New Phytol. (1999), 143, 45–51
Specific leaf area in barley : individual
leaves versus whole plants
S. G U N N*, J. F. F A R R A R, B. E. C O L L I S    M. N A S O N
School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK
Received 5 October 1998 ; accepted 8 April 1999

We have explored the relationships between specific leaf area calculated for a whole plant and its individual leaves.
Barley was grown in hydroponics in controlled environment cabinets. Plants were harvested on the basis of
physiological age (defined as the number of days after full expansion of leaves on the main stem) and the area and
weight of whole, fully expanded, leaves measured and specific leaf area (SLA) of individual leaves or whole plants
calculated. Specific leaf area calculated for individual leaves (SLAL) varied with leaf position and with leaf age after
full expansion whereas SLA calculated for whole plants (SLAP) varied with plant age. The same conclusions were
reached whether the results were based on total dry weight or dry weight minus soluble carbohydrates (‘ structural
weight ’). Transferring plants to shade on the day of full expansion of the third leaf on the main stem increased
the SLAP, and also SLAL of leaves 3 and 4 on the main stem (leaf 4 being the younger leaf of the two), because
of a decrease in the ‘ structural weight ’ of these leaves. However SLAL of leaf 2 (which was older than leaf 3) was
not affected by shading ; the effect was confined to leaves developing in the new conditions.
Key words : specific leaf area, barley, dry weight, structural dry weight, shade.

Specific leaf area (SLA) was introduced as a concept
in the analysis of whole plant growth and was defined
as the total leaf area divided by the total leaf weight
(Evans, 1972). Defined in this way, SLA has been
used to draw conclusions about the relative thickness
of leaves which in turn has led to questions about
how leaf structure is affected by environmental
conditions or experimental treatments. Therefore
there is a conceptual leap from a property of the
whole plant (traditional specific leaf area) to specific
changes in the structure and chemical composition of
individual leaves.
SLA has been correlated with variables as diverse
as net photosynthesis (McClendon, 1962), relative
growth rate (Atkins & Lambers, 1998 ; Poorter &
Van der Werf, 1998), yield (Singh et al., 1985) and
leaf structure (Cambridge & Lambers, 1998 ;
Pyankov et al., 1998). However there rarely seems to
be a clear distinction made between whole plant
SLA (i.e. a mean of all leaves, SLAP) and SLA of
individual leaves (SLAL) and how these relate to
*Author for correspondence (fax j44 1248 370731 ; e-mail
s.gunn!bangor.ac.uk).
Abbreviations : SLA, specific leaf area ; SLAL, specific leaf area of
individual leaf blades ; SLAP, SLA of the whole plant (mean) ;
dae, day after full expansion.
actual changes in leaf structure and chemical composition. This problem was noted by Garnier &
Freijsen (1994) who alluded to the paucity of data on
the relationship between SLAL and SLAP.
Furthermore there is the question of what controls
SLAL, and whether there are any mechanistic
explanations for effects of experimental treatment.
Correlations of SLAP with different variables between species are descriptive and do not lead to
mechanistic explanations (Shipley, 1995). A better
method is to manipulate SLAL within a single
species by altering the environment. For this approach to work, it is necessary to define exactly when
and where changes in SLAL occur so that changes
can be related to other parameters of plant growth.
The partitioning of carbon (C) between different
leaves and within leaves between transport carbohydrate (soluble sugars in the cytosol and phloem
vessels), storage carbohydrate (fructans in the vacuole and starch in the chloroplast) and structural C
(cell wall, lipid and protein) gives a basis for
understanding changes in leaf weight and hence
SLAL. These pools of C vary over different timescales
(from hours to days or weeks) and all may contribute
to changes in SLAL. Variation in inorganic compounds may also cause changes in SLAL (Heilmeier
& Monson, 1994 ; Van Arendonk & Poorter, 1994),
but the abundance of C make its partitioning and
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46
S. Gunn et al.
metabolism a strong candidate for controlling
changes in SLAL.
We wanted to determine whether SLAL and SLAP
can be used interchangeably in drawing conclusions
about individual leaves by posing the following
questions. (1) Does SLAL vary with leaf position
when leaves are harvested at a given developmental
stage ? (2) Does the SLAL of a leaf change with age
after it has fully expanded ? (3) Can SLAP be
predicted from SLAL ? (4) When plants are transferred from high to low light, what aspect of SLA
changes ? We also investigated the role of C in the
variation of SLAL with two further questions : (5). If
SLAL changes is this because of changes in leaf area
or leaf weight ? (6) Do the answers to the previous
questions change if the results are expressed on the
basis of total weight or structural weight ?
  
