Available online at www.sciencedirect.com
Environmental and Experimental Botany 63 (2008) 216–223
Solar irradiance level alters the growth of basil (Ocimum basilicum L.)
and its content of volatile oils
Xianmin Chang a,∗ , Peter G. Alderson b , Charles J. Wright b
a
b
Agronomy Institute, Orkney College, East Road, Kirkwall, Orkney KW15 1LX, UK
Division of Agricultural and Environmental Sciences, School of Biosciences, The University of Nottingham, Sutton Bonington Campus,
Loughborough, Leicestershire LE12 5RD, UK
Received 27 February 2007; received in revised form 3 October 2007; accepted 8 October 2007
Abstract
The experiments were commenced in March 2003 and repeated in June 2003 at Sutton Bonington Campus, the University of Nottingham, UK,
to investigate the effect of irradiance on plant growth and volatile oil content and composition in plants of basil. Four levels of irradiance were
provided in the glasshouse, i.e. no shade (control), 25, 50 and 75% glasshouse irradiance. It suggested that basil grows well in full sun, however it
can tolerate light shade. Heavy shading (75%) to provide a light integral of 5.3 moles m−2 d−1 resulted in shorter plants, lower weight, smaller leaf
area, less shoots and higher specific leaf area, and also strongly reduced the rate of photosynthesis. There was no difference in CO2 assimilation
rate between 24.9 moles m−2 d−1 light integrals (no shading) and 13.5 moles m−2 d−1 light integrals (25% shading). Shading effectively reduced
leaf temperature when air temperature was less than 30 ◦ C, but heavy shading (75%) could not reduce leaf temperature when air temperature was
above 36 ◦ C due to a limitation of free air convection. Consequently, leaf temperature increased. Heavy shading strongly reduced total volatile oil
content in fresh leaves, especially in older plants (shading treatment applied at the 3 leaf-pair growth stage). There were three chemical compounds
in basil leaves, namely linalool, eugenol and methyl eugenol, influenced by the shading treatments. Linalool and eugenol, which contribute to the
characteristic taste of basil, were significantly increased by high daily light integrals, whereas methyleugenol was increased by lower daily light
integrals. No differences in the relative content of 1,8-cineole, one of the key aromatic compounds of Ocimum species, were observed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ocimum basilicum; Irradiance; Growth; Volatile oils
1. Introduction
Light is the main factor controlling plant growth and development and, plants are affected in a complex manner by irradiance
at all stages of their growth. The architecture of plants is dependent on the quantity, direction, duration and quality of light
(Nilsen and Orcutt, 1996; Rich et al., 1987; Went, 1941). Photosynthesis is dependent on light for energy and for the induction
of enzymatic processes. High irradiance affects plant processes
directly through its effect on factors such as enzyme activity and
photosynthesis, but it also affects plant physiological processes
indirectly by its impact on thermal attributes of the tissues. Leaf
temperature is regulated primarily by the influences of net radiation, latent heat exchange, conduction and convection, and air
temperature. Some leaves may experience as much as 10–15 ◦ C
∗
Corresponding author. Tel.: +44 1856 569294; fax: +44 1856 569008.
E-mail address: xianmin.chang@orknry.uhi.ac.uk (X. Chang).
0098-8472/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.envexpbot.2007.10.017
difference between air and leaf temperature under certain conditions, however, most leaves experience a differential of 5 ◦ C
or less (Nilsen and Orcutt, 1996).
Plant responses to light include a variety of adaptations at
physiological (photosynthesis, nutrient uptake) and biochemical
(pigments, carbohydrates) levels. Such responses are translated
into alterations of growth rate, plant architecture and morphological characteristics (Peralta et al., 2002). Exposure of plants
to excessive light is a well-known cause of photoinhibition
(Sorrentino et al., 1997).
Light is a key factor in the ultimate production of many compounds because it supplies the energy needed to fix carbon. Light
intensity plays an important role in the biosynthesis of medicinally important metabolites, for example camptothecin (CPT),
an anti-cancer agent in the tree of joy (Camptotheca accuminata
L.), is produced at higher levels in shade conditions (Kaufman
et al., 1999). Plants of Rhynchelytrum repens, a tropical grass
species popularly used for diabetes treatment, growing under
natural irradiance showed increased total soluble carbohydrates
X. Chang et al. / Environmental and Experimental Botany 63 (2008) 216–223
and higher fluctuations in starch content compared with plants
cultivated under low irradiance (Souza et al., 2004). In contrast,
plants of Aloe vera under high irradiance produced more axillary shoots resulting in higher dry matter but there were only
minor effects on the concentration of soluble carbohydrates and
aloin in the leaf exudates, suggesting that there were no substantial effects of irradiance on either primary or secondary carbon
metabolites (Paez et al., 2000). For basil (Ocimum basilicum L.),
Lee et al. (1994) used three different shading nets to study the
effects of irradiance on plant size, weight and oil content in basil.
