Aust. J. Agric. Res., 1977, 28, 575-81
Determination of
Spikelet Number in Wheat. 11%
Effect of Varying Light Level
on Ear Development
M. S. Rahman, J. H. Wilson and Y. Aitken
School of Agriculture and Forestry, University of Melbourne.
Parkville, Vic. 3052.
Abstract
The effects of two light levels (0.98 and 4 . 9 0 cal cm-2 hr-') on rate of development and spikelet
number per ear were studied in eight wheat cultivars grown under a 16 hr photoperiod at 20°C.
The objective was to ascertain how light affects spikelet number.
At the lower light level the durations of the vegetative, spikelet and ear elongation phases were
greater, but the number of spikelets per ear, number of phytomers present at floral initiation, final
leaf number, number of phytomers that were converted into spikelets, apex length at floral initiation
and rate of spikelet initiation were smaller than at the higher light level. Responses to varying light
level for a11 these parameters were similar for different cultivars, but the sizes of the responses
differed. Within a given cultivar, an increase in spikelet number was associated with longer apices
at floral initiation and a higher rate of spikelet initiation. It was concluded that these two factors
are important determinants of spikelet number.
Introduction
Variation in the intensity of light received by growing wheat plants affects the
morphological and physiological characters and grain yield (Campbell and Read
1968; Campbell et al. 1969). Lucas (1971) studied the effect of increasing light
intensity on spikelet number per ear and rate of development in the wheat cultivar
Triple Dirk. Her evidence, although inconclusive, suggested that spikelet number
increased with increase in light intensity up to 50% of full daylight. The time to
terminal spikelet initiation and final leaf number were little affected. Friend et al.
(1963) and Friend ( 1 9 6 5 ~ observed
)
that low light intensity delayed floral initiation
(FI), heading and anthesis, but decreased spikelet number per ear in the wheat
cultivar Marquis.
This paper reports the effect of varying light level on spikelet number per ear
and on associated changes in developmental processes over a wide range of genotypes;
all previous studies have been conducted with one variety. This forms part of a
wider study designed to determine what factors control spikelet number in wheat.
Materials and Methods
For details of the eight cultivars used, see Rahman and Wilson (1977).
All plants were grown in CSIRO artificially lit cabinets (LB type; Morse and
Evans 1962) at a constant temperature of 20°C under a 16 hr photoperiod. Two
* Part I, Aust. J. Agric, Re$.,1977, 28, 565.
M. S. Rahman et at.
576
light levels were used: 0.98 and 4.90 cal cm-2 hr-l, equivalent to 300 and 1500 ft
candles, respectively (16 and 79 cal cm-2 day-'). For the higher light level, 28
fluorescent tubes were supplemented with four incandescent globes (each 60 W) and
for the lower level, six fluorescent tubes were supplemented with two incandescent
globes. A Kipp solarimeter was used for light measurement, and adjustment was
made for the 400-700 nm range. The lower light level was above the compensation
point for growth of wheat plants (Friend 1965b).
For details of sowing and subsequent growth, measurements (all on main sheet)
made, and the method of calculation of the rate of spikelet initiation and of the
number of phytomers converted into spikelets, see Rahman and Wilson (1977).
Results
The number of spikelets was lower at the lower light level in all cultivars (Table 1).
However, the effect of reducing light was different for different cultivars. Kogat,
which had the highest number (21 -5) at the higher level, had 16.8 (a reduction of
22%) at the lower level. Cultivar 8-23, which had the lowest number (16.3) at the
higher level, had 15.5 (a reduction of 5 %) at the lower level. On the other hand in
Gabo, which had a low number (16.5) at the higher level, the number fell to 11a8
(a reduction of 28 %).
Table 1. Number of spikelets per ear, rate of spikelet initiation, and apex length at FI
at two light levels (0.98 and 4.90 cal
hr-') in various wheat cultivars
grown at a 16 hr photoperiod and a constant temperature at 20°C
Cultivar
No. of spikelets
per ear
4.90
0.98
Rate of spikelet
initiation per day
4.90
0.98
Apex length (mm)
at FI
4.90
0.98
Triple Dirk
Gabo
Kalyansona
Kogat
Thatcher
Selkirk
8-23
8-27
A
N o terminal spikelet formed.
The rate of spikelet initiation was lower and the apex at floral initiation was shorter
at the lower level in all cultivars (Table 1). The cultivars differed widely in their
response to varying level for both the parameters. There were significant positive
correlations between rate of spikelet initiation and spikelet number (r = 0.67""")
and also between apex length and spikelet number (r = 0.76***).
