Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:

Tree Physiology 17, 445--452
© 1997 Heron Publishing----Victoria, Canada

Influence of irradiance on water relations and carbon flux during
rooting of Shorea leprosula leafy stem cuttings
H. AMINAH,1 J. MCP. DICK2 and J. GRACE3
1

Forest Research Institute Malaysia (FRIM), Kepong, 52109 Kuala Lumpur, Malaysia

2

Institute of Terrestrial Ecology, Bush Estate, Penicuik EH26 0QB, U.K.

3

Institute of Ecology and Resource Management, University of Edinburgh, Darwin Building, Mayfield Road, Edinburgh EH9 3JU, U.K.

Received May 26, 1995

Summary Single-node leafy stem cuttings of Shorea leprosula Miq. were subjected to a high, intermediate or low irradiance treatment for 16 weeks in an enclosed mist propagation

system. Before rooting, maximum photosynthesis of the cuttings occurred at an irradiance of 400 µmol m −2 s −1. Although
none of the irradiance treatments affected the number of roots
produced per cutting, the numbers of cuttings that formed roots
were 50 and 30% in the high irradiance (diurnal range of 0--658
µmol m −2 s −1) and low irradiance (diurnal range of 0--98 µmol
m −2 s −1) treatments, respectively, compared with 62% in the
intermediate irradiance treatment (diurnal range of 0--360
µmol m −2 s −1). Low rooting frequency of cuttings in the high
irradiance treatment was associated with water deficits (maximum leaf-to-air vapor pressure deficit (VPD) = 3.6 kPa),
whereas cuttings in the low irradiance treatment had a low
rooting frequency because they were below the light compensation point most of the time. In the intermediate irradiance
treatment, cuttings withstood a daily maximum VPD of 1--2
kPa and recovered overnight from the previous day’s deficit, as
indicated by higher relative water content (RWC) and stomatal
conductance (gs) in the morning than in the previous afternoon
and evening. Higher RWC and gs of cuttings in all treatments
on Days 14 and 21 compared with Day 8 probably indicated
recovery from water deficit following severance and insertion
of the cuttings in rooting medium. There were negative relationships between stem volume of cuttings and both number of
cuttings that rooted and number of roots per cutting.

cies from stem cuttings has been developed; however, rooting
of the cuttings is highly variable. Because irradiance has a
pronounced effect on rooting in many species, we hypothesized that the variability in rooting of S. leprosula cuttings is
associated with variations in irradiance during propagation.
During propagation, the primary effects of irradiance are on
assimilate production and water use of the cuttings (Eliasson
and Brunes 1980, Klass et al. 1985, Hartmann et al. 1990).
High irradiances increase leaf temperature causing an increase
in leaf-to-air vapor pressure difference, and thus an increase in
transpiration rate (Grange and Loach 1983a, 1983b, Loach
1988a, 1988b). Rapid transpiration is often lethal to cuttings
that have not rooted. Moreover, the warming of air is likely to
increase the saturation deficit over the leaf, exacerbating the
process of dessication (Evans 1952, Kemp 1952, Hess and
Snyder 1955, Loach 1988a, 1988b, Hartmann et al. 1990). To
reduce the effect of high irradiance on cuttings, propagators are
shaded (Loach 1977, Loach and Whalley 1978, Loach and Gay
1979); however, there have been few attempts to quantify the
effect of irradiance on dipterocarp cuttings during rooting

(Moura-Costa and Lundoh 1994, Smits et al. 1994).
To test our hypothesis, we examined the effects of irradiance
on leaf-to-air vapor pressure deficit (VPD), relative water
content (RWC), stomatal conductance (gs) and photosynthetic
rate (Pn) of S. leprosula stem cuttings during rooting on propagation beds.

Keywords: leaf-to-air vapor pressure deficit, photosynthesis,
relative water content, stomatal conductance, vegetative
propagation.

