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Effect of temperature integration on the growth and volatile oil
content of basil (Ocimum basilicum L.)
By XIANMIN CHANG
*
, PETER G. ALDERSON and CHARLES J. WRIGHT
Division of Agricultural and Environmental Sciences, School of Biosciences, The University of
Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, UK
(e-mail: changxianmin2002@yahoo.co.uk) (Accepted 23 June 2005)
SUMMARY
Sweet basil (Ocimum basilicum L.) is a warm climate plant. The optimum temperature for growth is 25°C and, at this
temperature, the volatile oil content of leaves is enhanced. Plants grown at 25°C for 2 weeks were taller and had a
higher dry matter content and larger leaves than plants grown at other temperatures. The total volatile oil contents in
fresh leaves from plants grown at 25°C or 30°C for 2 weeks were three times the levels found in leaves of plants grown
at 15°C. Temperature also affected the composition of the volatile oils. Warm conditions (25°C) resulted in the
accumulation of eugenol and cis-ocimene, whereas cooler temperature (15°C) resulted in more camphor and trans-
-
farnesene. There was no effect of temperature on the relative contents of 1,8-cineole and linalool. Treatments with
alternating temperatures, that supplied the same accumulated ‘day-degrees’ but with a different sequence of
temperatures, did not affect most plant growth parameters. In contrast, volatile oil content and composition were
strongly affected by the temperature regime during the final 2 weeks of growth. For example, the higher the
temperature before harvesting, the higher the volatile oil content and the greater the relative content of eugenol.
S
weet basil (Ocimum basilicum L.) is a herbaceous
species native to warm regions of Asia, Africa and
Iran, which emits a sweet and characteristic aroma. This
species is cultivated commercially in order to extract
volatile oils for use as aroma additives in food,
pharmaceuticals, cosmetics and other household
products. Therefore, both the yield and composition of
basil oils are important. Basil is cultivated under a range
of climatic and ecological conditions, but the most
favourable conditions are found in warm climates
(Hiltunen and Holm, 1999). The minimum temperature
for the growth of basil has been determined to be 10.9°C
(Chang, 2004), consequently basil grows best under
warm conditions. Putievsky (1983) showed that sweet
basil was not sensitive to temperature for germination,
and germinated well over a wide range of temperatures;
however, with increasing day-time temperatures
between 21°C and 30°C, the height of plants was
increased, and the highest yield of dry matter was
obtained at 30°C.
Previous research has focussed on the effects of
temperature on the morphology and oil yield of plants,
with limited information about volatile oil composition.
So far, there have been no publications on how
alternating temperatures affect plant growth, and the
yield or composition of volatile oils in basil.
This paper reports on experiments using constant
temperature or alternating temperature treatments. It
was hypothesised that warm temperatures would
increase plant height, leaf number, leaf area and plant
weight, as well as the yield of volatile oils, because the
rate of biochemical reactions generally doubles with
every 10°C increase in reaction temperature (Copeland,
2002). Using alternating temperature treatments, all
plants would be the same size but would differ in their
content and composition of volatile oils.
MATERIALS AND METHODS
Plant materials
Seeds of basil cv. ‘Basil Sweet Genovese’, which were
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.The mean daily temperature in
the glasshouse was set at 21° ± 3ºC for seed germination
and plant growth.
Constant temperature treatments
Temperature treatments were carried out in
controlled environment growth rooms at 15°C, 25°C
or 30°C maintained to within ± 1°C, with a 16 h
photoperiod, provided by 400 W high pressure mercury
vapour lamps (HLRG; Philips, The Netherlands). For the
1 week- and 2 week-treatments, plants at the four and six
leaf-pair, and at the three and four leaf-pair growth
stages were used, respectively. The experiment was
carried out in July 2002, and repeated in November 2002.
Irradiance (PAR) was kept constant between the
different temperature treatments (at 349.7, 351.2 and
382.8 µmoles m
–2
s
–1
) at 15°C, 25°C and 30°C respectively
(differences not significant).
Alternating temperature treatments
Three temperatures (15°C, 25°C and 30°C) were used
to provide six different alternating temperature
*Author for correspondence.
