Effect of Source-Sink Manipulation on Yield Components and Photosynthetic Characteristic of Wheat Cultivars (Triticum aestivum and T. durum L.)
To investigate the effect of source-sink manipulation on photosynthetic characteristics and yield components of wheat cultivars, a research was carried out in the greenhouse at the College of Agriculture, Shiraz University, Shiraz, Iran, during 2008-2009 growing season. Five wheat cultivars including Shiraz, Bahar, Pishtaz, Sistan (as bread wheat) and Yavaros (as durum wheat) were grown in 5 kg plastic pots. The source-sink manipulation treatments including defoliation of all leaves, defoliation of all leaves except the flag leaf, removal of 25% of spikelets and removal of 50% of spikelets were applied at anthesis. Results showed that number of grains per spike was significantly decreased by defoliation treatment in Shiraz, Bahar and Yavaros cultivars, so that in Shiraz and Bahar cultivars, source restriction reduced the number of grains per spike by 18.97 and 11.07%, respectively. In Shiraz cultivar, defoliation of all leaves decreased main stem grain yield by 40.75%, which demonstrated that Shiraz was very sensitive to source restriction. The little response of main shoot grain yield to defoliation in Pishtaz cultivar indicates high mobilization of photoassimilate from other parts of the crop to the grains. Under sink restriction conditions, wheat cultivars (except Pishtaz) had potential to increase their 100-grain weight; moreover, Pishtaz yield appeared to be more sink rather than source-limited. Removal of all leaves except the flag leaf at 8 and 18 DAA, in Pishtaz and Yavaros cultivars, had no significant effect on net photosynthesis rate (Pn) and in all cultivars except Pishtaz, sink restriction significantly decreased Pn rate. Further research, is recommended for improving our understanding on source-sink relationship in Iranian wheat cultivars.
Wheat (Triticum aestivum L.) is regarded as the most important cereal
crop of the world in view of both areas under cultivation and production level.
About two-thirds of the world population lives on wheat grain (Emam,
2007). In crops, the physiological basis of dry matter production depends
on the source-sink relationship, where the source is the potential capacity
for photosynthesis and the sink is the potential capacity to store or metabolize
the photosynthetic products. When the sink is small, higher yield could not
be achieved and even if the sink is large, the yield might not be high, when
the source capacity is limited (Alam et al., 2008;
Emam and Seghatoleslami, 2005; Borras
et al., 2004).
Optimizing the source and its proper utilization by the economic sink is important
for improvement of yield potential in wheat crop (Alam et
al., 2008; Shekoofa and Emam, 2008). According
to Richards (1996) the modern high yielding cultivars
of wheat are sink-limited and this has posed problem in yield increase. Borras
et al. (2004) reviewed the literature on the magnitude of the mean
grain weight response as a function of the assimilate availability during grain
filling period to test quantitatively whether source or sink limitation in wheat
grain growth period was predominant. They concluded that under most conditions
grain growth in wheat was apparently more sink, than source-limited.
Artificial reduction in grain number per spike (Borras
et al., 2004) or defoliation (Bingham et al.,
2006) or early use of chloromequat chloride (Emam and
Karimi, 1996; Emam and Dastfal, 1997; Emam
et al., 1997; Shekoofa and Emam, 2008) have
been employed in several studies to provide clear evidence on whether grain
yield in wheat is more source or sink limited (Emam, 2007).
Some researchers concluded that wheat final grain weight was limited by the
ability of the source to provide assimilate during grain filling. For example,
Fischer and Lambers (1978) reported that final weight
per grain was increased when grains per spike were reduced artificially, suggesting
a degree of limitation of grain weight by the photoassimilate supply during
grain filling. In contrast, data from other investigations have suggested that
the wheat yield is more sink-limited during the grain filling (Savin
and Slafer, 1991; Borras et al., 2004; Shekoofa
and Emam, 2008). Also, Aggarwal et al. (1990)
reported that whole plant defoliation treatments had no significant effects
on wheat grain yield in most cases. Finally, there are some reports suggesting
that grain growth in wheat might be limited by both assimilate supply (source
limitation) and assimilate demand (sink limitation) (Ahmadi
et al., 2009).