Materials and methods

Introduction
Shorea leprosula Miq. (Dipterocarpaceae) is an important timber species in southeastern Asia (Symington 1974). To sustain
the supply of harvestable timber, the species has been planted
in deforested areas (Azman et al. 1991), but these planting
programs have been hindered by poor supplies of viable seeds
(Tang 1971, Tamari 1976, Sasaki 1980). To overcome the
shortage of planting stock, a method for propagating the spe-

Materials and experimental design
In February 1994, 531 single-node, leafy stem cuttings were
taken from seven-month-old stock plants raised in 33% of full
sunlight as potted rooted cuttings at the Forest Research Institute of Malaysia. The stock plants comprised 17 clones. The
number of cuttings obtained per stock plant varied from 3 to 7
depending on availability of leaves on the node. Cuttings were
trimmed to 5 cm in length and 30 cm2 in leaf area. The base of
each cutting was treated with 20 µg of IBA in ethanol. The
alcohol at the cutting base was immediately evaporated in a
stream of air from a fan. The initial diameter and node position

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AMINAH, DICK AND GRACE

of each cutting were recorded, and the volume of each cutting
was calculated assuming a cylindrical shape. Prepared cuttings
were planted in cleaned river sand in propagators located in a
cutting shed.
Cuttings were subjected to one of three diurnal irradiance

regimes adjusted by means of black plastic netting: (1) high
irradiance (without net; 0--658 µmol m −2 s −1); (2) intermediate
irradiance (one layer of netting; 0--360 µmol m −2 s −1); and (3)
low irradiance (three layers of netting; 0--98 µmol m −2 s −1).
Cuttings in the high irradiance treatment were not in natural
sunlight because the roof of the cutting shed was shaded with
a translucent plastic sheet. The mean red:far red ratio in the
shaded propagators was 1.1 (SKR 110 660/730, Skye Instruments Ltd., Llandrindod Wells, U.K.), which is close to the
value of 1.2 of full sunlight. The cuttings were randomly
allocated to nine blocks and there were three blocks per irradiance treatment (117 cuttings per treatment). Each block comprised a closed polyethylene propagator (1 m length × 1 m
width × 0.8 m height) with a misting unit in the center that was
set to mist for 1 min every hour for 24 h per day. Each time the
propagators were opened, the cuttings were sprayed with a
hand sprayer to minimize sudden changes in humidity.
Microclimate in the propagator
Air and leaf temperatures were measured with thermocouples
(Type K chromel-alumel, T.C., Ltd., Uxbridge, U.K.); relative
humidity was measured with commercial humidity sensors
(MP 100 Rotronic probes, Campbell Scientific Ltd., Loughborough, U.K.) and irradiance was measured with quantum
sensors (Skye Instruments Ltd.). The sensors were placed in

the center of one randomly chosen block of cuttings in each
irradiance treatment. A polyvinyl chloride (PVC) tunnel was
made to protect the humidity sensors from direct contact with
water. To measure leaf temperature, thermocouples were supported on an aluminum label inserted in the rooting medium so
that the sensors touched the undersurface of the leaf. Signals
from the sensors were recorded by a data logger (21X micrologger, Campbell Scientific Ltd.). The data logger was
programed to scan each sensor every 60 s, and to calculate and
store mean readings every 5 min. Data were collected from
Day 1 to Day 25 of the experiment.
Relative water content (RWC)
Leaf RWC was determined as described by Beadle et al. (1987)
at 0900, 1300 and 1700 h on Days 1, 8, 14 and 21 on four leaf
discs per cutting from six cuttings per treatment. Leaf discs
(18 mm) were taken from cuttings with a cork borer, and the
fresh (FW), turgid (TW) and dry (DW) weights measured. The
turgid weight was determined after floating the discs in distilled water in covered vials for 24 h. Discs were blotted with
paper towel before weighing. Dry weight was obtained after
drying the discs at 80 °C for 48 h. Leaf RWC was calculated
as RWC = (FW − DW)/(TW − DW)100 .
Photosynthetic rate (Pn ) and stomatal conductance (gs)

The Pn and gs of cuttings were measured with an infrared gas
analyzer (LCA-3, Analytical Development Co., Hoddesdon,

U.K.). Each measurement of gas exchange was recorded when
the CO2 differential readings were stable for 30 to 40 s. At each
measurement, one leaf was measured per treatment per block.
Before each measurement, the leaf surface was blotted with
paper towel to remove free water on leaf surfaces. During each
measurement, additional artificial light supplied by an incandescent tungsten lamp was used to increase the photosynthetically active radiation (PAR) to ≥ 500 µmol m −2 s −1. Diurnal
changes in Pn were measured for 10 consecutive days after the
cuttings were planted in rooting medium. In another set of
experiments, Pn and gs were measured at 0900, 1300 and
1700 h on Days 8, 14 and 21 on four randomly chosen cuttings
per treatment per block. Dark respiration (Rd) was measured at
night and before dawn on the same cuttings.
Assessment of cuttings
Forty-five cuttings per treatment were randomly harvested at
0900 h on Day 1 and leaf and stem dry weights of each cutting
were determined after drying at 40 °C for 48 h.
Assessments of rooted, dead and unrooted cuttings as well

