EFFECTS OF EXTENDING THE DAY LENGTH WITH LOW-INTENSITY
LIGHT ON THE GROWTH OF WHEAT AND COCKS FOOT
By G. HOFSTRA,*t G. J. A. RYLE,*t and R. F. WILLIAMS*
[Manu8cript received September 5, 1968]
Summary
Extending the photoperiod with low-intensity light increased the yield and
net assimilation rate (E A ) of wheat.
After a "day" made up of 4 hr of natural light and 4 hr of low-intensity light,
E A was increased 18 % by 8 hr of 50 f.c. supplementary light, but not by a supplement
of 10 f.c. light.
After 8 hr of natural light, EA was increased 5·6% by 16 hr of 10 f.c. light
and 16% by 16 hr of 50 f.c. light. These effects were coniirmed by a gas analysis
study and shown to be due to large reductions in respiration. Rates of photosynthesis
were reduced by the long-day treatments.
Gas analysis studies with cocksfoot established patterns of response to supplementary lighting up to 100 f.c. intensity, and for a range of temperatures.
These studies show ~hat the energy supplied by low-intensity supplementary
lighting can make a significant contribution to the carbon assimilation and growth
of plants.
1. INTRODUCTION
In many photoperiod studies involving grasses, pronounced effects of lowintensity supplementary light on morphological characteristics have been observed.
Work in this field has been reviewed by Evans, Wardlaw, and Williams (1964).
Several workers have also observed increases in net assimilation rates (E A) when
the day length was extended with low-intensity light. Ryle (1966b) established
large positive effects of long days on E A for a simulated sward of cocksfoot, as well
as increases in the rate of production of new leaf surface. Friend, Helson, and Fisher
(1967) reported an increase in EA for Marquis wheat when their 8 hr day length was
extended by low-intensity light. More recently Williams and Williams (1968) reported
large positive effects on EA for wheat. These effects were greater at a low lightenergy level (4 hr natural light) than at a high energy level (8 hr natural light), but
morphological effects on leaf growth were greater at the high energy level.
Various explanations have been advanced to account for this effect on E A :
differences in leaf display resulting in the more efficient interception of light (Ryle
1966b); differences in the timing of stomatal opening, and sink-strength effects
resulting from the promotion of leaf and leaf-sheath growth (Williams and Williams
1968). Direct effects of the low-intensity light on photosynthesis and respiration
have been rather neglected in this context. Ryle (1966a, 1966b) attempted to avoid
the issue by equalizing the total light-energy supplied per day, and Williams and
* Division of Plant Industry, OSIRO, P.O. Box 109, Oanberra Oity, A.O.T. 2601.
t Present address: Department of Botany, University of Guelph, Guelph, Ontario, Oanada.
:\: Present address: The Grassland Research Institute, Hurley, Berkshire, U.K.
Aust. J. biol. Sci., 1969,22, 333-41
334
G. HOFSTRA, G. J. A. RYLE, AND R. F. WILLIAMS
Williams (1968) thought it unlikely that the small amount of additional energy could
account for all of their effect on EA.
The present studies were undertaken in an attempt to establish the physiological basis, for the increases in EA observed in wheat and cocksfoot.
II.
EXPERIMENTAL PROCEDURE
The work falls naturally into two sections: that in which the effects of varying intensities
of supplementary lighting were precisely defined using growth analysis procedures, and that in
which these effects were further examined by gas analysis techniques.
Ca) Growth Studie8
Two experiments were conducted with the lighting and temperature conditions set out
in Figure 1. Treatments Sand L50 of experiment 1 were similar to treatments 4S and 4L used
by Williams and Williams (1968), though with a photoperiod extension of 8 hr in L 50 instead
of 16 hr in 4L. Treatments Sand L50 of experiment 2 were the same as treatments 8S and 8L
of Williams and Williams (1968). In each experiment, a parallel long-day treatment, Llo, was
included, Vl"ith supplementary lighting of one-fifth the intensity normally used for this purpose in
CERES, the Canberra phytotron.
Experiment
S
Experiment 2
:.::::-;'::-
8'3012'304·30
a.m.
LIO
I
LS~
1
p.m.
p.m.