Plants were either shaded with white cheesecloth (21% shading), black cheesecloth (50%) or Gariso (a black material also
giving 50% shading) or were not shaded (control). Growth was
generally better in plots shaded with white cheesecloth and in
control plots than in the more heavily shaded plots. Plant height
was greatest under black cheesecloth, but leaf area was greatest under white cheesecloth. No significant difference in total
fresh weight (FW) was observed between plants under white
cheesecloth and the controls. The essential oil content of the
leaves and the total oil content/plant were higher in plots under
white cheesecloth than in control plots. It was suggested that
light shading was beneficial to growth and oil production. The
results were different, however, when the same shade percentage
was achieved with different shading materials probably due to
the shading materials influencing the light quality, especially the
R/FR ratio, which could lead to changes in plant morphology
(Went, 1941; Morelli and Ruberti, 2002). Thus, it is difficult to
conclude whether the effects observed were due to light quality
or light intensity.
The chemical composition of volatile oils in basil has been
investigated since the 1930s, and by 1999 approximately 190
chemical compounds had been identified. Due to their high relative content in leaves and extensive uses in plant protection,
food stocks and medicine during the past decades, eugenol, 1,8cineole and linalool have been studied extensively (Hiltunen and
Holm, 1999).
This paper reports the effect of irradiance on plant growth
and volatile oil content and composition in plants of sweet basil
using green ‘Rokolene’ shading nets with different percentage
light transmissions, since Rokolene netting has been previously
shown to have no effect on light quality (Wright and Sandrang,
1995). It was hypothesized that low irradiance would result in
shorter plants with reduced dry matter, fewer leaves and shoots
and a lower content and change in the composition of volatile
oils.
2. Materials and methods
2.1. Plant materials
Seeds of basil cv. ‘Basil Sweet Genovese’ obtained from
Nickerson-Zwaan Ltd. (Lincolnshire, UK) were sown on the
surface of Levington F2s compost (Fisons Horticulture Ltd.;
Ipswich, UK) in plastic trays. Seedlings with one pair of
unfolded leaves were transplanted to 12 cm diameter pots containing Levington M2A compost, and there was one plant per
pot. The mean daily temperature in the glasshouse was set
217
at 21 ± 3 ◦ C for seed germination and plant raising. Between
September and April, 16 h of supplementary lighting was provided by 400 W high-pressure sodium lamps (Poot Lichtenergie,
Holland). A shading experiment in a glasshouse was commenced
in March 2003 (first experiment) and repeated in June 2003
(second experiment).
Shading treatment design and data collection: Three levels
of reduced total irradiance were achieved by covering metal
support frames (140 cm × 70 cm × 70 cm) with green ‘Rokolene’ shading net of different percentage light transmissions.
In this way, four levels of irradiance were provided in the
glasshouse, i.e. no shade (control), 25, 50 and 75% glasshouse
irradiance, and a range of mean daily light integrals for each
shade treatment was calculated (Table 1). Plants at the 1 and
3 leaf-pair growth stages were placed in the shade treatments
for up to 2 weeks, and each treatment had 2 replicates with 36
plants.
The irradiance and temperature for each treatment were measured 50 cm above the benches using a data logger (Campbell CR
10, Campbell Scientific, Inc. Logan, UT) at intervals of 60 min.
Irradiance readings were subsequently converted to W m−2 .
Daily light integral (DLI) (moles m−2 d−1 ), i.e. the total quantity
of light delivered over the course of an entire day, was calculated
by the formula (Faust, 2002):
DLI (moles m−2 d−1 ) = [Sum of data logger reading each hour
× 0.0036] × 2
where 0.0036 is derived from 60 s/min × 60 min/h ÷
1,000,000 moles mole−1 .
Midday solar PPFD was also measured which was between
600 and 1600 moles m−2 s−1 in the glasshouses without shade
during the period of the experiment.
2.2. Leaf temperature
An infrared thermometer (Kane-May KM823, Comark Ltd.,
England) was used to measure the temperature of the third
and fourth pairs of leaves when shade treatment was applied
at the one leaf-pair and three leaf-pair growth stages respectively. There were ten replicates (ten individual plants) for each
treatment.