The numbers of double ridges present at floral initiation remained fairly
constant at 4.0 regardless of light level; the only departure from this figure was
with 8-27, which had 5.0 double ridges at the higher level, and with Triple Dirk
and Gabo, which had 3.0 double ridges at the lower level.
Spikelet Number in Wheat. I1
The durations of the vegetative, spikelet and elongation phases for the various
cultivars at the two light levels are given in Table 2. Reduction in light led to increases
Table 2. Durations of vegetative, spikelet, and elogation phases at two light levels (0.98 and
4.90 cal cm-2 hr-') in various wheat cultivars grown at a 16 hr photoperiod and a
constant temperature of 20°C
Cultivar
Triple Dirk
Gabo
Kalyansona
Kogat
Thatcher
Selkirk
8-23
8-27
Duration of vegetative
phase (days)
4.90
0.98
20.0
23.0
22.0
18.3
23.5
33.5
28.0
25.3
Duration of spikelet
phase (days)
4.90
0.98
24.5
33.5
26.8
26.8
27.0
49.5
40.8
46.3
11.0
10.8
10.8
12.8
13.3
16.3
12.0
13.8
(1 %)
0.78
No terminal spikelets formed or ears emerged.
q
E
a
e3
0.5
,
0.9
12.5
12.8
13.5
14.5
17.0
25.5
23.0
A
0.91
LSD
A
Duration of elongation
phase (days)
4.90
0.98
23.3
23.5
21.5
24.3
26.0
25.8
25.0
27.5
35.8
34.3
37.5
1.3
1.8
1
A
84.0
A
A
A
I
0.5
1.3
Rate of spikelet initiation per day
0 6
Apex length
0.7
(mm)
E
I
0
I
I
1
20
40
0
Duration
20
40
(days)
Fig. 1. Relation between (a) rate of spikelet initiation, (b) apex length at floral
initiation, (c) length of the vegetative phase and (d) length of the spikelet phase,
and spikelet number per ear in eight wheat cultivars at two light levels (0.98 and
hr-I). 0-0 Lower level, 0-0 Higher level. I , Triple Dirk;
4.90 cal
2, Gabo; 3, Kalyansona; 4, Kogat; 5, Thatcher; 6, Selkirk; 7, 8-23.
in the lengths of all three phases. Again there were substantial differences between
cultivars, both in the lengths of the phases at the higher level and in the effects of
M. S. Rahman et al.
reducing light on the lengths of the phases. Within cultivars, the effects of reducing
the light on the lengths of the phases seemed to be largely independent of one another.
For example, in Thatcher, reduction in light level caused only small increases in the
duration of the vegetative and spikelet phases, but lengthened the elongation phase
from 26.0 days to 84.0 days. Cultivar 8-27 failed to initiate a terminal spikelet, and
three others (Kogat, Selkirk, 8-23) failed to produce ears at the lower level.
Advantage has been taken of the effects of varying light level on the rate of spikelet
initiation, apex length at FI, and lengths of the vegetative and spikelet phases to plot
the relation between spikelet number and each of these parameters (Fig. 1). Spikelet
number is seen to increase with increase in the rate of spikelet initiation and increase
in length of the apex, but to decrease with increase in lengths of the vegetative and
spikelet phases. In the absence of information to the contrary, the relations are shown
as linear, but we have no evidence that they are not curved. While the elevations of
the regression lines vary markedly between cultivars, the slopes for the different
cultivars for each of the relations, spikelet number with respect to rate of spikelet
initiation and apex length, are remarkably similar. However, with spikelet number in
respect to lengths of the vegetative and spikelet phases, there is no constancy between
cultivars either in elevation (mean spikelet number) or slope of the relations.
Table 3. Number of phytomers present at FI, final leaf number, and number of phytomers that
were converted into spikelets at two light levels (0.98 and 4.90 cal ~ m hr-')
- ~ in
various wheat cultivars grown at 16 hr photoperiod and a constant temperature of 20°C
Cultivar
No. of phytomers
present at FI
4.90
0.98
Final leaf number
0.35
0.51
4.90
0.98
No. of phytomers
converted to spikelets
4.90
0.98
Triple Dirk
Gabo
Kalyansona
Kogat
Thatcher
Selkirk
8-23
8-27
LSD
A
(1 %)
0.27
Could not be determined.
The number of phytomers present at FI, the final leaf number and the number of
phytomers that were converted into spikelets are given in Table 3. Reduction in
light was accompanied by decreases in all these parameters in most cultivars, but in
Selkirk, 8-23 and 8-27 the numbers of phytomers and leaves either increased or
changed little.
Discussion
Control o f Spikelet Number
The results of the present investigation show that the direct effect of light can be
real and substantial. This accords with the findings of Friend et al. (1963), Friend
Spikelet Number in Wheat. I1
(1965a), and Lucas (1971) who have observed that spikelet number increases with
increase in light intensity.