12 '30
8 ·30
t?,;,·:::'.~
<b
20"C-~-----
S
a.m.
EI--
LS~
:.I
IS"C-----,l>
f:': ,,<ti,'<:,,',····......
8 '30
LIO
4'30
8 '30
p.m.
~
""-20"C--~-----IS"C
lC',':'. i::.::""""'''·
----i>
'''':':'''';'':'''''::':'-::::':J
C=:J
Natural light
~ Low-intensity supplementary light (10 I.e.)
_
Darkness
[<:,·n
Standard supplementary light ( 50 I.e.)
Fig. I.-Treatment diagrams for experiments 1 and 2. Within each experiment the
"day" conditions are the same for all treatments. S, short-day control; Llo, long day
with low-intensity supplementary lighting; L50, long day with standard supplementary
lighting.
Many experiments in controlled environments are conducted on a single treatment-single
cabinet basis because of limitations of space. Such experiments are thus conducted with inadequate statistical control. Where, as in the present experiments, rather small quantitative
effects have to be determined with precision, sound experimental design is essential. Accordingly,
the experiments were set up in three CERES, C-type cabinets, each with three compartments.
These compartments were known to have differing natural-light characteristics (e.g. the northern
compartments receive more light than the others), so the three treatments were allotted in a 3 by 3
Latin square design, in which the cabinets were the rows and the compartments the columns.
A spring wheat (Triticum ae8tivum L. cv. Gabo) was the main test plant in each experiment,
but a second spring wheat (cv. Nabawa) and two winter wheats (cv. Winter Raven and Winter
Minflor) were also used in experiment 1. The seeds were soaked for 24 hr, selected for uniformity
of germination, and sown in perlite. Harvests were taken 10, 17, and 24 days after seed soaking.
Leaf areas were determined from length and width measurements (Williams and Williams 1968)
in experiment 1, and on an air-flow planimeter in experiment 2. Dry weights were determined
after drying at 80°C.
In experiment 1 three replicates of four plants were harvested from each cabinet compartment, and in experiment 2 there were four replicates of four plants. Shoot apices were dissected
on day 24.
EFFEOT OF PHOTOPERIOD ON PLANT GROWTH
335
In experiment 1 there were two degrees of freedom each for treatment, cabinet, compartment, and error (plot error)_ The replication of three gave 18 degrees of freedom for a within-plot
error. Out of a total of 13 analyses for a range of growth-response attributes, only four gave
variance ratios (plot error/within-plot error) in excess of unity-none were significantly so. In
these circumstances pooled errors with 20 degrees of freedom were used. In experiment 2 there
were 27 degrees of freedom for within-plot error, but we were less fortunate in that the variance
ratios were often in excess of unity, sometimes significantly so. The minimum significant difference
values for this experiment were based on the plot error, pooled only with the terms for cabinet
or compartment as appropriate.
(b) Gas Analysis Studie8
One experiment was conducted with wheat and two with S37 cocksfoot (Dactylis glomerata
L.). Long days produce very large morphogenetic effects on vegetative growth in cocksfoot
(Ryle 1966a). In each case, potted seedlings were enclosed in a gas-tight Perspex box through
which air could be passed at rates up to 30 litres per minute. Samples of air drawn off before and
after pa,ssing through the plant chamber were compared for 002 content in a Grubb Parsons infrared
gas analyser sensitive to O· 5 p.p.m. of 002. Air temperature was measured with a shaded thermocouple in the exit line from the plant chamber, and plant temperature with a thermocouple
inserted into the leaf sheath of one tiller.
For experiment 3 the wheat plants were grown with those of experiment 2. On three
successive days about the time of harvest 2 (day 17) of that experiment, the gas exchange of
three pots of four plants of the three treatments was measured for 24 hr under growth conditions
simulating those of experiment 2 but using artificial light.
For experiments 4 and 5 the cocksfoot plants were grown for 7 weeks, in pots containing
a mixture of perlite and vermiculite, in a controlled-environment cabinet. The plants received
8 hr of naturalligh't each day at a temperature of 25°0; during the 16 hr dark period the temperature was 20°0.