2.3. Photosynthesis
An infrared gas analyser (CIRAS I, Scotrail and the
University of Strathclyde, Scotland) was used to measure photosynthesis rate. Measurements were made on the 5th pair of
leaves of plants that had received 2 weeks of shading treatments
at the 3 leaf-pair growth stage. The rate was calculated from the
difference between the CO2 concentration entering (Cin ) and
leaving (Cout ) and the flow rate through the cuvette using the
following formula:
11A = Cin × W − Cout × (W + E)
= −[W × (Cout − Cin ) + Cout × E]
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X. Chang et al. / Environmental and Experimental Botany 63 (2008) 216–223
Table 1
Mean daily light integrals (moles m−2 d−1 ) during shading treatments
Experiment
Date (2003)
Control
25% shading
50% shading
75% shading
First
Second
09/April–06/May
02/July–23/July
19.8
24.9
12.2
13.5
8.8
11.3
4.3
5.3
From the known amounts of the pure chemicals and the peak
areas, calibration formulae were established.
For plants that received the shade treatment at the one leafpair and three leaf-pair growth stages, leaf samples for volatile
oil analysis were collected from the third and fifth pairs of leaves
respectively. Three samples were analysed for each replicate of
the irradiance treatments.
Chemical compounds were identified on the basis of relative
retention times, using standard samples and comparing peaks
with a Library (Wiley7n.L, supplied by Agilent Technologies,
USA).
where A is the rate of CO2 exchange in the cuvette
(moles m−2 s−1 ), Cin the CO2 concentration of air into cuvette
(moles mole−1 ), Cout the CO2 concentration of air leaving
cuvette (moles mole−1 ), W the mass flow of dry air per unit leaf
area (moles m−2 s−1 ), E the transpiration rate (moles m−2 s−1 ),
and Cout − Cin is the CO2 difference (CO2 Diff.) that was calculated and displayed by the CIRAS.
2.4. Measurement of plant growth
At the end of the treatments, 18 plants of each treatment were
harvested for growth analysis and, plant height, plant weight
(fresh and dry), leaf weight (fresh and dry), number of shoots,
number of leaf-pairs on the main stem, leaf area and specific leaf
area were recorded.
2.6. Statistical analysis
Using Genstat, an analysis of variance (ANOVA) table was
calculated for each growth parameter and for volatile oil content
and composition. Differences between treatments were assessed
using the F-test, and the Least Significant Difference (LSD) was
calculated at the 0.05 probability level.
2.5. Extraction and identification of volatile oils
Samples (5 g) of fresh leaf harvested from the third or fifth
pair of leaves of plants were analysed for volatile oils using
TD–GC/MS as described by Chang (2004) and Chang et al.
(2005).
External standards were used for calibration of the TD/GC.
Standard solutions (0.025, 2.5, 5 and 7.5%) of 1,8-cineole,
linalool plus eugenol were prepared by mixing, and ethyl acetate
was selected as solvent. There were three replicates for each
density and 1 l was injected in each Universal tube (replicate).
3. Results
3.1. Plant growth
Differences in plant morphology as well as plant and leaf
weights (fresh and dry) were observed following 2 weeks of
shading treatments. Since both experiments produced simi-
Table 2
Growth parametersa of basil plants after 2 weeks of shading treatments applied at 1 and 3 leaf-pair growth stages
Growth parameter
Growth stage
Mean daily light integral (moles m−2 d−1 )
24.9
20.3
45.3
11.3
Plant height (cm)
1
3
Plant dry weight (g)
1
3
0.98
3.24
0.81
3.01
0.67
2.21
Leaf dry weight (g)
1
3
0.67
2.01
0.57
1.81
Number of axillary shoots
1
3
2.5
7.0
Number of leaf-pairs
1
3
Leaf area (cm2 )
Specific leaf area (cm2 g−1 )
a
21.0
43.0
13.5
19.1
40.7
SED
P
5.3
10.6
24.5
1.377
1.577
<0.001
<0.001
0.21
0.88
0.067
0.149
<0.001
<0.001
0.45
1.39
0.15
0.67
0.040
0.079
<0.001
<0.001
1.5
6.5
1.5
6.0
0
2.3
0.408
0.465
<0.001
<0.001
4.0
6.0
4.0
6.0
4.0
6.0
3.4
5.2
0.102
0.118
<0.001
<0.001
1
3
237.1
700.0
233.7
714.0
213.7
629.0
110.4
356.0
18.690
37.900
<0.001
<0.001
1
3
351.8
363.5
409.3
393.8
477.0
453.5
731.0
533.7
49.200
36.300
<0.001
<0.001
Plant growth parameters are expressed on a per plant basis. SED: Standard error of the difference; P: probability.
X. Chang et al. / Environmental and Experimental Botany 63 (2008) 216–223
lar results, the data presented is from the second experiment
(Table 2).