Source and sink manipulation might be regulated by plant physiological processes
such as net photosynthesis and features as stomatal conductance and transpiration
rate of wheat (Ahmadi et al., 2009; Rohi
and Siose Mardeh, 2008) however, the direction and magnitude of the regulation
varies with time and cultivar (Rohi and Siose Mardeh, 2008;
Ahmadi and Joudi, 2007). The grain growth of wheat mainly
depends on formation, translocation, partitioning and accumulation of photosynthates
during the grain filling. Therefore, both photosynthetic activity of leaves
(source) and storage ability of grains after anthesis (sink) are factors limiting
the grain yield of wheat (Wang et al., 1997;
Detrimental effects of defoliation on yield components might be related directly
to reductions on the photosynthetic capacity of the remaining tissue (Macedo
et al., 2006). Zhu et al. (2004) reported
that defoliation of wheat at late tillering increased main shoot grain yield
and harvest index 7.3 and 10.7%, respectively and enhanced stomatal conductance
and net photosynthesis rate of remaining leaves at anthesis. In another investigation
on wheat, Yin et al. (1998) found that at the
initial grain filling stage, the large-grain cultivar was sensitive to source
reduction, leading to an increased net photosynthesis rate by 10%, however,
source reduction had no impact on small grain cultivar.
The effects of source and sink manipulation on Iranian wheat cultivars has not yet been fully understood. The present study was undertaken to investigate the effect of source and sink manipulation on yield components and photosynthetic characteristics of five wheat cultivars.
MATERIALS AND METHODS
In order to investigate the effect of source-sink manipulation on photosynthetic
characteristics and yield components of five wheat cultivars, a pot experiment
was carried out in the greenhouse at the College of Agriculture, Shiraz University,
(29° 50' N, 52° 46' E) 12 km North of Shiraz, Iran, on a fine mixed,
mesic typic Calcixerpets soil during 2008-2009 growing season. Five wheat cultivars
including Shiraz, Bahar, Pishtaz, Sistan (as bread wheats) and Yavaros (as durum
wheat) were grown in 5 kg plastic pots filled with fertilized soil at 20 mg
kg-1 nitrogen as urea. A completely randomized design with four replications
was used. Ten uniform seeds of each cultivar were sown in each pot, on 21 December
2008 and thinned to five seedlings at two-leaf stage. The pots were watered
when necessary to avoid water stress. The greenhouse temperature was 25°C
(±5), with 70% (±10) relative humidity and light intensity varied
in the range of 600-1000 μmol/m2/sec.
The source-sink manipulation treatments including control, defoliation of all
leaves, defoliation of all leaves except the flag leaf, removal of 25% of spikelets
(one out of every four spikelets) and removal of 50% of spikelets(every alternate
spikelets) were applied at anthesis on 12 March 2009. Also, at anthesis, the
plants were de-tillerd to avoid becoming alternative sinks for mobilized carbohydrates
Measurements of the net photosynthesis rate (Pn) and closely related processes, such as stomatal conductance (gs), intercellular CO2 concentration (Ci) and transpiration rate (E), were recorded from the flag leaf of main shoot on each plant using a portable photosynthesis system (IGRA model LCA4-ADC, Hoddeson, UK) at 8 days (initial grain filling stage) and 18 days (rapid grain filling stage) after anthesis (DAA).
At physiological maturity, plants of each pot were harvested and oven-dried
at 80°C, then the number of grains per spike, main shoot yield, 100-grain
weight and spike compactness (spikelet number per spike divided by spike length)
was measured (Emam, 2007). The collected data were subjected
to analysis of variance and the means were separated with LSD test (p = 0.05)
using SAS software (SAS, 2000).