In experiment 4, rates of 00 2 evolution were measured in the dark and in a range of intensities of incandescent light up to 100 f.c. All light intensities were measured at plant height
inside the Perspex chamber with an EEL photometer. Varying intensities of incandescent light
were obtained by altering the wattage of the four incandescent bulbs or by covering the plant
chamber with Sarlon shades. For experiment 5, the effect of temperature on 002 evolution in dim
light and in the dark was investigated by altering the temperature of the enclosing controlledenvironment cabinet over the range 13-29°0.
III.
RESULTS
(a) Growth Studie8
(i) Experiment 1
The mean yields for the three harvests are given in Ta:ble 1. These show a
positive and increasing effect of the Lso treatment (Fig. 1) but no significant effect
of LIO over S. Leaf areas per plant were affected in much the same way, though
relatively less than the dry weights_
Growth analysis in terms of net assimilation rate (E A) and leaf area ratio (F A)
showed that the increase in yield with Lso was due to a highly significant increase
in EA for harvest interval 1-3 (Table 1). By harvest 3, a small negative effect on
F A had developed in Lso but not in L IO .
The treatments of this experiment had rather small morphogenetic effects.
There were no significant effects on either the lengths or areas at maturity of the
first three leaves. However, there were significant though rather small effects on the
336
G. HOFSTRA, G. J. A. RYLE, AND R. F. WILLIAMS
lengths of leaf sheaths by harvest 3 (Table 2). Even LlO had a small effect on leaf
sheath 3. Dissection of the apices on day 24 showed all apices of S to be vegetative;
those of L 10 and L50 had passed the double-ridge stage of floral induction, L50 being
more advanced than L 10 .
TABLE
EXPERIMENT
1:
1
YIELD AND GROWTH ANALYSIS FOR WHEAT,
cv.
GABO
Total Dry Weight
(mg per plant)
Net Assimilation Leaf Area
Rate, EA
Ratio, FA
A
(d'm2 g-I)
(mg dm- 2 day-I)
Treatment
I
\
Harvest 1 Harvest 2 Harvest 3 for Harvest
for Harvest 3
(day 24)
Interval 1-3
(day 24)
(day 10)
(day 17)
S
26·8
52·1
96·7
37·5
2·98
LID
28·1
51·6
98·2
37·0
3·00
L5D
28·5
57·3
119·8
44·3
2·73
Minimum significant differences:
5%
n.s.
2·34
8·48
4·35
0·149
1%
n.s.
3·20
11·52
5·93
0·203
0·1%
n.s.
4·33
15·50
8·02
0·271
Since E A was increased by L50 and not by LlO and there was no significant effect
of L 10 on yield after 24 days of growth, it appears that the extra energy of long-day
treatment L50 is mainly responsible for this effect.
TABLE
EXPERIMENT
1:
2
LENGTHS OF LEAF SHEATHS AT HARVEST
3
Measurements are in millimetres
Treatment
Leaf Sheath 1
Leaf Sheath 2
Leaf Sheath 3
S
44
70
85
LID
46
74
92
L5D
46
79
103
Minimum significant differences:
5%
n.S.
4·9
6·2
1%
n.S.
6·7
8·4
0·1%
n.s.
9·1
11·4
The yields on day 24 for the other wheat cultivars included in this experiment
were also greatest with L 50 , and gave no significant responses with L 10 . Final yields
337
EFFECT OF PHOTOPERIOD ON PLANT GROWTH
for Winter Raven, a winter wheat grown here without vernalization, were 104·6,
106·3, and 127·4 mg for S, L lO, and Lso respectively. Floral initiation would thus
seem not to have been involved in the effect.
Since the morphogenetic effects of the long-day treatments on leaf growth
were small with all cultivars, it could be argued that there had been little opportunity
for the expression of any possible sink-strength effect of the extra growth. It is
likely, for all three treatments, that carbohydrate levels were low and limiting growth.
(ii) Experiment 2
In this experiment the natural-light period was increased to 8 hr and the
supplementary light period to 16 hr (Fig. 1) in order to obtain greater effects on leaf
and sheath size.
The mean yields for the three harvests are given in Table 3. The final yields
were much higher than in experiment 1 but, as explained in Section II, it was more
difficult to establish significant effects. There were consistent but non-significant
increases with L 10 , and greater increases with Lso (P<O·Ol on day 24).