Journal of Horticultural Science & Biotechnology (2005) 80 (5) 593–598
593_JHSB_80_5.ps 11/8/05 10:56 am Page 593
Temperature effects on volatile oils in basil
treatments (Table I) with the same irradiance as in the
constant temperature treatments. This experiment was
carried out in January 2003.
Plant growth parameters
Plant height, plant fresh weight (FW) and dry weight
(DW), leaf FW and DW, number of shoots, number of
leaf-pairs on the main stem, leaf area and specific leaf
area (SLA) were measured at the end of each treatment.
Photosynthesis, stomatal conductance and transpiration
An infra-red gas analyser (CIRAS I; Scotrail and the
University of Strathclyde, Scotland) was used to measure
photosynthesis, stomatal conductance and transpiration.
Analysis of leaves for volatile oil content and
composition
Freshly-harvested leaf material (5 g) from the fifth
pair of leaves on each sample of three plants was
homogenised immediately in a modified airtight blender
connected to a Universal Tube (UT) packed with
Tenax/Carbograph ITD/Carbaxen 1000 (Markes
International Ltd., Pontyclun, Rhondha Cynon Taff,
Wales, UK) and to a supply of nitrogen gas. Each leaf
sample was blended for 5 s and the volatile oils collected
for 5 min by passing nitrogen gas at 1.03 10
4
N m
–2
through the blender to prevent oxidation and to carry
the volatile oils into the UT. The UTs were designed for
an analyte volatility range of n-C2 to n-C30, with weak
(Tenax), medium (Carbograph ITD) and very strong
(Carbaxen 1000) sorbent strengths.
A Thermal Desorption (TD) unit (Markes
International Ltd.) with an Agilent 6890N Network GC
and an Agilent 5973 Network MSD (Agilent
Technologies, Alpharetta, GA, USA) were used to
identify the volatile compounds present. Thermal
desorption conditions were as follows: prepurge time,
1 min; tube desorption, 5 min; trap low temperature,
–10°C; trap high temperature, 275°C; flow path
temperature, 150°C; minimum carrier pressure,
6.89 10
4
N m
–2
. Capillary GC/MS measurements were
carried out on a DB-5MS (0.25 mm 25 m 0.25 µm)
column coupled directly to the Agilent 5973 Network
MSD. GC/MS conditions were as follows: flow rate of
helium gas through the column, 0.6 ml min
–1
; average
velocity, 30 cm s
–1
; pressure, 2500 N m
–2
;oven
programme, initial temperature 40°C, ramped at 10°C
min
–1
to 180°C and held for 10 min; carrier gas, helium;
electron energy, 70 eV; mass spectra, 1306 EMV
(electron multiplier voltage); scanning speed, 3.39 scans
s
–1
from 35 to 250 m/z and split ratio, 390:1.
Chemical compounds were identified on the basis of
their relative retention times, using standards and
comparing peaks with a library (Wiley7n.L; supplied by
Agilent Technologies, USA). Each analysis was
performed in triplicate. The relative contents of
individual compounds were used to compare the overall
composition of the volatile oil, and the sum of the peak
areas was used to compare total oil contents in basil
leaves under different conditions.
Calibration for 1,8-cineole, linalool and eugenol
External standards were used to calibrate the TD/GC.
Standard solutions [0.025%, 2.5%, 5% and 7.5% (v/v)] of
1,8-cineole, linalool and eugenol dissolved in ethyl
acetate were prepared by mixing. Ethyl acetate was
selected as the solvent because it is compatible with
Tenax. There were three replicates for each
concentration and 1 µl was injected into each replicate
UT. From the known amounts of the pure chemicals and
the peak areas, calibration formulae were established as
follows:
for 1,8-cineole: y = 1063902 x - 2090073; R
2
= 0.9797;
for linalool: y = 998844 x - 975067; R
2
= 0.9856;
and for eugenol: y = 1483637 x - 4444830; R
2
= 0.9846;
where: y = weight of volatile oil and x = peak area.
Experimental design and statistical analysis
There were three replicates of 18 plants for each
treatment. Growth parameters and data for volatile oil
contents and compositions were subjected to analysis of
variance (ANOVA) using GenStat 2003–04. Differences
between treatments were assessed using the F-test, and
Least Significant Differences (LSD) were calculated at
the 0.05 probability level (P = 0.05).