Plant height was increased by increasing the light integral
from 5.3 to 11.3 moles m−2 d−1 , for plants treated at both the
1 and 3 leaf-pair growth stages. Light integrals greater than
11.3 moles m−2 d−1 applied at the 1 leaf-pair growth stage and
13.5 moles m−2 d−1 applied at the 3 leaf-pair growth stage did
not increase plant height further, suggesting a light saturated
response at these levels. The fresh and dry weights of plants
and leaves were significantly increased with increases in light
integral for 2 weeks at both plant ages, however, the differences
between 24.9 and 13.5 moles m−2 d−1 were not significant, suggesting a saturation of the light level. Compared with treatments
applied at the 1 leaf-pair growth stage, differences in the effects
of the 11.3 and 13.5 moles m−2 d−1 light integrals applied at the
3 leaf-pair growth stage were significant. With increasing light
integrals, numbers of axillary shoots increased by 2.5 and 4.7
when treatments were applied at the 1 and 3 leaf-pair growth
stages respectively. Shoot numbers were significantly lower in
plants under the 5.3 moles m−2 d−1 light integral compared with
plants at the 24.9 moles m−2 d−1 light integral, and there was
more than threefold difference when treatments were applied
at the 3 leaf-pair growth stage. Differences between plants at
the 24.9, 13.5 and 11.3 moles m−2 d−1 light integrals were not
significant.
The number of leaves on the plant main stem did not vary with
light integrals from 11.3 to 24.9 moles m−2 d−1 , remaining constant at 4 pairs and 6 pairs of leaves with the treatments applied
at the 1 and 3 leaf-pair growth stages respectively. However,
differences in leaf number between 5.3 and 11.3 moles m−2 d−1
were significant and there was one leaf-pair less on plant main
stems under 5.3 moles m−2 d−1 compared with the higher light
integrals.
Total leaf areas were enhanced significantly with increases
of the light integrals; however, there was no difference between
the 13.5 and 24.9 moles m−2 d−1 light integrals. Although
leaf numbers per main stem were the same in the 11.3
and 24.9 moles m−2 d−1 treatments, the differences in shoot
numbers, leaf numbers in shoots and individual leaf sizes
resulted in differences in total leaf area between the 11.3 and
13.5 moles m−2 d−1 light integrals, especially when treatments
were applied at the 3 leaf-pair growth stage. The areas of individual leaves were strongly affected by shading treatments,
depending on when the treatments were applied. For treatments
applied at the 1 leaf-pair growth stage, significant differences
were detected from the 1st pair of leaves to the 4th pair of leaves,
because the 1st pair of leaves were present but not fully expanded
when the treatment started. When treatments were applied at
the 3 leaf-pair growth stage, the 3rd pair of leaves was not fully
expanded so, as expected, differences between the 3rd–5th pairs
of leaves were significant.
Specific leaf area was calculated by dividing total leaf
area by total leaf dry weight. For treatments applied at the
1 leaf-pair growth stage, specific leaf area declined more
than twofold when light integrals were increased from 5.3
to 24.9 moles m−2 d−1 . However, there was no difference
between the 13.5 moles m−2 d−1 treatment and the 24.9 and
219
Fig. 1. Photosynthesis rate under different solar irradiances.
11.3 moles m−2 d−1 treatments. For treatments applied at the 3
leaf-pair growth stage, the trend was the same as with the treatments applied at the 1 leaf-pair growth stage, but the change
was less. Although plants were at different growth stages, specific leaf areas for both sets of plants at the 24.9 moles m−2 d−1
light integral (control) were very similar; however, there was a
big difference at the 5.3 moles m−2 d−1 light integral. This suggests that, in terms of specific leaf area, young plants were more
sensitive to daily light integral than older plants.
3.2. Photosynthesis
Photosynthesis was significantly reduced by shading
(P < 0.001), with the CO2 assimilation rate ranging from 15.1
to 37.7 moles m−2 s−1 between the 440 moles m−2 s−1 (75%
shading) and 1387 moles m−2 s−1 (control) solar irradiance
respectively (Fig. 1). The CO2 assimilation rate increased greatly
with increase in irradiance from the lowest level, but this
increase declined at higher levels of irradiance and there was no
significant difference between the 1187 moles m−2 s−1 (25%
shading) and 1387 moles m−2 s−1 (control) treatments.
3.3. Leaf temperature
Data for leaf and air temperatures were collected at
10:00 a.m., 2:00 p.m. and 4:00 p.m. on April 16, April 22, July 9
and July 15. Since the highest air temperatures in the glasshouse
on 16 and 22 April were less than 30 ◦ C, and the trend in changes
in leaf temperature under different shade conditions was the
same as for the data collected on 9 and 15 July before 2:00 p.m.,
the data from 15 July is presented.