as number of roots per cutting were made weekly until
Week 16. A cutting was considered rooted when it produced a
root of 1 cm or more in length, and a cutting was considered
dead when the whole stem turned brown. The mean accumulated number of roots per rooted cutting was calculated by
dividing the total number of roots produced by the total number of rooted cuttings.
Statistical analyses
Analysis of variance was used to test for significant differences
in mean accumulated number of roots per rooted cutting at
Week 16. Analysis of deviance for stepwise regression was
used to determine which variables were significantly associated with accumulated rooting at Week 16. In the regression
analysis, the association between each variable and rooting
was indicated by regression coefficients and this association is
presented graphically. Points on the graph were obtained by
grouping observed rooting data with cutting volume (at intervals of 0.25 cm3); the line was constructed by connecting
rooting values of individual cuttings computed from the multiple regression model. Because other factors or variables were
also fitted, the line drawn through the data points is not smooth.
For the diurnal Pn data, a nonrectangular hyperbola was
fitted using the model described by Jarvis et al. (1985), with
measured values of PAR, Pn and gs as the input variables. The
parameter optimization utility of the Genstat 5 software package (Payne et al. 1987) was used to estimate parameter values

of (i) dark respiration (Rd); (ii) mesophyll conductance (gm );
(iii) the initial slope of the Pn versus PAR curve (α, apparent
quantum efficiency); and (iv) the convexity coefficient (θ),
which defines the degree of curvature between the initial slope
and the asymptotic value of Pn. With the parameter estimates,
Pn versus PAR curves were plotted using values for the mean
gs measured on leaves of cuttings for each irradiance treatment.
Maximum Pn (Pn,max ) was calculated as:
Pn = (Ci − Γ)gm ,

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447

where Γ is the CO2 compensation concentration and was assumed to be 50 µmol mol −1 (Dick et al. 1991) and Ci (internal
CO2 concentration) was the mean value obtained for PAR
higher than 400, 300 and 300 µmol m −2 s −1 for high, intermediate and low irradiance treatments, respectively. These PAR
values were chosen because there was no significant difference

in Ci with increasing PAR values.
Treatment differences between the Pn versus PAR curves
were determined by a combined curve analysis of variance
(Ross 1981), which tests the reduction in residual variance
obtained by fitting a set of individual curves that are then
compared with the residual variance from the common curve
determined from combining the Pn data obtained at all PAR
values.
Results were considered to be significant at P ≤ 0.05.
Results
Environmental conditions in the propagators averaged over a
25-day period are given in Table 1. The PAR values in the low,
intermediate and high irradiance treatments were 20, 83 and
152 µmol m −2 s −1, respectively. Mean maximum VPD was
highest in the high irradiance treatment and lowest in the low
irradiance treatment. The daily maximum VPD in each treatment is shown in Figure 1.
At the start of the experiment, the mean diameters of the
cuttings were 0.34, 0.37 and 0.34 cm, and the mean volumes
of the cuttings were 0.50, 0.57 and 0.49 cm3 for cuttings in the
high, intermediate and low irradiance treatments, respectively.

The corresponding values for mean leaf weight were 0.21, 0.20
and 0.21 g and for mean stem weight 0.16, 0.18 and 0.18 g.
The irradiance treatments had a significant effect on the
number of cuttings that rooted. Fewer cuttings rooted in the
low irradiance treatment than in the other treatments (Figure 2a). Rooting frequency was negatively correlated with
initial cutting volume (Table 2 and Figure 2b).
Leaf shedding and mortality of cuttings were significantly
higher in the low irradiance treatment than in the other treat-

Figure 1. Daily maximum vapor pressure deficit (VPD) measured
from Days 1 to 25 of the experiment in (a) high irradiance, (b)
intermediate irradiance, and (c) low irradiance treatment propagators.
Maximum VPD per day was calculated as a 5-min average.