TABLE
EXPERIMENT
2:
3
YIELD AND GROWTH ANALYSIS FOR WHEAT,
Total Dry Weight
(mg per plant)
Treatment
"r
\
Harvest 1 Harvest 2 Harvest 3
(day 10)
(day 17)
(day 24)
cv.
GABO
Net Assimilation
Rate, EA
(mg dm- 2 day-I)
for Harvest
Interval 1-3
Leaf Area
Ratio, FA
(dm2 g-I)
for Harvest 3
(day 24)
S
32·1
91·8
261·5
75·0
2·39
LlO
34·2
102·5
298·4
79·2
2·11
Lso
34·0
108·1
334·5
87·0
1·96
Minimum significant differences:
n.s.
n.s.
5%
42·2
3·97
0·133
1%
n.s.
n.s.
70·0
6·59
0·201
0·1%
n.s.
n.s.
130·9
12·32
0·323
Growth analysis showed that, as in experiment 1, the effects of treatment on
final yield were attributable to highly significant effects on E A for harvest interval
1-3 (Table 3). Even with the rather conservative statistical test available for this
experiment, the increase with L 10 was significant (P<O·05), and that with Lso
almost attained the O· 1 % level. During the same time interval the effects of the
long-day treatments on F A developed in the reverse direction. The values for harvest 3
are given in Table 3.
The morphogenetic effects of treatment were much greater than in experiment 1.
There were effects on the lengths of the first three leaves, and still greater effects on
G. HOFSTRA, G. J. A. RYLE, AND R. F. WILLIAMS
338
the lengths of the leaf sheaths (Table 4). Since the leaf-sheath measurements were
made from the base of the plant in all cases, they include the lengths of the internodes
below the specified leaf. Even for leaf sheath 3, however, these would contribute
less than 5 mm to the values given for L 10 and L 50 . It will be noted that the long-day
effects on the lengths of leaves and leaf sheaths were not significantly affected by the
intensity of the supplementary lighting.
TABLE
EXPERIMENT
2:
4
LENGTHS AT MATURITY OF LEAF BLADES AND LEAF SHEATHS
Measurements are in millimetres
Treatment
Blade 1
S
82
146
L 10
93
L50
90
Blade 2
Sheath 1
Sheath 2
Sheath 3
210
28
40
66
171
251
36
63
96
171
249
37
67
100
Minimum significant differences:
22·4
5·6
5%
1%
0·1%
Blade 3
6·4
4·6
3·2
9·4
9·3
33·9
9·7
7·0
4·9
14·2
17·5
54·4
15·7
11·2
7·9
22·8
Dissection of the apices on day 24 again showed all apices of S to be vegetative ;
those of L 10 and L50 all had inflorescences about 4 mm long and with stamen primordia
differentiating.
In spite of the fact that morphogenetic effects of some magnitude were produced
by the two long-day treatments, there seems little doubt that the quantitative effects
of treatment-expressed in yield increments and net assimilation rates-were
determined primarily by the extra energy of the supplementary lighting of the longday treatments. In particular, it will be noted that E A for L 10 was 5·6% higher than
that for S, and that EA for L50 was 16% higher. Any hypothetical sink-strength
effect common to LlO and L50 must have been quite small.
(b) Gas Analysis Studies
(i) Experiment 3
This gas analysis study on plants grown concurrently with those of experiment 2
gave results which show a remarkable agreement with the growth analysis results
of that experiment. Thus Table 5 shows the net CO 2 gain per unit area to have been
increased by 7·4 and 14·1 % respectively for LlO and L50 compared with increases
in EA of 5·6 and 16·0% in experiment 2. The table further shows these gains to
have resulted from large reductions in CO 2 evolution during the "night" period.
To some extent these effects were offset by reductions in the rates of photosynthesis
339
EFFECT OF PHOTOPERIOD ON PLANT GROWTH
(Table 5). Incidentally there was no effect of these diverse treatments on the induction
time for photosynthesis at the beginning ofthe "day" period, as was thought possible
by Williams and Williams (1968). The induction period was about 35 min for all
treatments.
