Vol.2, No.3, 283-290 (2011)
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
Agricultural Scienc es
Pollination dynamics, grain weight and grain cell
number within the inflorescence and spikelet in oat and
Ari Rajala, Pirjo Peltonen-Sainio
MTT Agrifood Research Finland, Jokioinen, Finland; Corresponding Author: ari.rajala@mtt.fi
Received 3 December 2010; revised 23 May 2011; accepted 7 July 2011.
Oat (Avena sativa L.) and wheat (Triticum aes-
tivum L.) vary in the structure of their inflores-
cences and also in how pollination proceeds
within the inflorescence. In both species the
grain position in the spikelet determines grain
weight potential. Primary grains in oat and
proximal grains in wheat weigh more than sec-
ondary and distal grains. This variation in grain
weight can potentially result from differences in
post-pollination cell division in the grain. In this
study pollination duration and dynamics were
analyzed from head samples collected at two-
day intervals, starting from the pollination of the
most advanced floret. The number of grain cells
was determined for individual grains throughout
the inflorescence, starting from the pollination
event. When mature, grain position in the spikelet
and spike was noted and grain weight assessed.
Pollination advance in oat proceeded from the
uppermost primary floret towards the basal
spikelet s in ten to eleven day s. Within the spi kelet,
the primary floret was pollina ted on av erage one
day earlier than the secondary floret. In wheat,
pollination duration was four to five days, start-
ing from the proximal florets in the mid-section
of the inflorescence progressing towards the
apical and basal spikelets. Proximal florets were
pollinated one to two days earlier than distal
florets. Maximum cell number in primary grains
exceeded that of secondary grains in two oat
cultivars. Similarly, primary grains were heavier
than secondary grains. Cell number and single
grain weight were correlated in terms of grain
position in the spikelet (primary – secondary)
and cultivar. Oat cultivar Belinda had a higher
single grain weight than Fiia, which was also
expressed as larger grain cell number. In w heat,
proximal grains had higher maximum cell
numbers and were also heavier than distal
grains. This grain weight gradient was apparent
throughout the inflorescence. Consequently,
grain cell number is one of the possible regu-
lators of grain-filling capacity in both cereal
Keywords: Cell Number; Distal Grain; Filling
Potential; Floret; Oat; Pollination; Primary Grain;
Proximal Grain; Secondary Grain; Whea t
Oat and wheat have different inflorescence structures
and the rate of development also differs between the
crops. The development in the oat panicle proceeds from
the uppermost terminal spikelet downwards to the base
of the panicle [1-3], while in wheat the inflorescence
develops more synchronously [4,5]. In oat the develop-
mental stages of spikelets and timing of pollination dif-
fer markedly within the panicle. For example, Bonnett
recorded 18 days for the spikelet initiation phase to
reach the basal parts of the panicle [6]. Somewhat
shorter intra-panicle durations (7 - 12 days) were re-
corded for pollination [3,6]. In wheat, the development
proceeds from the mid-section of the spike towards the
apical and basal sections and initiation of the terminal
spikelet at the top of the spike terminates the spikelet
initiation phase [4,5]. Pollination begins in proximal
florets situated in the middle of the spike and reaches the
distal grains in the basal part of the spike within three to
seven days [4,5]. However, most distal florets fail to set
grain if anthesis takes place more than three days after
pollination in the most advanced proximal florets [5].
The oat panicle probably exhibits greater plasticity to
respond to favorable conditions through increased floret
and grain set than the wheat spike, which has a terminal
spikelet [7]. It also takes a long time for both floral ini-
A. Rajala et al. / Agricultural Science 2 (2011) 283-290
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tiation and advance of pollination downwards in the
panicle [3,6]. This probably exposes different parts of
panicle to various growing conditions during floral de-
velopment and grain filling, which may result in within
panicle heterogeneity in grain weight. Also, a long pol-
lination period may increase the potential exposure of
the oat florets to Fusarium infections. Oat samples ex-
pressed the highest mycotoxin levels when compared
with wheat and barley (Hordeum vulgare L.) under
Scandinavian growing conditions [8].