Effects of source-sink manipulation on wheat yield components: The number
of grains per spike was found to be affected significantly by defoliation of
all leaves and all leaves except the flag leaf treatments in three cultivars
of Shiraz, Bahar and Yavaros (Table 1). It was observed that
in Shiraz and Bahar cultivars, source restriction caused reduction in the number
of grains per spike by 18.97 and 11.07% compared to control, respectively. In
all cultivars, except Pishtaz, the main shoot grain yield was decreased significantly
by defoliation treatments (Table 1). In Shiraz cultivar, defoliation
of all leaves decreased main shoot grain yield by 40.75%, compared to control
and this demonstrated that Shiraz cultivar was sensitive to source restriction.
of source or sink manipulation on yield components of wheat cultivars
with the same letter in each column are not significantly different, using
LSD test (0.05)
of spike compactness in wheat cultivars (Vertical bar represented ±SE)
In the present investigation, removal of all leaves decreased the grain yield
and its components more drastically, compared to leaving the flag leaf treatment
On the other hand, comparison of spike compactness in wheat cultivars showed that, Pishtaz had the minimum spike compactness (1.35 compared to 2.44 spikelet cm-1 in Yavaros, Fig. 1). This might be associated with more sunlight penetration to the spike of this cultivar. Yavaros cultivar had maximum spike compactness (Fig. 1) and defoliation treatment affected yield and yield components of this cultivar negatively (Table 1). Furthermore, Pishtaz cultivar had large awns (8.2 cm) and awn photosynthesis might have contributed to grain filling under source restriction conditions. In Shiraz, Bahar, Sistan and Yavaros cultivars, 100-grain weight was significantly affected by defoliation of all leaves. In contrast, in Pishtaz the 100-grain weight was not affected by source restriction (Table 1).
Response to sink manipulation, by removal of 50 and 25% of spikelets, is shown in Table 1. As expected, in all cultivars, the number of grains per spike was decreased (24.36 in Sistan to 50.12% in Bahar cultivar) significantly by sink restriction treatments. In the present investigation, removal of 50% of spikelets caused reduction in number of grains per spike and main stem yield by 50.12 and 48.30% in Bahar cultivar, respectively.
Effects of source-sink manipulation on photosynthetic characteristics:
In all cultivars except Pishtaz, sink restriction significantly decreased Pn
rate (Table 2). Among the wheat cultivars, Yavaros showed
minimum Pn rate after removing 50% of spikelets at 18 DAA (Table
2). In Pishtaz, stomatal conductance (gs) of flag leaf was not
affected by sink or source limitation after 18 DAA (Table 2).
of source or sink manipulation on net photosynthesis rate (Pn),
stomatal conductance (gs), intercellular CO2 concentration
(Ci) and transpiration rate (E) of wheat cultivars at 8 and
18 days after anthesis (DAA)
with the same letter in each column are not significantly different, using
LSD test (0.05)
between photosynthesis rate (Pn) of flag leaf with (a) stomatal
conductance and (b) intercellular CO2 concentration of wheat
cultivars at 8 and 18 days after anthesis (DAA) under sink or source manipulation
In addition, in other cultivars at 18 DAA, sink restriction decreased and source
restriction enhanced gs of the flag leaf. A significant positive
relationship was found between Pn rate and gs at 18 DAA (R2
= 0.62) (Fig. 2a). Also, the relationship between Pn
rate and Ci at 8 (R2 = 0.68) and 18 DAA (R2 = 0.71) was
highly significant (Fig. 2b).
Among the cultivars, Sistan had the least reduction in the number of grains
per spike (0.43 to 0.91%) under source restriction treatments after anthesis
(Table 1). In the similar study, Chowdhary
et al. (1999) reported that removal of all leaves in spring wheat
caused reduction of 17.17 and 13.27% for number of grains per spike and 100-grain
weight, respectively. Also, Singh and Singh (2002) studied
the effects of defoliation of all leaves in wheat and reported 30 to 40% reduction
in grain yield of the main shoot. Furthermore, Zhenlin et
al. (1998) observed that removing one-half of the wheat leaves decreased
main shoot yield by 15%.
The small response of main shoot yield to defoliation in Pishtaz cultivar (Table
1), suggests that photoassimilate supply by other parts of the crop has
probably met most of the demand by the grains in this cultivar. Ahmadi
et al. (2009) also noticed that Ghods wheat cultivar had a large
spike with long awns and photoassimillate translocation from the leaf sheath,
peduncle and lower internodes could support yield under source restriction.
In this study, it appeared that the solar radiation could be utilized more efficiently
in Pishtaz cultivar by the spike parts (i.e., glumes, lemma, etc.) when the
compactness of the ear is lower.