The mean air temperature from 9:00 to 10:00 a.m. was
19.4 ◦ C and there was no significant difference in leaf temperatures between shade treatments measured at 10:00 a.m. (Table 3).
When leaf temperatures were measured at 2:00 p.m. while the
mean air temperature was 30.3 ◦ C (from 1:00 to 2:00 p.m.), it
was found that leaf temperature increased with increasing irradiance and there were differences between the shading treatments.
220
X. Chang et al. / Environmental and Experimental Botany 63 (2008) 216–223
Table 3
Leaf temperatures (◦ C) of basil plants under different shading conditions and
the time of day
Time of day (TAir )
10.00 (19.4)
14.00 (30.3)
16.00 (36.0)
Treatment (shading)
Control
25%
50%
75%
24.8
27.6
30.7
25.1
27.5
29.1
24.0
26.7
28.7
24.7
26.0
30.8
SED
P
0.484
0.613
0.465
0.271
0.050
<0.001
TAir : Air temperature (◦ C); SED: standard error of the difference; P: probability.
At 4:00 p.m. when the mean air temperature was 36.0 ◦ C (from
3:00 to 4:00 p.m.), there was a highly significant difference in
leaf temperatures under the different shading treatments.
At the air temperature of 30.3 ◦ C, leaf temperature was
decreased by high level of shading. In contrast, at 36 ◦ C air
temperature, 75% shading resulted in leaf temperatures significantly higher than with 25 and 50% shading, but not higher than
the control. At the lower air temperature of 19.4 ◦ C, leaf temperature was consistently higher than air temperature, however,
at higher air temperatures, leaf temperature increased but was
lower than the air temperature. At the high air temperature of
30 ◦ C, shading effectively reduced leaf temperature, however,
at the higher air temperature of 36 ◦ C, heavy shading could not
reduce leaf temperature.
3.4. Volatile oils
Shading of basil plants led to a reduction in the oil content
(P < 0.001) and changes in oil composition in the leaves.
In treatments applied at the 3 leaf-pair growth stage, there
was a fivefold reduction in volatile oil content with the
5.3 moles m−2 d−1 light integral (75% shading) compared with
the 24.9 moles m−2 d−1 light integral (no shading) (Fig. 2).
Treatments applied at the 1 leaf-pair growth stage showed no
significant difference between 13.5 moles m−2 d−1 light integrals (25% shading) and 24.9 moles m−2 d−1 light integrals (no
shading).
The principal phenylpropanoids detected were eugenol and
methyl eugenol, while the major terpenoids were 1,8-cineole,
linalool, cis-ocimene and trans-␣-bergamotene. Small amounts
of ␣-pinene, -pinene, myrecene and camphor were also
Fig. 2. Total volatile oil content in 5 g fresh leaf samples after 2 weeks of shading
treatments applied at the 1 and 3 leaf-pair growth stages.
Fig. 3. Relative content of selected volatile compounds after 2 weeks of shading
treatments applied at the 1 leaf-pair growth stage.
detected. When plants were treated at the 1 leaf-pair growth
stage, shading led to a reduction in the relative contents
of linalool and eugenol but methyl eugenol was markedly
increased (Fig. 3). Linalool was reduced by a factor of 4
in plants grown under the 5.3 moles m−2 d−1 light integral
compared with 24.9 moles m−2 d−1 light integral (no shading)
(P = 0.007), and eugenol by a factor of 3 (P = 0.033). Differences with 13.5 moles m−2 d−1 light integrals (25% shading)
and 11.5 moles m−2 d−1 light integrals (50% shading) were
not significant. The biggest treatment effect was observed
with methyl eugenol (P < 0.001), with sixfold enhancement by
5.3 moles m−2 d−1 light integral (75% shading) compared with
24.9 moles m−2 d−1 (no shading).
For treatments applied at the 3 leaf-pair growth stage, differences in the relative contents of both linalool and eugenol
with 13.5 (25% shading), 11.3 moles m−2 d−1 light integrals
(50% shading) and 5.3 moles m−2 d−1 light integrals (75% shading) were not significant, however, with 24.9 moles m−2 d−1
light integral (no shading) their relative contents were higher
(P < 0.05) (Fig. 4). Methyl eugenol was again enhanced by shading (P < 0.001), increasing more than fivefold from 0.34% with
no shading to 1.89% with 5.3 moles m−2 d−1 light integral (75%
shading), however, the relative content of methyl eugenol was
significantly less than that in plants at the 1 leaf-pair growth
stage. These results showed that shading of basil plants significantly affected the volatile oil composition and that young plants
contained a higher relative content of methyl eugenol. In addi-
Fig. 4. Relative content of selected volatile compounds after 2 weeks of shading
treatments applied at the 3 leaf-pair growth stage.