The single grain weight is dependent on the position
of the spikelet in the head and on the grain position in
the spikelet, the latter being the more important factor in
both species. In oat, grains situated at the top or upper
section of the panicle are typically slightly heavier than
grains produced in the lower section of the panicle,
whereas the primary grain clearly outperforms secondary
and tertiary grains [3,9,11-13]. In wheat, spikelets situ-
ated in the mid-section of the spike produce heavier
grains compared with apical and basal spikelets [14-17],
while within the spikelet proximal grains clearly exceed
the distal grains in single grain weight [15,17-19].
The primary grain domination seems to be very strong
trait in oat as even elimination of the primary grain fails
to enhance grain-filling capacity of the secondary grain
such that it reaches that of the primary grain [3,11]. In
wheat, elimination of the proximal grains within the
spikelet increased the occurrence and dry weight of the
remaining (third and fourth) grain above that of the
grains of intact spikelets [4,18,20]. On the other hand,
sink size reductions in wheat, induced by removing
whole spikelets, resulted only in modest, if any, effects
on weight of the remaining grains [14,21,22].
Several factors contribute to within inflorescence
variation in individual grain weight and possibly interact.
Intra-inflorescence and intra–spikelet differences in de-
velopment rate and time of pollination potentially affect
the duration of the grain-filling period and grain-filling
capacity of oat [3,6,11]. Differences in vascular transport
capacity within the spike and spikelet may regulate the
filling capacity of the grain. Primary grain in oat and
proximal grains in wheat have more efficient assimilate
transport systems compared with secondary and distal
grains [6,10,18,23].
Ultimately, the grain itself could play an active role in
the grain-filling process [24]. The rate and duration of
post-pollination cell division and consequent number of
cells formed in the grain could set the limit for the indi-
vidual grain-filling capacity [24]. A positive relationship
between grain cell number and grain weight was re-
ported for wheat [20,25] and oat [3]. Thus, grain growth
is controlled by grain characteristics developed after
pollination and prior the active grain-filling process.
Both genetic and environmental factors have an effect on
grain growth capacity. Genetic factors set certain limits
to grain size, while the environmental conditions control
the degree to which this potential is fulfilled [24]. There
are also indications that growing conditions prior to pol-
lination may determine a grain’s potential to accumulate
carbohydrates. Calderini et al. [26] reported that expo-
sure to high temperature during floret development
(prior to anthesis) resulted in reduction in carpel weight
at anthesis and in final grain weight. Similarly, geno-
types of lighter carpel weight produced lighter grains
The aim of the study was to characterize pollination
dynamics within the inflorescence and spikelet in two
oat and wheat cultivars differing in growing time re-
quirement. Also, grain position effect on single grain
weight within the inflorescence and spikelet was evalu-
ated. Further aim was to determine potential association
between grain cell number and single grain weight in
two oat and a wheat cultivar.
2.1. Pollination Dynamics
The inflorescence samples for pollination dynamics
studies were collected from field experiments carried out
in Jokioinen (60˚48 N, 23˚28 E) in 2009. Early and late
maturing cultivars of oat and wheat were selected to
study the potential effect of earliness on pollination pro-
gress. At the onset of heading, 40 plants per plot at an
identical development stage were marked with adhesive
tape for sampling (one plot per cultivar). The pollination
of the most advanced floret was determined according to
Waddington et al. [28]. The most advanced floret of the
main shoot inflorescence was identified and when the
stigmatic branches were wide open and clusters of pollen
were visibly attached to them, pollination was deter-
mined to be taking place. From that date forward inflo-
rescence samples (4 per collection time) were collected
three times a week to study the progress of pollination in
different positions of the inflorescence and within the
spikelet. The oat cultivars were Eemeli (early type) and
Belinda (late type), and wheat cultivars were Anniina
(early type) and Amaretto (late type). Duration from pol-
lination of the most advanced floret to other florets was
determined in days and as growing degree days (ºCd,
base temperature 5˚C).