Table 1. Environmental data in the enclosed mist propagators subjected to different irradiances from Days 1 to 25. Data for each variable were
calculated as a 5-min average. The mean value for each variable was calculated over a 12-h period for each measurement period.
Variable

High irradiance

Intermediate irradiance

Low irradiance

Mean

Range

Mean

Range

Mean

Range

Time period = 0705 to 1855 h
Relative humidity (%)
Air temperature (°C)
Leaf temperature (°C)
Vapor pressure deficit (kPa)
Irradiance (µmol m −2 s −1)

93.86
31.74
32.08
0.49
152.40

50.74--100
22.23--44.79
22.04--46.35
0--3.62
0--658.30

98.88
30.43
31.15
0.23
83.22

67.97--100
22.53--41.82
22.09--42.70
0--1.87
0--359.90

99.86
30.08
30.38
0.03
19.76

78.94--100
22.04--40.85
22.04--41.7
0--0.89
0--98.00

Time period = 1900 to 0700 h
Relative humidity (%)
Air temperature (°C)
Leaf temperature (°C)
Vapor pressure deficit (kPa)
Irradiance (µmol m −2 s −1)

99.23
26.67
26.44
0.02
0.52

91.48--100
22.37--33.25
22.08--33.11
0--0.41
0--17.83

99.91
26.57
26.37
0.06
0.42

86.17--100
22.72--32.79
22.09--32.72
0--0.13
0--11.89

100
26.49
26.37
0.002
0.26

100--100
22.08--32.87
22.11--32.50
0--0.13
0--2.97

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AMINAH, DICK AND GRACE

ments (Figures 3a and 3c). Regression analysis indicated that
mortality and cutting volume were positively associated (Figure 3b). The number of roots per rooted cutting was not
significantly affected by the irradiance treatments (Figure 4a);
however, it was significantly and negatively affected by initial
cutting volume (Figure 4b).

The RWC of the cuttings decreased with increasing irradiance (Figure 5a). Across all treatments, RWC varied during the
day with a high of 86% at 0900 h, falling to 83% at 1300 h and
recovering slightly at 1700 h. On Day 8 after insertion of
cuttings in rooting medium, RWC was 78% compared with
values of 87 and 86% on Days 14 and 21, respectively. Stomatal conductance, gs, followed the same diurnal pattern as
RWC. It was highest at 0900 h with a value of 302 mmol
m −2 s −1 followed by 115 and 123 mmol m −2 s −1 at 1300 and
1700 h, respectively.
Photosynthetic rate of cuttings was significantly lower in the
low irradiance treatment than in the other irradiance treatments
(Figure 5b) and the carbon balance was negative in cuttings in
the low irradiance treatment. Combined curve analysis of
variance of curves fitted using the theoretical model of Jarvis
et al. (1985) indicated that there were significant treatment

Figure 2. (a) Effect of irradiance on rooting rate of stem cuttings
(n = 60 per treatment). (b) Relationship between rooting and cutting
volume of stem cuttings. Circles represent groups of observed data and
line was drawn by connecting predicted values computed from the
multiple regression model.

Table 2. Analysis of deviance for a stepwise regression to determine
the effects of irradiance treatments and morphological characteristics
on rooting ability of stem cuttings at Week 16 (n = 60 per treatment;
* = significant at P ≤ 0.01; and ns = not significant at P ≤ 0.05).
Source

Degrees
of freedom

Deviance

Mean
deviance

Deviance
ratio

Blocks
Irradiances
Cutting volume
Residual
Total

2
2
1
174
179

3.26
12.86
10.60
222.25
248.97

1.63
6.43
10.60
1.28

1.27 ns
5.02*
8.28*1

1

Regression coefficient.

Figure 3. (a) Effect of irradiance on percentage of dead stem cuttings
(n = 60 per treatment). (b) Relationship between dead cuttings and
cutting volume. Circles represent groups of observed data and line was
drawn by connecting predicted values computed from the multiple
regression model. (c) Effect of irradiance on the rate of leaf shedding
of stem cuttings (n = 60 per treatment).

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IRRADIANCE EFFECTS ON SHOREA CUTTINGS

449

Figure 5. (a) Effect of irradiance on mean RWC of stem cuttings before
rooting (n = 288 per treatment). (b) Effect of irradiance on mean Pn of
stem cuttings before rooting; (n = 144 per treatment). Means with the
same letters are not significantly different at P ≤ 0.05.
Figure 4. (a) Effect of irradiance on rate of root production per stem
cutting (n = 60 per treatment). (b) Relationship between root number
per stem cutting and cutting volume. Circles represent groups of
observed data and line was drawn by connecting predicted values
computed from the multiple regression model.

differences in the overall predicted Pn curves (Table 3). The
PAR required for maximum photosynthesis was around 400
µmol m −2 s −1, and higher PAR did not result in further increases in Pn of cuttings (Figures 6a--c). Estimated values of
gm, α, Pn,max and Rd of cuttings in each irradiance treatment are
given in Table 4. Dark respiration rate was significantly higher
in cuttings in the high irradiance treatment than in the other
treatments.