X. Chang et al. / Environmental and Experimental Botany 63 (2008) 216–223
221
as when treatments were applied at the 1 leaf-pair growth
stage, however, the yields of the three oils were much higher
(Fig. 6).
4. Discussion
Fig. 5. Content of selected volatile oils after 2 weeks of shading treatments
applied at the 1 leaf-pair growth stage.
tion, plants at the younger growth stage were more sensitive to
different shading conditions.
Based on peak areas and calibration curves for selected
individual chemicals, the yields of the three main chemicals
(1,8-cineole, linalool and eugenol) were calculated (Fig. 5).
Although differences in the relative content of 1,8-cineole
in basil leaves when treatments were applied at the 1 leaf-pair
growth stage were not significant, the quantity of 1,8-cineole in
fresh leaves was significantly decreased by shading (P < 0.001),
ranging from 1.29 g g−1 with 5.3 moles m−2 d−1 light integral (75% shading) to 9.24 g g−1 with 24.9 moles m−2 d−1
light integral (no shading), i.e. a sevenfold difference. For
linalool, when treatments were applied at the 1 leaf-pair growth
stage, due to the differences in relative content combined
with the differences in total content, differences between the
shading treatments were significant (P < 0.001), ranging from
0.25 g g−1 with 5.3 moles m−2 d−1 light integral (75% shading) to 8.11 g g−1 with 24.9 moles m−2 d−1 light integral
(no shading), i.e. a factor of 33 times. However, differences
between 13.5 moles m−2 d−1 light integrals (25% shading) and
11.3 moles m−2 d−1 light integrals (50% shading) were not significant. The yield of eugenol from fresh leaves increased by
a factor of 9 from 0.19 g g−1 with 5.3 moles m−2 d−1 light
integral (75% shading) to 1.67 g g−1 with 24.9 moles m−2 d−1
light integral (no shading).
When shading treatments were applied at the 3 leaf-pair
growth stage, a similar but less marked trend was observed
Fig. 6. Content of selected volatile oils after 2 weeks of shading treatments
applied at the 3 leaf-pair growth stage.
The absence of any differences in plant height, weight,
shoot number, leaf weight and leaf area between plants in the
24.9 moles m−2 d−1 (no shade) and 13.5 moles m−2 d−1 (25%
shade) treatments may be explained by the latter possessing
a greater conversion efficiency of light utilization for increasing plant size and dry matter. This is supported by previous
reports for cineraria (Yeh, 1996) and strawberry (Wright and
Sandrang, 1995). Commonly, dry matter accumulated by annual
plants increases linearly with the amount of solar radiation intercepted, but shade-plants have a higher quantum efficiency due
to a decrease in the ratio of the respiration rate to the rate of
photosynthesis (Yeh, 1996).
In general, plant responses to light include a variety of photosynthetic and biochemical adaptations that are translated into
alterations of plant growth and architecture (Peralta et al., 2002).
Also, reductions in irradiance lead to a reduction in the rate of
photosynthesis and therefore a reduction in the rate of growth
(Corree, 1983). This is supported by the current findings that the
treatments at 11.3 moles m−2 d−1 light integrals (50% shade)
and 5.3 moles m−2 d−1 light integrals (75% shade) significantly
reduced plant size and weight, both in the young and old plants.
As hypothesized, plant height, dry matter and leaf area increased
with increasing irradiance between 5.4 moles m−2 d−1 light integrals (75% shading) and 13.5 moles m−2 d−1 light integrals
(25% shading), which suggests that, although basil grows well
under sunny conditions (Putievsky and Galambosi, 1999), it can
tolerate light but not heavy shade.
Specific leaf area decreased as daily light integral increased
up to 13.5 moles m−2 d−1 , and thereafter remained fairly constant. This could be characterised as a sun-adaptation feature.
The reduced thickness of the leaves in shaded conditions may be
associated with smaller cell volume. The present results support
the findings of Yeh (1996) for cineraria plants.
As expected, photosynthesis in the basil plants increased
with increasing irradiance. The initial part of the light response
curve is linear because light is the dominant limiting factor. On
reaching light saturation, the curve becomes horizontal because
resources other than light become limiting (Nilsen and Orcutt,
1996).