2.2. Grain Cell Number
The cell number component of the study comprised
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two different experiments carried out for research pur-
poses other than potential association between grain cell
number and grain weight reported in this paper. Oat cul-
tivars Belinda and Fiia were grown at the Finnish Offi-
cial Variety Trial site at Jokioinen in 2006. Plot size in
all field experiments was 10 m2. Common cultivation
practices were applied in experiments. Oat was sown at
rate of 500 and wheat at rate of 600 viable seeds per m2.
Compound NPK-fertilizer (20-3-8) was applied at rate of
80 kg·N/ha for oat and 100 kg N/ha for wheat. Plots had
been ploughed in autumn, cultivated in spring and seed
and fertilizer were applied with placement drill. Spring
wheat cultivar Amaretto was grown in 2007 in a large,
plastic covered greenhouse (20 × 30 m) with automatic
control of temperature and watering. Amaretto was sown
with a placement drill (10 sowing rows, 12.5 cm apart)
in plots of 7.5 m2 using 350 viable seeds per m2. NPK-
fertilizer (21-3-9) was applied at rate of 120 kg N/ha.
Marking the plants to be collected and determination of
pollination was carried out as described previously (one
plot per cultivar). Oat panicle samples were collected 1,
4, 7, 10, 14 and 18 days after pollination (DAP) and
wheat ears 2, 4, 6, 8, 10, 12 and 16 DAP. Inflorescences
were fixed for 24 hours in acetic acid/ethanol (75/25,
vol/vol) and thereafter stored in 70% ethanol under cool
and dark conditions. Advance of pollination in different
positions of the inflorescences was determined and the
cell numbers were assessed in pollinated grains. When
grain cell nuclei number started to decline and starch
accumulation was clearly visible, counting ceased. Cell
number of the developing grains was determined ac-
cording to the slightly modified procedure introduced by
Tuberosa et al. [29]. The individual inflorescences, stored
in ethanol, were rinsed three times with deionised water
and placed in deionised water for 5 min at room tem-
perature. After removal of glumes, paleas and lemmas,
caryopsis were placed in 1 M HCl and kept for 30 min
on ice and 20 min at 60˚C in a water-bath. After three
rinses with deionised water, samples were placed in
Feulgen’s reagent (Schiff) for 4 h in the dark at room
temperature. After stain removal with three rinses, sam-
ples were macerated in a sodium-acetate-buffered (pH
4.8) solution of cellulase (Onozuka R10) of 5% (1 - 4
DAP) and 10% (8 - 12 DAP) for 17 h at 37˚C. Stained
cell nuclei were counted in a haemocytometer (Fuchs-
Rosenthal) at 50× magnification. Numbers of nuclei in
four separate blocks of squares (total of 64 squares) were
counted from two separate sub-samples.
Mature inflorescences were collected for single grain
analyses. Each grain from the inflorescence was scaled
and the position within the inflorescence and within the
spikelet was marked. Inflorescences collected for both
cell number and single grain analyses were chosen to be
identically structured in order to reduce the inconsis-
tency caused by varying inflorescence structure. Mean
values and standard errors for the means were calculated
using MS-Excel 2000.
3.1. Pollination Dynamics
In oat advance in pollination proceeded from the up-
permost primary floret towards the basal spikelets (Fig-
ure 1). Within the spikelet, on average, the primary flo-
ret was pollinated one day prior to the secondary floret.
The last of the basal spikelets was pollinated ten to
eleven days later than the uppermost spikelet, which is
equivalent to 100 to 110 ºCd (Figure 1). In wheat, polli-
nation advanced faster than in oat and by four to five
days after pollination of the most advanced proximal
grains in the mid-section of the ear all florets throughout
the ear had been pollinated. This is equivalent to 40 to
50 ºCd (Table 1). Apical spikelets tended to be polli-
nated prior to basal spikelets in both wheat cultivars. No
marked cultivar differences in pollination dynamics were
recorded for either species (Figure 1, Table 1).
