.
High grain yield correlated with low protein percentage in inbred lines,
but in crosses the relationship was positive and significant. This result
shows the advantage of the possibility of selection for increasing grain
yield and protein percentage and that some of the current crosses combine
both high yield and protein percentage. In addition, the lack of evidence
of significant correlation between N uptake and NUE implies that within
this pool of inbred lines and crosses it is possible for combining high
N uptake efficiency with high internal use efficiency.
The previous results are in accordance with those obtained by different
studies on cereal crops. Gungula et al. (2005) recorded a positive
association between lower percentage of leaf senescence and high maize
grain yield. They attributed such positive relationship to the higher
photosynthetic capacity of green leaves and the ability of the plants
to capture more solar energy during grain filling. Muurinen et al.
(2006) recorded a positive correlation for total plant biomass and grain
yield with nitrogen uptake and also observed positive correlation between
NUE and HI. Di Fonzo et al. (1982) mentioned that under low level
of N fertilizer, maize grain yield is correlated with the NHI, on the
other hand.
CONCLUSION
It could be concluded from the previous results that growing maize plants
under N stress resulted in the reduction in grain yield and other related
traits, but raised the N use efficiently for dry matter and grain yield
production. Inbred lines showed recorded mean values for all measured
traits compared with their crosses. There is, in some cases, simultaneous
decrease in yield and increases in NUEb, since some inbred
lines (1, 8 12 and 14) represented low yielding ability although utilized
N in efficient way for biomass production. In addition, it is not necessarily
the case that a genotype with high nitrogen uptake record high NUE for
biomass production (1, 11 and 14 vs 8) or vice versa as confirmed by Singh
and Arora (2001). In developing countries the high cost and the little
or no access to fertilizer input, makes farmers attempt to use minimal
dose, in such cases it is advantage to cropping high N efficient genotypes
under limited N input where high N input maize would not give some appreciable
returns (Kogbe and Adediran, 2003). In addition, cultivating N-efficient
genotypes would reduce the hazards of environmental pollution. Furthermore,
some genotypes (inbred lines and crosses) showed differential in N use
efficiency for dry matter {1, 11 and 13 and (4x1), (8x5), (9x10), (9x11
and 9x12)} and/or grain production {4, 13, 15 and 9; (4x1), 8x7), (9x11)
and (9x10)}, this gives an ample chance to select genotypes to be used
either for grain production or as a source for nutritive value for animal
feed, as maize stover has high crop residue quality in terms of crude
protein and digestibility than sorghum.
The inbred lines and crosses, that showed desirable tendency towards
efficient and inefficient use of N, could be further subject to physiological
analyses to study the pattern of enzyme activities controlling N-uptake
and translocation. Building the relationship between the enzymes activity
and responsible proteins in one hand and the intensity of such relationship
among the parents and their crosses on the other hand is of important.
Also the obtained crosses could be re-evaluated with the local cultivated
hybrids at different levels of N in multi-location experiments to compare
their yielding ability.
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