The more interesting outcome of varying light level has been its effect on correlations between various parameters and spikelet number. Spikelet number has
usually been found to be positively correlated with the length of the vegetative phase.
Lucas (1972) claimed that the length of the vegetative phase is a direct determinant
of spikelet number in wheat. However, in the present study, reducing lig4t increased
the length of the vegetative phase but decreased the number of spikelets, i.e. reversed
the usual relation between spikelet number and length of the vegetative phase.
Another factor which has been proposed as a determinant of spikelet number is
the length of the spikelet phase. This has usually been found to be positively
correlated with spikelet number (Rawson 1970). However, in the present study,
lengthening of the spikelet phase by reducing light has resulted in a decrease in
spikelet number, which is again a reversal of the usual trend.
It seems clear therefore that neither the length of the vegetative phase nor the
length of the spikelet phase can in themselves be critical determinants of spikelet
number.
Apex length at floral initiation and rate of spikelet initiation are the other factors
that might be considered. In experiments with photoperiod, apex length has been
shown to be positively associated with the duration of the vegetative phase (Lucas
1972; Rahman and Wilson 1977). Both Lucas and Rahman and Wilson have concluded that apex length at floral initiation is an important determinant of spikelet
number. The results of the present study, in which there was a significant positive
correlation between apex length and spikelet number (r = 0.76), confirm these
findings, but because a considerable proportion of the variation in spikelet number
remains unexplained (r2= 0.58) the conclusion that other factors are involved is
also suggested.
In their study involving changes in photoperiod Rahman and Wilson (1977)
concluded that the rate of spikelet initiation is an important determinant of the
spikelet number. In the present study, there was a significant positive correlation
(r = 0.67) between rate of initiation and spikelet number.
The number of phytomers present at floral initiation which become converted to
spikelets has also been shown to vary and to be a potential factor, although one of
secondary importance, in determining spikelet number (Rahman and Wilson 1977);
here the number of phytomers converted to spikelets varied from 1.0 in Selkirk at
the lower light level to 4.0 in 8-27 a t the higher level, and there was a positive
association between the number of phytomers converted to spikelets and final spikelet
number. Within cultivars the number of phytomers converted to spikelets was greater
at the higher light level. It seems possible that the proportions of phytomers, uncommitted at floral initiation, which become leaves or spikelets may depend on the
relative rates of differentiation of leaves and spikelets, i.e. an increase in rate of
spikelet initiation (differentiation) relative to rate of leaf differentiation might result
in a greater proportion of uncommitted nodes supporting spikelets. Although in
- ~
this experiment increasing the light level from 0.98 to 4.90 cal ~ r n hr-hesulted
in increases in both rate of leaf initiation (final leaf number divided by the length
of the vegetative phase) and rate of spikelet initiation, at the higher level rate of spikelet
initiation was about three times as fast as leaf initiation, but at the lower level it was
only about twice as fast.
M. S. Rahman et al.
Practical Implications
The cultivars differed greatly in their tolerance to low light, Kogat, Thatcher,
Selkirk, 8-23 and 8-27 being more affected in the elongation phase. In Australia
and India (latitude 27-37") wheat is grown during winter and therefore under low
light intensity and short photoperiod. In higher latitudes (50-65") such as in northern
Europe, Canada and Alaska, and in the high altitudes of Afghanistan (72000 m),
wheat is grown during summer and therefore under high light intensity and long
photoperiod. The present results suggest that the wheats selected for early maturity
in the winter-spring growing seasons of moderate latitudes were unwittingly selected
for tolerance to low light intensity as well as to short photoperiod.
The finding that wheat cultivars differ in their reaction to low light should be
considered in breeding programs. Although the light level available in the field
during the Australian winter is far higher than the low level used in the present
study, light intensity could become critical because of shading from weeds or mutual
shading by the wheat seedlings themselves during the spikelet initiation phase, and
thus could restrict spikelet number. This aspect of competition: should be considered
in regard to both the management of the crops and the possibility of producing
cultivars with higher spikelet number which are more tolerant of low light intensity.
The experiment was conducted under only two light levels, the higher level being
80% of mean daily light in winter. Furthermore the photoperiod used was higher
than that of the mean winter months in southern Australia. The responses to light
observed may be smaller than might be observed if a greater range of light level was
used. Experiments with a greater range of light levels and shorter photoperiods
(e.g. 12 hr) might be rewarding.
Acknowledgments
The authors are grateful to Mr G. M. Halloran of the School of Agriculture and
Forestry, University of Melbourne, for his helpful comments on the manuscript.
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58 1
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Manuscript received 16 November 1976