3.2. Cell Number and Grain Weight
In wheat, proximal grains had higher maximum cell
number and were also heavier than distal grains (Figure
2). This was the case throughout the ear. Usually, there
were two proximal and one or two distal grains in each
of the spikelets studied. However, in some spikelets, also
the fifth, the most central floret, seemed to be pollinated.
These were found exclusively, though rarely, in the
spikelets in the mid-section of the ear. The recorded cell
number of these florets was always low, averaging only
24,000 cells per grain. These developing grains were
apparently aborted during the early phase of grain filling
Table 1. Required growing degree days (ºCd, from sowing,
base temperature 5˚C) for floret pollination in different ear
positions of wheat cultivars Anniina and Amaretto.
Anniina Amaretto
proximal graindistal grain proximal graindistal grain
spikelet ºCd ºCd ºCd ºCd
1 573 599 613 626
2 568 599 603 619
3 563 595 599 619
4 553 579 595 613
5 558 584 595 619
6 568 589 608 626
7 589 599 619 639
8 595 646 647
9 653
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Eemeli Belinda
Figure 1. Schematic presentation of oat panicle and required growing degree days (ºCd, from sowing, base temperature 5˚C) for
floret pollination in different panicle positions of cultivars Eemeli and Belinda. Value represents the mean of four panicles.
Figure 2. Single grain weight (SGW) and maximum grain cell
number in proximal and distal grains in wheat cv. Amaretto.
Bars indicate standard error of means. Figure 3. Single grain weight (SGW) of proximal (blue line)
and distal (red line) grains in wheat cv. Amaretto. Spikelet 1 to
9 represents top and basal positions in the spike. Bars indicate
standard error of means.
and were not found in the mature ears. Grain weight
varied within the wheat spikelet and ear (Figure 3).
Proximal grains were heavier than distal grains and the
grains produced in spikelets of the mid-ear weighed
more. Proximal grains produced 74% of the total ear
both oat cultivars maximum cell number in primary
grains exceeded that of secondary grains and the differ-
ence was apparent in all branches throughout the pani-
cle. Similarly, primary grains weighed more than secon-
ary grains (Table 2). This resulted in primary grain
In oat, lower whorls tended to have lower grain cell
numbers than the uppermost whorls (data not shown). In d
Openly accessible at
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Table 2. Single grain weight (SGW) and standard error of means (stderr) of primary and secondary grain of oat cultivars Belinda
and Fiia.
Belinda Fiia
primary grain secondary grain primary grain secondary grain
SGW stderr SGW stderr SGW stderr SGW stderr
whorl mg mg mg mg
1 48,9 2,03 26,5 0,28 46,3 1,17 28,7 1,44
2 45,0 5,16 29,5 1,58 41,3 1,45 22,2 3,58
3 49,9 1,18 29,4 1,85 39,1 2,84 25,0 1,07
4 44,2 2,31 28,7 1,12 39,5 1,79 24,7 0,94
5 41,3 1,03 29,0 0,98 38,2 0,90 23,1 0,93
Whorls 1 to 5 represent top and basal positions in the panicle.
domination in yield formation. In panicles of Belinda
and Fiia, 61% and 62% of the yield was produced by the
primary grains, respectively. Cell number and single
grain weight were correlated in terms of grain position in
the spikelet (primary – secondary) and cultivar. Belinda
had higher single grain weight and higher grain cell
number compared with Fiia (Figure 4).
The contribution of different whorls to panicle yield
was similar in both oat cultivars and most yields (66%)
was produced in the two lowest whorls of the panicle.
There was also a tendency for lower whorls to produce
slightly lighter primary and secondary grains in both
cultivars (Table 2).