RESULTS
Plant growth parameters
Constant temperature treatments: Similar results were
obtained for the 1 week- and 2 week-treatments. Plants
at 25°C were taller, heavier (both FW and DW) and had
594
TABLE I
Alternating temperature treatment regimes and accumulated ‘day-degrees’
First week Second week Third week
Treatment °C ADD* °C ADD °C ADD
1 15 28.7 30 162.4 25 261.1
2 25 98.7 15 127.4 30 261.1
3 30 133.7 25 232.4 15 261.1
4 30 133.7 15 162.4 25 261.1
5 25 98.7 30 232.4 15 261.1
6 15 28.7 25 127.4 30 261.1
*ADD, Accumulated ‘Day-Degrees’.
T
ABLE II
Comparison of plant growth parameters following 1 week of constant temperature treatment applied at the six leaf-pair growth stage
Temperature (°C)
Growth parameter 15°C 25°C 30°C SED* Probability (P)
Plant height (cm) 38.2 48.7 48.2 1.71 < 0.001
Number of leaf pairs 7.0 8.0 8.0 – –
Number of shoots 6.0 8.0 8.0 – –
Total leaf area 679.0 969.0 917.0 51.80 < 0.001
Plant fresh weight (g) 37.6 47.8 47.9 1.80 < 0.001
Plant dry weight (g) 5.5 7.1 6.8 0.28 < 0.001
Leaf fresh weight (g) 21.5 25.8 25.6 1.13 < 0.001
Leaf dry weight (g) 4.0 4.7 4.3 0.18 0.006
Specific leaf area (cm
2
g
–1
) 169.7 206.0 213.2 9.40 < 0.001
Leaf water content (%) 81.4 81.8 83.3 0.21 < 0.001
*SED, standard error of the difference (n = 6).
594_JHSB_80_5.ps 11/8/05 10:56 am Page 594
X. CHANG,P.G.ALDERSON and C. J. WRIGHT
larger leaf areas, more leaves and more shoots compared
to plants held at 15°C. For some parameters, the
differences between the 25°C and the 30°C treatments
were not significant (Table II).
After 1 week, plants in the 30°C treatment were
tallest, but leaf FWs at 25°C were the highest.
Comparing plants at two growth stages, plants with six
leaf-pairs showed no significant difference between
25°C and 30°C; however, those with four leaf-pairs
showed significant differences, suggesting that young
plants were more sensitive to temperature. Leaf DWs at
25°C were the highest, and the differences were
significant between plants at the two growth stages. Leaf
water-content increased with temperature, which may
explain the significant difference in leaf DWs between
25°C and 30°C. Leaf area was highest for plants grown
at 25°C; however plants at the four leaf-pair growth
stage did not show any difference in leaf area between
25°C and 30°C. SLA (total leaf area divided by total leaf
DW) increased with increasing temperature after
1 week of treatment. Compared with plants grown at
15°C, more lateral shoots were produced at 25°C and at
30°C. Plants grown at 15°C had paler (more yellow)
leaves after only 2–3 d compared with plants grown at
25°C or 30°C.
Alternating temperature treatment: After 1 week, plants
grown at warmer temperatures (25°C or 30°C) were
taller and had more leaves. However, after 2 weeks, there
were no differences in the number of leaf-pairs or plant
height between treatments with the same accumulated
‘day-degrees’. With higher accumulated ‘day-degrees’,
plants were taller and had more leaves and more shoots.
After 3 weeks, plants in all treatments had received
the same accumulated ‘day-degrees’, but in different
orders, and no significant differences were seen in the
numbers of shoots and FWs between treatments
(Table III). However, due to differences in the water-
content of plants at different temperatures, the
differences in DWs between treatments were highly
significant. With cool conditions (15°C) in the last week
and warm temperatures (25°C or 30°C) during the first 2
weeks, plants had higher DWs, whereas cool conditions
(15°C) in the first week and warm conditions (25°C or
30°C) in the last 2 weeks resulted in taller plants. Whole
plant and leaf water-contents were strongly affected by
the last temperature experienced. Plants that had 15°C as
their last temperature had the lowest water content,
while those at 30°C had the highest water content.