Leaf temperature is regulated primarily by the influence of
net radiation, latent heat exchange, conduction and convection
and air temperature (Nilsen and Orcutt, 1996). Any limitation
of free convection reduces plant growth by limiting heat and gas
exchanges between plant leaves and the ambient air (Kitaya et
al., 2001). In this study, when the air temperature was 19.4 ◦ C at
10:00 a.m., the leaf temperature exceeded the air temperature by
as much as 4.6–5.7 ◦ C because, during periods of intense irradiance and/or high tissue irradiance absorption, tissue temperature
can increase dramatically compared with the air temperature
(Nilsen and Orcutt, 1996). With the air temperature increasing to
30.3 ◦ C at 2:00 p.m., leaf temperature was effectively reduced by
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X. Chang et al. / Environmental and Experimental Botany 63 (2008) 216–223
shading. When the air temperature was 36 ◦ C, light shading (25
and 50%) significantly reduced leaf temperature compared with
no shading, but heavy shading (75%) resulted in increased leaf
temperature. With the irradiance increasing during this period,
leaf temperature was increased, and light shading reduced leaf
temperature through its effect on light reduction and therefore
reduced leaf temperature as stated above. Heavy shading (75%)
restricted air movement due to the less porous shading net around
the plants, with the consequent decrease in conductance of heat
and gases in the leaf boundary layer resulting in higher leaf temperature; however, such critical air temperature conditions are
not common during the growing season in England. The plants
under 75% shading made less growth due to the reduced irradiance and also probably due to the restricted free air convection
retarding heat and gas exchanges between leaves and the ambient air (Kitaya et al., 2001). This research suggested that further
shade experiments should consider using clear netting rather
than no netting as a control, or improving netting materials to
prevent the restricted free air convection.
The composition of volatile oils was strongly affected by light
intensity. There were higher relative contents of linalool and
eugenol under higher daily light integrals, and higher shading
resulted in more methyl eugenol. Phenylalanine is the precursor
for both eugenol and methyl eugenol and, under certain conditions, the methylation of eugenol to methyl eugenol, is catalysed
by eugenol O-methyl transferase (EOMT) (Gang et al., 2001).
In the present study, lower irradiance resulted in the accumulation of methyl eugenol, but how irradiance affects the activity
of EOMT requires further investigation.
Linalool is a common acyclic monoterpenoid compound that
can be synthesized from ␣-pinene or -pinene (Bauer et al.,
2001). In plants, the enzyme linalool synthase catalyzes geranyl pyrophosphate (GPP) to linalool (Kaufman et al., 1999).
In addition, GPP can be catalysed by 1,8-cineole synthase to
1,8-cineole (Gang et al., 2001; Kaufman et al., 1999) and ␣terpieol (Gang et al., 2001) catalysed by monoterpene cyclase
to pinenes, 3-carene and limonene (Kaufman et al., 1999). In the
present study, there were no differences in the relative contents
of ␣-pinene, -pinene, 3-carene and 1,8-cineole under different shading treatments. Significant differences were only found
for linalool, suggesting that further research should be carried
out to determine how the light intensity affects the activity of
linalool synthase and thereby regulating the relative content of
linalool. Some other chemicals maintained a relatively stable
level under different conditions, for example the relative contents of pinenes, 3-carene and limonene did not change with
different shading conditions, temperatures and supplementary
UV-B or red light (Chang, 2004).
The highest dry matter and volatile oil content were obtained
under the highest daily light integrals, which suggests that,
as secondary metabolites, the synthesis of volatile oils has
a very close relationship with primary metabolism, i.e. the
more photosynthates produced, the more secondary metabolites accumulated. Precursors are usually derived from basic
metabolic pathways (Wink, 1999). Both phenylalanine and GPP
are derived from sucrose catalysed by a series of enzymes, such
as sucrose synthase, glucose-6-phosphate dehydrogenase, ger-
anyl pyrophosphate synthase and shikimate kinase (Gang et al.,
2001).
5. Conclusion
This research suggested that heavy shading (75%) resulted
in shorter plants, lower weight, smaller leaf area, less shoots
and higher specific leaf area. Due to irradiance reduction,
heavy shading (75%) strongly reduced photosynthesis rate and,
effectively reduced leaf temperature when air temperature was
less than 30 ◦ C. However, it could not reduce leaf temperature when air temperature was above 36 ◦ C. Total volatile oil
content in fresh leaves was significantly reduced by heavy
shading, the young plants, however, were less sensitive to
shading than the older plants. Higher irradiance significantly
increased the relative contents of linalool and eugenol, whereas
methyleugenol was increased by lower irradiance. However,
there were no differences in the relative content of 1,8-cineole
observed.
Acknowledgment
The senior author gratefully acknowledges financial support
from the Division of Agricultural and Environmental Sciences
of the University of Nottingham.