Removal of all leaves in Shiraz, was associated with the maximum reduction
of 100-grain weight (26.88%). In a study with 20 wheat cultivars, Alam
et al. (2008) found that Agrani and SAN-127 wheat cultivars showed
high reduction in 100-grain weight, however, SAN-119, Shotabdi and Agrani cultivars
were highly affected by defoliation treatments for number of grains per spike.
Also, the cultivars SAN-119, Agrani and Shotabdi showed a significant decrease
in grain yield of main shoot by defoliation treatments.
Under sink restriction conditions, the 100-grain weight of all cultivars except
Pishtaz, was increased compared to control. This finding was in agreement with
the result of Simmons et al. (1982), who reported
that reduction in kernel number per spike of wheat increased the final seed
weight, whereas defoliation reduced it. Alam et al.
(2008) also declared that removal of 50% of spikelets decreased the number
of grains per spike and main shoot yield by 41.03 and 37.01%, respectively;
it also increased the 100-grain weight by 9.44%. Furthermore, they reported
that removal of 25% of spikelets reduced the number of grains per spike and
main shoot yield by 25.13 and 23.38%, respectively. This treatment also increased
100-grain weight by 4.08%. Roy and Salahuddin (1994)
studied the effect of spikelet removal at anthesis in wheat and reported that
spikelet removal increased the mean grain weight by 14%. Present results showed
that under sink restriction conditions, all wheat cultivars except Pishtaz,
had potential for further increase in their sink size i.e. increased 100-grain
weight. Indeed, the lower response of Pishtaz cultivar to changes in assimilate
availability might suggest that grain yield of this cultivar is more regulated
by sink rather than the source size (Table 1).
Similar to our results, Ma et al. (1990) in
an investigation with wheat crop, found that the partial degraining (removal
of spikelets) decreased the number of grains per spike significantly (by 51%).
Also, Ferdous and Shamsuddin (2001) also reported that
removal of 50% of spikelets in spring wheat crop decreased the number of grains
per spike and grain yield by 47.56 and 42.03%, respectively.
Indeed, the enhanced gs of the flag leaf following source restriction,
was due to feed back effect of the demand for assimilate by the sink (i.e.,
developing grains), also noted by other researchers (Ahmadi
and Joudi, 2007; Emam and Seghatoleslami, 2005). Ahmadi
and Joudi (2007) reported that leaf removal appeared to stimulate Pn
rate and gs of the remaining flag leaves. Similar results have been
reported by Wang et al. (1997), who declared
that source restriction by partial defoliation of Winter wheat plants increased
Pn rate of most leaves, however, the range of increase differed among
cultivars. While, some cultivars like Lumai and Shannong showed slightly (not
more than 10%) increase in Pn rate, in others, such as Hesheng and
DO41 cultivars, defoliation markedly increased Pn rate. Our results
suggest that photosynthetic characteristics including Pn rate, gs,
intercellular CO2 concentration (Ci) and transpiration
rate (E) differed among the cultivars and in Pishtaz at 8 and 18 DAA, Ci
and E of flag leaf was not affected by source or sink restriction (Table
According to our results, Rohi and Siose Mardeh, (2008)
in the similar study with 20 wheat genotypes declared that Pn rate
of flag leaf had strong positive relationship with gs (R2
= 0.77) and Ci (R2 = 0.73) under drought stress conditions.
Also, Koc et al. (2003) reported that increased
Pn rate, was associated with increased gs particularly at rapid grain
filling stage, in durum wheat cultivars.
Generally, the present investigation demonstrated that in all cultivars, except Pishtaz, reduced source size was associated with reduced sink development, including grain number per spike, mean grain weight and consequently the grain yield. On the other hand, reducing sink size, by removal of spikelets had an increasing effect on mean grain weight, indicating the possibility of further increase in grain weight, if sufficient source is provided. Photosynthesis characteristics of flag leaf of Shiraz, Bahar, Sistan and Yavaros, were sensitive to source or sink manipulation. Pishtaz wheat cultivar, responded to source or sink manipulation slightly. This cultivar had large spike with long awns and the low spike compactness, probably photosynthesis by spike during the grain growth has had an important role in carbohydrate supply to the grain growth in defoliated plants, which is worthy of further explorations.
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