At the time of harvest, all plants were at the ‘early
flower initiation’ growth stage, and no further leaves
would be produced. Normally there are six-to-seven
pairs of leaves on the main stem of this cultivar but, when
plants were grown under cool conditions (15°C) for the
first week, then moved to higher temperatures (25°C or
30°C), there was approx. one leaf-pair less than under
other conditions. Differences in leaf area between
treatments were highly significant, but it was not possible
to draw any conclusions. SLA was strongly affected by
the last temperature experienced. The higher the
temperature, the larger the SLA.
Volatile oil compounds
Thirty-six chemical compounds were identified in basil
leaf extracts (Table IV). The principal phenyl-propanoid
detected was eugenol, while the major terpenoids were
1,8-cineole, linalool, cis-ocimene and trans-
-
bergamotene, with small amounts of pinenes, myrcene
and camphor. There were significant effects of
temperature on the quantity and composition of volatile
oils present in leaf extracts following constant
temperature or alternating temperature treatments.
595
TABLE III
Comparison of plant growth after alternating temperature treatments over 3 weeks
Treatment (°C) (weeks 1-2-3)
Growth parameter 15-30-25 25-15-30 30-25-15 30-15-25 25-30-15 15-25-30 SED* Probability (P)
Plant height (cm) 35.7 32.0 33.5 32.3 33.8 34.2 1.10 0.031
Number of leaf pairs 5.0 6.0 6.2 6.2 6.3 5.8 0.18 < 0.001
Number of shoots 8.0 6.7 7.3 6.7 7.7 7.3 0.60 0.198
Total leaf area 655.0 537.0 625.0 554.0 520.0 514.0 41.70 0.005
Plant fresh weight (g) 35.3 29.7 33.0 32.0 32.3 29.2 2.41 0.156
Plant dry weight (g) 4.7 3.4 5.7 4.1 5.4 3.4 0.36 < 0.001
Leaf fresh weight (g) 20.7 18.7 19.7 18.6 18.4 17.5 1.34 0.252
Leaf dry weight (g) 3.7 2.3 4.1 2.8 4.0 2.3 0.26 < 0.001
Specific leaf area (cm
2
g
–1
) 177.0 233.4 152.0 197.8 130.0 223.0 10.80 < 0.001
Plant water content (%) 86.7 88.6 82.7 87.2 83.3 88.4 0.87 < 0.001
Leaf water content (%) 82.1 87.3 79.2 85.0 78.3 86.9 0.35 < 0.001
*SED, standard error of the difference (n = 6).
T
ABLE IV
Volatile compounds identified in basil leaf extracts
Chemical RT* (min) Chemical RT (min) Chemical RT (min)
Thujene 4.165
-terpinene 6.051 Methyl eugenol 11.103
-pinene 4.275
-terpinolene 6.377 Trans-caryophyllene 11.384
Camphene 4.481 Linalool 6.704 Trans-
-bergamotene 11.570
Sabinene 4.822
-ocimene 7.095
-guaiene 11.625
-pinene 4.752 Camphor 7.396
-cadinene 11.735
Myrcene 5.035
-terpineol 8.053 Trans-
-farnesene 11.769
-Phellandrene 5.254 Nerol 8.966
-humulene 11.851
-3-carene 5.334 Fenchyl acetate 9.463 Germacrene-D 11.212
-terpinene 5.434
-cubebene 10.376
-selinene 12.297
p-cymene 5.530 Eugenol 10.486 Germacrene-B 12.413
1,8-cineole 5.660
-copaene 10.767
-cadinene 12.614
Cis-ocimene 5.725
-elemene 10.978
-sesquiphellandrene 12.714
*RT, retention time.
595_JHSB_80_5.ps 11/8/05 10:57 am Page 595
Temperature effects on volatile oils in basil
Constant temperature treatment: Differences in total oil
contents between temperatures for both 1 week- and
2 week-treatments were highly significant (Figure 1).