Discussion and conclusions
The low irradiance treatment significantly reduced the number
of S. leprosula stem cuttings that rooted (0--98 µmol m −2 s −1;
about 5% full sunlight). Poor rooting at low irradiances has
been observed previously. For example, Eliasson and Brunes
(1980) reported that cuttings of Populus tremula L. × tremuloides Michx. and Salix caprea L. × viminalis L. did not root
at irradiances of less than 2 W m −2 (about 0.3% full sunlight).
Similarly, only 9% rooting was observed in Prosopis alba
Griseb. cuttings grown at irradiances of less than 150 µmol m −2
s −1 (about 8% full sunlight) (Klass et al. 1985). Both Eliasson
and Brunes (1980) and Klass et al. (1985) concluded that

Table 3. Combined curve analysis of variance (Ross 1981) for Pn
curves of stem cutting curves fitted using the theoretical model of
Jarvis et al. (1985). Measurements of Pn were made for 10 days after
cuttings were planted in rooting beds in a high, intermediate or low
irradiance treatment propagator. During measurements, the cuttings
were artificially illuminated by a tungsten lamp to increase the maximum irradiance in each treatment to ≥ 500 µmol m −2 s −1 (n = 276 per
treatment; * = significantly different at P ≤ 0.01; ns = not significantly
different at P ≤ 0.05; treatment degrees of freedom = 4; and residual
degrees of freedom = 544).
Curve comparisons

Sum of squares

Mean square

F-value

High versus
intermediate irradiance
Residual

0.00
243.20

0.00
0.45

0.00 ns

High versus
low irradiance
Residual

5.42
198.60

1.36
0.37

3.68*

Intermediate versus
low irradiance
Residual

15.42
222.58

3.86
0.41

9.41*

current photosynthate was essential for rooting, although no
measurements of photosynthesis were made in either study. In
our study, poor rooting in the low irradiance treatment was

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AMINAH, DICK AND GRACE

Table 4. Estimated parameter values from the theoretical model of Jarvis et al. (1985) and maximum photosynthetic rate (Pn,max ). Measurements
of photosynthetic rate (Pn) were made for 10 days after cuttings were planted in rooting beds in a high, intermediate or low irradiance treatment
propagator. During measurements, the cuttings were artificially illuminated by a tungsten lamp to increase the maximum irradiance in each
treatment to ≥ 500 µmol m −2 s −1 (n = 228 and 48 for Pn and Rd measurements, respectively; α = initial slope of Pn /PAR curve (apparent quantum
efficiency); gm = mesophyll conductance; Rd = dark respiration; and θ = convexity coefficient).
Treatment

α
(mol CO2 photon −1)

gm
(mol CO2 m −2 s −1)

Rd
(µmol CO2 m −2 s −1)

θ

Pn,max
(µmol CO2 m −2 s −1)

High irradiance
Intermediate irradiance
Low irradiance

0.06
0.03
0.04

0.0092
0.0083
0.0083

0.85
0.77
0.71

0.00
0.94
0.13

2.54
2.29
2.43

Figure 6. Curves of Pn versus PAR for stem cuttings before rooting at
(a) high irradiance, (b) intermediate irradiance, and (c) low irradiance
(n = 228). Measurements were made for a period of 10 days; maximum irradiance for each treatment was achieved with a tungsten lamp.

associated with low Pn. Because the rooting of cuttings of
several tree species is influenced by the carbon balance of the
cuttings at the time they are severed and inserted in rooting
medium, it has generally been concluded that photosynthesis
must precede the onset of rooting (Davis 1988, Leakey and
Storeton-West 1992, Newton et al. 1992, Hoad and Leakey