4.1. Advance of Pollination within the
Pollination advance (Figure 1 and Table 1) was simi-
lar to that reported for oat grown in the USA [6] and for
Figure 4. Single grain weight (SGW) and maximum grain cell
number in primary and secondary grain of oat cultivars Belinda
and Fiia. Bars indicate standard error of means.
wheat grown in Australia [4,5]. An apparent floret hier-
archy in pollination was evident for both species: the
primary floret in oat and proximal floret in wheat were
pollinated prior to the accompanying florets irrespective
of the spikelet position in the inflorescence (Figure 1
and Table 1). Apical development rate in cereals is
characteristically accelerated under long day conditions
[30,31]. However, when comparing the results of this
study and published results, pollination seemed to pro-
ceed rather similarly regardless of day length. Pollina-
tion in oat proceeded from the uppermost primary floret
towards the basal spikelets (Figure 1).
A similar phenomenon was apparent also horizontally
at the whorl level. The pollination proceeded from the
outermost spikelet towards the spikelets situated closest
to the panicle rachis (Figure 1). In general, it seems that
oat spikelets are pollinated more or less in the same or-
der as they appear from the flag leaf sheath at panicle
In wheat the pollination was more synchronous in
terms of days and cumulative degree days when com-
pared with oat. It took four to five days to complete pol-
lination in wheat compared with ten to eleven days in oat.
A relatively long pollination period in oat potentially
exposes the grains in different positions of the panicle
and whorl to a range of environmental conditions. This
may be reflected in varying grain-filling capacity and
yielding potential in the panicle. Moreover, a long polli-
nation period can increase the risk of Fusariu m infection.
The infection is favored by precipitation or a humid mi-
croclimate during pollination and early grain filling [8,
32]. Oat has twice as long a pollination period as wheat,
and therefore there is potentially a higher risk of Fusa-
rium infection in some parts of the panicle. The advance
of pollination was similar in early and late maturing cul-
tivars in both species (Figure 1, Table 1).
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4.2. Grain Weight and Grain Position in
Spikelet and Inflorescence
Grain weight was primarily dependent on grain posi-
tion within the spikelet for both species. Proximal grain
for wheat and primary grain for oat were heavier than
their counterpart grains (Figure 3, Table 2 ). This is in
accordance with earlier reports for both species [3,13,15-
17]. Higher grain weight resulted in primary and proxi-
mal grain domination in yield production: 61% and 74%
of the grain yield was produced by primary and proximal
grain in oat and wheat, respectively.
These values, recorded for cultivars adapted to ex-
tremely short growing seasons, are well in line with pub-
lished values, in which primary grains of oat were re-
ported to produce 60% to 70% of the panicle grain yield
[9,12] and up to 80% of the proximal grains in wheat [15,
16]. Accordingly, the gradient in grain yield potential
within the spikelet appears similar irrespective of grow-
ing conditions. Grain position within the ear in wheat
had an apparent effect on grain weight (Figure 3). Both
proximal and distal grains in the mid-section of the ear
outperformed their counterpart grains in the apical and
basal sections. Typically, wheat grains produced in the
mid-section of the ear were the heaviest [14-16]; while
in oat, the upper whorls produced heavier grains than the
lower whorls (Table 2). This is in accordance with pub-
lished data [3, 10,12,13]. Accordingly, the individual
grain filling capacity at the different positions in the
panicle and whorl slightly decreases towards the base.
But when grain- filling capacity is surveyed at the whorl
level, the assimilate transport capacity or the sink
strength of the whorl appears to be directly dependent on
the number of grains in the whorl. This was evident in
figures for the grain yield produced in each whorl. The
two lowest whorls produced 68% of the grains and 66%
of the grain yield. The greater number and area of vas-
cular tissue in the lower whorls facilitate almost as effi-
cient assimilate transport at grain level as in top whorls
[10]. The slight grain weight gradient in favor of grains
in the uppermost whorls is likely to be due to increased
vascular transport capacity per grain [10]. The grain
weight gradient from primary to secondary and proximal
to distal grains was clear for both species (Figures 2-4).
This difference is likely attributable to the differences in
assimilate transport capacity within the grains in the
spikelet in both species [6,10,18,23].