Leaves grown at 25°C produced the highest oil content
and, compared with 1 week of treatment, much more oil
was produced after 2 weeks of treatment. After 2 weeks,
leaves at 25°C and 30°C contained three-times as much
oil as those at 15°C; whereas, after 1 week, the increase
was only 25%. There was no difference in the oil content
of plants grown at 25°C or at 30°C.
After both 1 week and 2 weeks of treatment, there
were no differences in the relative contents of 1,8-cineole
or linalool, the two main compounds in basil leaves. In
contrast, eugenol, (
-terpinene, cis-ocimene,
-terpinene,
trans-sabinene hydrate,
-cadinene and
-terpinenol
were enhanced at 25°C and 30°C, while camphor, trans-
-farnesene and trans-
-bergamotene were enhanced at
15°C.
Three main compounds, 1,8-cineole, linalool and
eugenol, were selected to calculate their absolute
contents using TD/GC peak areas and calibration
formulae. Plants grown at 25°C produced the highest
yields of these three volatile oils, and differences after 1
or 2 weeks were highly significant (Table V). Although
the oil contents in leaves of plants grown at 30°C for
2 weeks were lower than in leaves grown at 25°C, they
were still significantly higher than in leaves grown at
15°C. Overall, plants at 15°C produced less volatile oils
(P < 0.001) and the effect of temperature was much less
after 1 week than after 2 weeks of treatment. Although
there were no differences in the relative contents of
1,8-cineole or linalool, differences in the absolute
contents of these compounds in fresh leaves were highly
significant after 1 week and 2 weeks of treatment, with
the difference being larger after 2 weeks.
Alternating temperature treatment:Volatile oil contents in
leaves were strongly affected by alternating temperature
treatments, with plants grown in warm conditions for
2 weeks prior to harvest (i.e., 15°–30°–25°C;
15°–25°–30°C) having the highest oil content (Table VI).
Thirty-six compounds were identified, of which sabinene,
-pinene, myrcene,
-phellandrene,
-terpinene,
1,8-cineole, cis-ocimene,
-terpinolene, linalool and
eugenol showed significant differences in their relative
contents under the different treatments. All are
monoterpenes, except eugenol. In contrast to the
constant temperature treatments examined, the three
major compounds (1,8-cineole, linalool and eugenol)
showed significant differences after 3 weeks of
alternating temperature treatments.As with the constant
temperature treatments, the higher the temperature
before harvest, the higher the eugenol content of the
leaves. Warmer temperatures (25°C or 30°C) in the first
week followed by 15°C in the second week, resulted in a
decrease in the relative content of 1,8-cineole. When
plants were grown initially at 30°C, the relative content
596
5.31
10.54
5.37
4.33
10.85
3.97
2
4
6
8
10
12
14
10 15 20 25 30
Temperature (
o
C)
Sum of peak area (x 10
8
)
FIG.1
Total volatile oil content in 5 g of fresh leaves after 1 week () or 2
weeks () of various constant temperature treatments applied at the
four leaf-pair growth stage. Peak areas from the GC/MS were integrated
and summed without specific units. Mean values are shown for each
data point.
T
ABLE V
Concentrations of selected molecules (µg per 5 g per fresh leaf extract) after 1 or 2 weeks of constant temperature treatment applied at the four leaf-pair
growth stage
1,8-cineole Linalool Eugenol
Temperature (°C) 1 week 2 weeks 1 week 2 weeks 1 week 2 weeks
15°C 120.0 90.5 69.0 74.0 40.0 40.0
25°C 141.0 449.8 113.0 311.0 60.0 179.0
30°C 150.4 226.2 98.8 199.0 62.6 107.6
SED* 9.4 17.4 18.4 30.9 9.3 7.7
Probability (P) < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
*SED, standard error of the difference (n = 3).
T
ABLE VI
Comparison of total volatile oil contents in 5 g of fresh leaves after various alternating temperature treatments
+
Treatment (°C)
+
15-30-25 25-15-30 30-25-15 30-15-25 25-30-15 15-25-30 SED* Probability (P)
Sum of peak area ( 10
8
) 9.08 6.38 6.57 6.97 6.48 9.36 0.867 < 0.001
*SED, standard error of the difference (n = 3).
+
Weekly temperatures over the 3 weeks are shown.