TABLE
EXPERIMENT
3:
5
GAS ANALYSIS DATA FOR WHEAT,
cv.
GABO
Treatment
S
Leaf area (dm 2 )*
Net CO 2 assimilated in 8 hr day (mg)*
Net CO 2 evolved in 16 hr "night" (mg)*
Net C02 gain in 24 hr period (mg)*
Net CO 2 gain per unit area (mg dm- 2 day-I)
Percentage increase over treatment S
Rate of photosynthesis by "day" (mg dm- 2 hr- I )
Percentage decrease below treatment S
Rate of respiration at "night" (mg dm- 2 hr- I )
Percentage decrease below treatment S
* Values for
1·47
175·7
48·1
127·6
86·8
14·94
2·05
LlO
2·66
290·2
42·0
248·2
93·2
7·4
13·64
8·7
0·99
52
L50
2·25
244·4
22·2
222·2
99·0
14·1
13'58
9·1
0·62
70
12 plants.
That the lO f.c. supplementary lighting should reduce the 00 2 evolution rate
by more than 50% is quite remarkable, and seems further to reduce the likelihood that
sink-strength effects are of importance under these conditions of light and temperature.
(ii) Experiment 4
Respiration of cocksfoot plants at 21-23°0 varied between 2 and 5 mg g-l hr- 1
according to the time which had elapsed since exposure to high-intensity light, and
to the duration of the light period. Figure 2 shows the effects of light of low intensity
on the 00 2 evolution of whole plants, for two representative runs. Incandescent
light intensities within the range 30-60 f.c. clearly offset a considerable proportion
of the respiratory 00210ss, while compensation was achieved at 100-120 f.c. Essentially the same response as is shown in Figure 2 was obtained whether the variation
in light intensity was obtained by varying the wattage of the incandescent bulbs
used or by shading with Sarlon of various textures and colours.
(iii) Experiment 5
The effect oftemperature on whole-plant dark respiration and on 00 2 evolution
in light of 60 f.c. is shown in Figure 3. As in experiment 4, the absolute rates of 00 2
evolution varied between plants and with the time at which the measurements
were taken. The relation was curvilinear, though its exact form varied according to
whether the measurements were made as temperature increased or decreased. The
relation was essentially the same for all plants examined, and the low-intensity light
reduced the evolution of 00 2 by about 1· 5 mg g-l hr- 1 over the range of temperatures
examined. The compensation point was reached at 17°0 in the example of Figure 3,
but occurred over the range 13-17°0.
G. HOFSTRA, G. J. A. RYLE, AND R. F. WILLIAMS
340
The large effects of light of low intensity on the CO 2 evolution of whole plants
in these experiments indicates that such light energy is very efficiently used. The
3·5,
I
Fig. 2
Fig. 3
30l
2·5
'I
~
..c
'I
2·0
co
co
E
c
..8
1·5
0
j.Q
..2
>
OJ
0'"
U
0·5
of.
5'0
I
~I
."
'~
'0
4·0
o~
"i
1:
'Ico
co
E
.2
:a I
>
0
Cmn,,~.,">" ~O'"-, ,
-------
~
25
50
75
.
';; 20
"<~
-05~
3·0
~Il
+
U
°,
100
I
-j.Q
I
125
10
Light intensity (I.e.)
-.---
_ ... "..,.,..,,-
IS
I
,,/
/
.
/
I
I
",/1'
I
I
I
I
I
0/
0
/
/
/
/
/
/
/
"
/
",'"
,.,'/"
Compensation
,.;,;,,0
----
......... -
_ _ 0 ' ....
""
/
/
/
I
I
I
I~
20
25
30
Temperature ("C)
Fig. 2.-Effects of low.intensity incandescent lighting on C02 evolution by whole
cocksfoot plants per gram dry weight. Leaf temperature 22°C. e, 0 Morning runs .
• , 0 Afternoon runs.