1993, Mesen 1993). Furthermore, it has been postulated that
low Pn indirectly reduces rooting by slowing basipetal transport of auxin and other rooting cofactors that may originate
from leaves and buds (cf. Davis 1988).
In certain species, e.g., Hibiscus rosa sinensis L., leafy
cuttings root equally well in darkness or moderate irradiance
(van Overbeek et al. 1946), indicating that current photosynthate is not a prerequisite for rooting of cuttings of all species.
However, Hibiscus cuttings root within one week, which may
explain the independence of rooting on current photosynthate.
By contrast, in species characterized by a longer rooting period, even cuttings with substantial carbohydrate reserves,
such as Picea abies L. (Karst.), may require additional photosynthate to compensate for the depletion of carbohydrate reserves during the rooting process (Strömquist and Eliasson
1979). We conclude that S. leprosula stem cuttings require
current photosynthate because the rooting period is 6 to 8
weeks in this species. Furthermore, mortality of cuttings was
high (63%) in the low irradiance treatment and was associated
with a high percentage of leaf shedding (60%).
The irradiance treatments resulted in differences in the estimated parameters obtained from the Pn model. The gm value
was slightly higher in the high irradiance treatment than in the
other treatments. In general, the estimated gm values for
S. leprosula (mean gm = 0.008 mol CO2 m −2 s −1) were comparable to those for other climax species; e.g., Blighia sapida K.
König and Strombosia pustulata Oliv. (Riddoch et al. 1991).
The value of α in the high irradiance treatment was slightly
greater than in the other treatments, suggesting that the quantum efficiency of S. leprosula is sensitive to differences in
irradiance. These results are in contrast with those obtained by
Riddoch et al. (1991), Ramos and Grace (1990), and Kwesiga
et al. (1986) who found that α is insensitive to environmental
conditions. The rate of dark respiration was lower at low
irradiance than at high irradiance. Similarly, Bazzaz and Picket
(1980) observed that respiration rates of shade-acclimatized
leaves of climax species were lower than those of unshaded
leaves of pioneer species.
Lower rooting at high irradiance than at intermediate irradiance could be the result of water deficits in cuttings in the
higher irradiance treatment. Also, at high irradiance, carbohydrates may be used for maintenance respiration rather than for
root formation (Hartmann et al. 1990). Mesen (1993) showed
that rooting of Cordia alliodora (Ruiz & Pav.) Oken. stem

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IRRADIANCE EFFECTS ON SHOREA CUTTINGS

cuttings was significantly reduced at high irradiances (0--1460
µmol m −2 s −1) compared with low irradiances (0--339 µmol
m −2 s −1), and concluded that the reduction in rooting was
associated with water deficits in the cuttings and with damage
to the photosynthetic apparatus as indicated by a decline in leaf
chlorophyll fluorescence.
Rooting of S. leprosula stem cuttings was negatively associated with the initial volume and diameter of the cuttings. The
larger cuttings may have undergone secondary growth and
greater lignification than the smaller cuttings, and lignin is
known to create a physical barrier to root initiation (Hartmann
et al. 1990). Lignified cuttings often root poorly and die when
their carbohydrate reserves are depleted (Hartmann et al.
1990). An alternative explanation for the negative relationship
between rooting and the initial volume of cuttings is that
rooting may not depend on the carbohydrate reserves of the
cutting. It is also possible that the carbohydrate reserve was not
converted to sugar for root initiation.
At night (1900--0700 h), VPD in all irradiance treatments
was low; however, periods of temporary water deficit occurred
during the day, especially in the intermediate and high irradiance treatment propagators. These temporary water deficits
did not affect rooting of S. leprosula stem cuttings in the
intermediate irradiance treatment and even in the high irradiance treatment, 50% of the cuttings rooted despite a daytime
maximum VPD of 3.6 kPa. Overnight recovery from these
water deficits was reflected in a high RWC and gs the following
morning. Increased RWC on Days 14 and 21 compared with
Day 8 probably reflects recovery of the cuttings from a water
deficit experienced at the time of severance and insertion into
the rooting medium (cf. Mesen 1993, Newton and Jones 1993).
We observed large reductions in rooting and high mortality
of S. leprosula stem cuttings grown in low irradiance and
conclude that a reduction in photosynthesis may be the cause.
Because photosynthetic activity of stem cuttings of S. leprosula saturated at a PAR of about 400 µmol m −2 s −1 (about 20%
of full sunlight), we conclude that an irradiance of 0--360 µmol
m −2 s −1 (about 0--18% full sunlight; propagator with one layer
of netting) is most suitable for the propagation of cuttings of
this species because it kept VPD low while supporting photosynthesis at about 80% of its maximum rate. Similar irradiance
regimes are optimal for the rooting of other dipterocarp cuttings. For example, more than 60% rooting of S. acuminata
Dyer and S. parvifolia Dyer cuttings was obtained at an average irradiance of 27 W m −2 (about 3% of full sunlight, Noraini
and Ling 1993); and 73 to 88% of Dryobalanops lanceolata
Burck stem cuttings rooted in 290 µmol m −2 s −1 (about 15%
full sunlight, Moura-Costa and Lundoh 1994).

Acknowledgments
We thank Mr. R.I. Smith for statistical advice, and the Overseas
Development Administration (ODA) and the British High Commission for financial support.

451

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