0
5
10
15
20
25
30
15-30-25 25-15-30 30-25-15 30-15-25 25-30-15 15-25-30
Temp eratures (
o
C)
Percentage (%)
1, 8-cineole
linalool
eugenol
FIG.2
Relative contents of three selected volatile oil molecules in basil plants
after various alternating temperature treatments (±SE). Weekly
temperature patterns over the 3 weeks as shown.
596_JHSB_80_5.ps 11/8/05 10:57 am Page 596
X. CHANG,P.G.ALDERSON and C. J. WRIGHT
of linalool was significantly higher than in the other
treatments (Figure 2).
Phtotosynthesis, transpiration and stomatal conductance
Plants grown at 25°C had the highest photosynthetic
rate and stomatal conductance to water vapour. The
highest transpiration rate, however, was measured at
30°C; but there was no significant difference between
25°C and 30°C (Table VII).
DISCUSSION
Temperature had clear effects on the growth
parameters of basil plants. Plant weight, plant height and
leaf area were significantly decreased at 15°C compared
to 25°C or 30°C. The primary biological effects of
temperature are to change rates of enzyme reactions,
metabolite transport and diffusion. At low temperatures
these processes are slower than at high temperatures. For
example, the rate of supply of phosphate ions to
chloroplasts at low temperatures could limit
photosynthesis, and hence reduce tissue growth (Lawlor,
1993). Cooler conditions also lead to decreased stomatal
conductance and reduced net rates of photosynthesis,
thereby inhibiting growth. The lower plant water content
observed at 15°C, compared to 25°C or 30°C, may have
been due to a lower transpiration rate and lower
stomatal conductance under the cooler conditions, thus
limiting water uptake from the soil.
The lack of any differences in the weight, height and
total leaf area of plants grown at 25°C or 30°C may result
from increasing photo-respiration at higher
temperatures. Photo-respiration is a minor component of
net photosynthesis under cool conditions; but, with
increasing temperature, the photosynthetic rate rises and
photo-respiration increases more than gross
photosynthesis (particularly above 30°C), resulting in a
small increase in net photosynthesis. This result supports
the findings of Pogany et al. (1968; cited in Hiltunen and
Holm, 1999) from a growth chamber experiment, which
measured the fastest growth rate of basil at 27°C.
The yellowing of leaves of plants from the second or
third day of treatment at 15°C was probably due to
changes in carotenoid and chlorophyll contents. At low
temperatures, the levels of carotenoids, plastoquinones
and cytochromes are increased in leaves (Lawlor, 1993)
and, shortly after a temperature drop, leaves may contain
lower concentrations of both chlorophyll a and
chlorophyll b (Vågen et al., 2003).
Plant leaf area was also strongly affected by
temperature (i.e., the higher the temperature, the larger
the leaf area). With plants grown at 15°C, SLA was
decreased because the leaves were thicker. Tomato
leaves have also been shown to be smaller and thicker at
lower temperatures. Hoek et al. (1993) attributed the
slower growth of leaves to a lower rate of cell division
and/or to lower numbers of leaf cells.A similar change in
leaf morphology occurred in basil; however, this requires
further study to determine the rates of cell division.
Plant height was strongly affected by accumulated
‘day-degrees’. During the 1 or 2 weeks of alternating
temperature treatments, plants that had received more
accumulated ‘day-degrees’ were taller. After 3 weeks, all
plants had received the same accumulated ‘day-degrees’
and, although there were still differences in plant height,
the differences were much less than during the first
2 weeks.
There were no differences in plant FWs and leaf FWs
between alternating temperature treatments but,
because cooler temperatures can result in lower water
contents (Nilsen and Orcutt, 1996), there were significant
differences in plant DWs and leaf DWs between
treatments. Plants that received more accumulated
‘day-degrees’ in the first 2 weeks produced more plant
DW.