Fig. 3.-Effects of low-intensity lighting and temperature on CO 2 evolution by whole
cocksfoot plants per gram dry weight. e Darkness. 0 60 f.c.
practical importance of this effect is demonstrated in Table 6, which shows the net
carbon fixation of whole cocksfoot plants receiving equal amounts of radiant energy
during either a long or short, day. Because the supplementary light is used about
TABLE 6
SHORT-TERM MEASUREMENTS OF C02 EXCHANGE* IN DIFFERENT LIGHT REGIMES AND IN DARKNESS
IN WHOLE PLANTS OF S37 COCKSFOOT
There were five replicate measurements per treatment, with three plants per measurement;
temperature 22-24°C. Other growth conditions as described in Section II(b)
Duration Long-day Light
(hr)
Conditions
C02 Exchange
(mg CO 2 /g/hr)
Short-day Light
Conditions
C02 Exchange
(mg C02/g/hr)
Significance
of Difference
8
Fluorescent
(2200 f.c.)
+32·1
Fluorescent
+ incandescent
(2260 f.c.)
+32·6
P<O'OI
8
Incandescent
(60 f.c.)
-1·86
Dark
-4·55
P<O'OOl
8
Dark
-4·55
Dark
-4·55
* Calculated net C02 uptake per day was 206 mg C02/g under long-day and 189 mg C02/g
under short-day conditions.
EFFECT OF PHOTOPERIOD ON PLANT GROWTH
341
five times more efficiently for assimilation when given during the dark than when
given as a supplement to the main light, the long-day plant fixes 17 mg more CO 2
each day than the corresponding short-day plant. Furthermore, there is no accurate
empirical method of determining the amount of extra light short-day plants need
to overcome this advantage, since it depends on the structure of the plant canopy in
relation to light interception, the energy conversion efficiency, temperature, and
possibly other factors.
IV.
DISCUSSION
There seems little doubt that the increases in yield and net assimilation rate
when the day length was extended by low-intensity light were determined primarily
by the extra energy of the supplementary lighting. In experiment 1 the increases
were accompanied by quite small morphogenetic effects on leaf growth. In experiment
2, where large effects on leaf and sheath lengths were recorded, there was room for
a small sink-strength effect, but the extra energy of the supplementary lighting
could still account for most of the effect of treatment on EA. The gas exchange
measurements indicate the high efficiency with which the low-intensity light can
be used to offset respiration.
That the photosynthetic rates for the long-day wheat plants were slightly lower
than for the short-day plants was somewhat unexpected. This effect could be due to
differences in light interception or to differences in leaf structure and chlorophyll
content. That the promotion of leaf and sheath growth by long days did not constitute a very strong sink in these conditions should not be taken as an indication
that it would not be effective when photosynthesis itself was less limited by the
environment (e.g. at lower temperatures).
The unexpectedly large effects of supplementary lighting on yield and E A suggest
that care should be taken in interpreting the results of photoperiodic studies, especially where indices of response are quantitative rather than qualitative in character.
These studies show that the energy supplied by such lighting can make a significant
contribution to the carbon assimilation and growth of plants.
V.
ACKNOWLEDGMENTS
The authors acknowledge their indebtedness to Dr. A. T. Pugsley; Director,
Agricultural Research Institute, Wagga Wagga, N.S.W., for seed samples of the
winter wheats used in experiment 1; to Dr. L. T. Evans for constructive criticism of
the manuscript; and to Misses F. Lin and L. Walker for technical assistance. The work
was carried out in CERES, the Canberra phytotron, while one of the authors (G.H.)
was supported by a Canadian National Research Council Postdoctoral Fellowship,
and another (G.J.A.R.) held a Stapledon Memorial Fellowship.
VI.
REFERENCES
EVANS, L. T., WARDLAW, 1. F., and WILLIAMS, C. N. (1964).-In "Grasses and Grasslands".
(Ed. C. Barnard.) pp. 102-25. (Macmillan & Co., Ltd.: London.)
FRIEND, D. J. C., HELSON, V. A., and FISHER, J. E. (1967).-Oan. J. Bot. 45, 117-31.
RYLE, G. J. A. (1966a).-Ann. appl. Biol. 57, 257-68.
RYLE, G. J. A. (1966b).-Ann. appl. Biol. 57, 269-79.
WILLIAMS, R. F., and WILLIAMS, C. N. (1968).-Aust. J. biol. Sci. 21, 835-54.