The filling capacity of the secondary grains in oat is
apparently restricted. Elimination of the primary grain
improved the single grain weight of the secondary grains,
but they invariably failed to reach the weight of the in-
tact primary grains [3,11]. Thus, the potential weight of
the secondary grain must be restricted as it always fails
to reach that of the primary grains, even when there is
reduced assimilate competition within the spikelet and
thereby surplus assimilate availability for secondary
grain. In oat, the number and area of vascular tissue
transporting assimilates decreases towards the top of the
panicle [6,10]. Nevertheless, the heaviest grains are
typically produced in the top part of the panicle [3,11,13]
because of the lower grain number at the apex of the
panicle and greater vascular transport capacity per grain
4.3. Grain Cell Number and Grain Weight
When individual grain cell number was recorded and
maximum cell number and grain weight of a particular
grain were compared, no correlation was established
(data not shown). However, when the relationship be-
tween grain cell number and single grain weight was
investigated at primary-secondary and proximal-distal
grain level; there was an apparent correlation for both
species (Figures 2 and 4). According to these results it
seems that the number of cells formed in the endosperm
affects the potential weight of the grain within the
spikelet. Similar indications were reported in for wheat
previously [33-35], and for oat [3] and other cereal spe-
cies [36-39]. Egli [24] emphasized the role of the seed
itself in establishing filling capacity of the individual
seed. According to Egli [24] genotype defines the limit
for potential grain size, while the growing conditions
regulate the degree to which this potential is fulfilled. In
this study, it was not possible to measure the environ-
mental effect, but the genotype effect was evident in oat:
Belinda clearly outperformed Fiia in terms of grain cell
number and grain weight (Figure 4). A similar differ-
ence was noted also in our earlier preliminary work [3].
In addition, barley and wheat genotypes with higher
grain weight have had greater grain cell numbers [25,34,
39]. Thus, these findings support the hypothesis that
cultivar differences in grain characteristics are associated
with grain-filling capacity. Grain cell number is an at-
tractive and a likely candidate for determining potential
grain weight. Single grain weight is known to be cultivar
dependent and more stable over contrasting growing
conditions when compared with grain number per plant
or unit land area, which fluctuate more [40,41]. This
supports the hypothesis that cultivar differences in single
grain weight could originate from limit induced by grain
setting for grain growth potential.
This study revealed differences in numbers of grain
cells at different spikelet positions. Possibly differences
in assimilate transport capacity within the spikelet also
control the grain-filling capacity through post-pollina-
tion cell division. Another possible reason relates to
hormonal control as cell division in plant tissue is hor-
monally regulated. Peak concentrations of hormones,
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their relative concentrations, timing of occurrence during
the growth cycle and change in tissue sensitivity to hor-
mones play a complex role in plant development and
growth processes [42].
Higher cytokinin and auxin levels were shown to be
associated with higher grain weight in cereals [42-46].
Growing conditions, especially stresses, alter hormone
balance. For example, drought-induced increase in ab-
scisic acid concentration inhibited cell division in de-
veloping grains, reducing grain weight in wheat and
maize (Zea mays) [36,37,47]. Thus, observed differences
in grain cell number could be hormonally regulated.
Oat and wheat differed markedly in how pollination
proceeded within the inflorescence. The entire wheat ear
was pollinated in four to five days, whereas in the oat
panicle the pollination period lasted ten to eleven days.
For both species, pollination advanced similarly in early
and late maturing cultivars adapted to the short growing
season of northern Europe. The position of the grain in
the spikelet strongly determined the grain weight poten-
tial .in both species. Grain weight and number of cells
formed in the grain seemed to associate at grain position
level. Hence, grain cell number may establish dry matter
accumulation potential of the grain.
This study was partly financed by EU INCO-MED project WatNit-
MED (INCO-CT-2004-509107), Management Improvements of Water
and Nitrogen Use Efficiency of Mediterranean Strategic Crops (Wheat
and Barley). The authors are grateful for the technical assistance of
Aino Lahti and Leila Salo. Dr Jonathan Robinson is acknowledged for
linguistic revision.
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