Temperature also affected basil leaf and flower
initiation. Prior to harvest, all plants had initiated
flowers, so no new leaves would be produced on the main
stem. Plants possessed fewer leaves below the initiated
flowers when grown at 15°C in the first week of
treatment. In the first 2 weeks of treatment, as with plant
height and number of shoots, the number of leaf-pairs
was strongly affected by accumulated ‘day-degrees’ and,
with higher accumulated ‘day-degrees’, more leaves were
produced. In other plants (e.g., carnation), low
temperature has been shown to stimulate the initiation
of flowers, and increased temperature normally results in
more rapid development, with more leaf-pairs initiated
prior to flower initiation (Beisland and Kristoffersen,
1969).
Temperature is an important factor that regulates
plant metabolism, with rates of enzyme activity
increasing up to about 40°C, after which most plant
enzymes become inactivated or even damaged
irreversibly (Kaufman et al., 1999). At high temperatures,
the largest class of plant secondary metabolites, the
terpenoids, play a role in stabilising membranes (Wink,
1999), which may explain why the total terpenoid
content in leaves was enhanced. After 2 weeks of
constant temperature treatment at 25°C or 30°C, in
addition to an increased content of eugenol, there was
more than a three-fold increase in total volatile oil
content.
As plants grown at 25°C had higher leaf FWs than
those grown at 15°C, and their volatile oil content per
unit weight was higher, after 2 weeks at 25°C they had
produced seven-times as much volatile oil as those at
15°C.
Of the three major volatile compounds compared
between alternating temperature treatments, eugenol
content was most strongly affected by temperature prior
to harvest. The highest eugenol content was recorded
597
TABLE VII
Photosynthesis, transpiration and stomatal conductance of leaves grown at different temperatures for 2 weeks at the four leaf-pair growth stage
Temperature (°C)
Parameter 15°C 25°C 30°C SED* Probability (P)
Photosynthetic rate (µmol m
–2
s
–1
) 3.92 6.00 5.10 0.55 0.003
Transpiration rate (mmol m
–2
s
–1
) 0.26 1.38 1.79 0.18 < 0.001
Stomatal conductance (mmol m
–2
s
–1
) 34.90 78.50 75.20 9.84 < 0.001
*SED, standard error of the difference (n = 12).
597_JHSB_80_5.ps 11/8/05 10:57 am Page 597
Temperature effects on volatile oils in basil
under warm conditions 1 week before harvest. In
contrast, although the contents of 1,8-cineole and
linalool were significantly different between treatments,
it was difficult to draw any conclusion. Linalool and 1,8-
cineole have the same precursor, geranyl pyrophosphate
(GPP), and the enzymes linalool synthase (Kanfman,
1999) and 1,8-cineole synthase (Gang, 2001) that catalyse
the conversion of GPP directly into linalool or 1,8-
cineole have been identified. However, how
environmental factors stimulate these enzyme activities
is still not clear.
Further research should focus on secondary metabolism
pathways for the synthesis of isoprenoids and phenyl
propanoids in basil plants using peltate glands as a model
system. These glands have been shown to produce both
isoprenoid and phenyl propanoid derivatives (Gang,2001).
CONCLUSIONS
Plant height, plant DW and numbers of shoots were
strongly affected by accumulated ‘day-degrees’. The
higher the accumulated ‘day-degrees’, the greater the
values of these three parameters.Growth of young plants
under cool conditions may reduce the number of
leaf-pairs produced. SLA was strongly affected by the
last temperature experienced during cultivation: the
higher the temperature, the larger the SLA. Both the
relative and absolute contents of eugenol were also
strongly affected by the last temperature experienced:
the higher the temperature, the more eugenol was
produced. Although the contents of 1,8-cineole and
linalool differed between treatments, the mechanisms
and/or conditions that increased or reduced their relative
levels remain unclear. The oil content of fresh basil
leaves, and the oil yield per plant, were greatly enhanced
by higher temperatures 2 weeks before harvest. Oil
contents, both in fresh leaf samples and as total yield per
plant, were strongly enhanced by accumulated ‘day-
degrees’ 2 weeks before harvest, and significantly
reduced by low temperatures (15°C) 1 week before
harvest. For maximum oil production, it is therefore
recommended to grow basil plants under warm
conditions (25°C or 30°C), especially during the 2 weeks
prior to harvest.
The senior author gratefully acknowledges financial
assistance from the Division of Agricultural and
Environmental Sciences of the University of Nottingham.
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