References
Bauer, K., Garbe, D., Surburg, H., 2001. Common Fragrance and Flavour
Materials, Preparation, Properties and Uses, fourth ed. Wiley-VCH,
Weinheim.
Chang, X., 2004. Effects of light and temperature on volatile oil compounds
and growth in basil (Ocimum basilicum L.). Ph.D. Thesis, University of
Nottingham, UK.
Chang, X., Alderson, P.G., Wright, C.J., 2005. Effect of temperature of integration on the growth and volatile oil Content of basil (Ocimum basilicum L.).
J. Hortic. Sci. Biotechnol. 80, 593–598.
Corree, W.J., 1983. Growth and morphogenesis of sun and shade plants. I: The
influence of light intensity. Acta Bot. Neerl. 32, 49–62.
Faust, J. E., 2002. First Research Report. Light Management in Greenhouses.
I. Daily Light Integral: A Useful Tool for the U.S. Floriculture Industry.
http://www.firstinfloriculture.org/pdf/2002-5 LightManagement pt 1.pdf.
Gang, D.R., Wang, J., Dudareva, N., Nam, K.H., Simon, J.E., Lewinsihn, E.,
Putievsky, E., 2001. An investigation of the storage and biosynthesis of
phenylpropenes in sweet basil. Plant Physiol. 125, 539–555.
Hiltunen, R., Holm, Y. (Eds.), 1999. Basil: The Genus Ocimum. Harwood
Academic Publishers.
Kaufman, P.B., Cseke, L.J., Warber, S., Duke, J.A., Briemann, H.L (Eds.), 1999.
Natural Products from Plants. CRC Press, Boca Raton, FL.
Kitaya, Y., Kawai, J., Tsuruyama, H.T., Akahashi, A., Tani, A., Goto, E., Saito,
T., Kiyota, M., 2001. The effect of gravity on surface temperature and net
photosynthetic rate of plant leaves. Adv. Space Res. 28, 659–664.
Lee, B.S., Seo, B.S., Chung, S.J., 1994. Shading effects on growth and essential
oil content of hydroponically grown sweet basil (Ocimum basilicum L.). J.
Kor. Soc. Hortic. Sci. 35, 95–102.
Morelli, G., Ruberti, I., 2002. Light and shade in the photocontrol of Arabidopsis
growth. Trends Plant Sci. 7, 399–404.
Nilsen, E.T., Orcutt, D.M. (Eds.), 1996. The Physiology of Plants Under Stress,
Abiotic Factors. John Wiley & Sons Inc..
Paez, A., Gebre, G.M., Gonzalez, M.E., Tschaplinski, T.J., 2000. Growth, soluble carbohydrates and aloin concentration of Aloe vera plants exposed to
three irradiance levels. Environ. Exp. Bot. 44, 133–139.
X. Chang et al. / Environmental and Experimental Botany 63 (2008) 216–223
Peralta, G., Pérez-Looréns, J.L., Hernández, I., Vergara, J.J., 2002. Effects of
light availability on growth, architecture and nutrient content of the seagrass
Zostera noltii Hornem. J. Exp. Mar. Biol. Ecol. 269, 9–26.
Putievsky, E., Galambosi, B., 1999. Production systems of basils. In: Hiltunen,
R., Holm, Y. (Eds.), Basil: The Genus Ocimum. Harwood Academic Publishers, pp. 39–65.
Rich, T.C.G., Whitelam, G.C., Smith, H., 1987. Analysis of growth rates during
phototropism: modifications by separate light-growth responses. Plant Cell
Environ. 10, 303–311.
Sorrentino, G., Cerio, L., Alvino, A., 1997. Effect of shading and air temperature
on leaf photosynthesis, fluorescence and growth in lily plants. Sci. HorticAmsterdam 69, 259–273.
223
Souza, A., De Paula, A.C.C.F.F., Figueiredo-Ribeiro, R.C.L., 2004. Effects of
irradiance on non-structural carbohydrates, growth and hypoglycemic activity of Rhynchelytrum repens (Willd) C. E. Hubb. (Poaceae). Braz. J. Biol.
64 (3B), 697–706.
Went, F.W., 1941. Effect of light on stem and leaf growth. Am. J. Bot. 28, 83–
95.
Wink, M., 1999. Biochemistry of Plant Secondary Metabolism. Annual Plant
Review, vol. 2. Sheffield Academic Press and CRC Press.
Wright, C.J., Sandrang, A.K., 1995. Efficiency of light utilization in the strawberry (Fragaria × ananassa) cv. Hapil. J. Hortic. Sci. 70, 705–711.
Yeh, D.M., 1996. Manipulation and predictive modelling of flowering in
cineraria. Ph.D. Thesis, University of Nottingham, UK.