Plant growth
Seeds of Hordeum vulgare L. (barley) cv. Klaxon
were germinated and grown in controlled environment chambers (Sanyo Gallenkamp PG660, Leicester, UK), at a mean CO concentration of 350 µmol
#
CO mol−", at 20mC with a 16 h photoperiod, a
#
photon flux density of 400 µmol m−# s−" at plant
height, supplied by halogen metal halide bulbs
supplemented with tungsten filament bulbs and a
vapour pressure deficit of 0.7 kPa. Air was drawn
into the cabinets through a modified inlet port from
a fan (Type-3MS11, Air Control Installations,
Chard, UK) at 60 dm$ min−" which produced 5.5
changes of air h−".
Sixty plants were grown in two troughs each
containing 7 dm$ of solution aerated at 1.2 dm$
min−". Plants were spaced to minimize shading
between plants and there was little shading within
each plant. The temperature of the solution was not
controlled but wasp1mC of the air temperature.
Solutions were changed every 3 or 4 d. The plants
were grown in full strength Long Ashton solution
(mol m−$, full strength) ; KNO (4), Ca(NO ) ;4H O
$
$#
#
(4), NaH PO ;2H O (1.33), MgSO ;7H O (1.5),
# %
#
%
#
FeEDTA Na (0.1), MnSO ;4H O (0.01), CuSO ;
%
#
%
5H O (0.001), ZnSO ;7H O (0.001), H BO (0.05),
#
%
#
$ $
Na MoO ;2H O (0.004), NaCl (0.1), Na SiO ;
#
%
#
#
$
5H O (0.05).
#
Transfer to low light
On the day of full expansion of leaf three on the main
stem, 10 plants were transferred to low light (90
µmol m−# s−" at plant height). Neutral density filters
(layers of muslin) were supported on a wire frame
such that air could circulate freely over the plants :
shaded and unshaded plants were grown in the same
trough.
Dry weight and leaf area
Plants were harvested (a) 2 dae of successive leaves
on the main stem, (b) 2 dae of leaf 4 (low light) or (c)
every day after full expansion of the third leaf on the
main stem from the day of full expansion (0 dae) to
5 dae. At each harvest four replicate plants were
divided into ; main stem leaf blades which were fully
expanded, and tiller leaf blades which were fully
expanded. Leaf area was measured on a flatbed
scanner with computer software (Delta T Devices,
Cambridge, UK) before leaves were dried in an oven
at 70mC for 48 h.
‘ Structural weight ’
‘ Structural weight ’ was calculated as total dry weight
minus soluble carbohydrates. Soluble carbohydrates
were extracted from dried fully expanded leaf blades
in 5 ml 80% ethanol at 60mC overnight, followed by
5 ml 40% ethanol at 60mC for 2 h and then 5 ml
distilled water at 60mC for 2 h and the three extracts
combined and made up to 20 ml. A further extraction
in 5 ml distilled water at 60mC for 2 h did not contain
significant amounts of carbohydrate. Soluble carbohydrates were determined by the phenol sulphuric
method (Dubois et al., 1956), with sucrose as the
standard. Starch was not determined as it is a very
small proportion (2.5p0.5%) of the carbohydrate
of barley leaves grown under these conditions
(B. Collis, unpublished).
Specific leaf area
Specific leaf area of individual leaf blades (leaf SLA,
SLAL) was calculated as :
SLAL l
individual leaf area
individual leaf weight
whereas whole plant SLA (SLAP) was calculated as :
SLAP l
Σ (area of all fully expanded leaves)
Σ (weight of all fully expanded leaves)
but excluding senescent leaves.
Means were compared by ANOVA using the
computer package SPSS (version 7, SPSS, Chicago,
US). Levine’s test was used to test for the equality of
variances and significant interactions were compared
using Tukey’s honestly significant test. Data are
shown as means of four replicatesp1 SE.
Allometric coefficients
Allometric coefficients were calculated for the relationships between the natural logarithms of leaf
area and leaf dry weight by geometric mean regression (Gunn et al., 1999). Two relationships were
determined for ; all fully expanded leaves individu-
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SLA of leaf and plant
47
Specific leaf area of individual leaf blades (SLAL)
The SLAL of leaf 4 on the main stem was smaller
than for leaves 1, 2 or 3, whether expressed on total
(P 0.001) or structural weight basis (P 0.001,
Fig. 1) whereas SLAL of leaf 3 was lower than leaf 2
when expressed on a structural basis. The SLAL of
leaf 3 was lower between 2–5 than 0–1 d after full
expansion when expressed on a total (P 0.01) or
structural weight basis (P 0.01, Fig. 2a) because of
an increase in the structural dry weight of the leaf
(Fig. 2b). Leaf area was unaffected by shading (Fig.
2b).
SLAL (cm2 mg–1 d. wt)
0.4
0.3
0.2
0.1
0.0
0
1
2
3
4
5
70
20
(b)
60
15
50
40
10
30
20
5
10
0
0
0
1
2
3
4
Days after full expansion
5
Fig. 2. The effect of leaf age on (a) the specific leaf area
(SLAL, cm# mg−") of leaf 3 on the main stem and (b) the
structural dry weight (mg) and leaf area (cm#) of barley
(Hordeum vulgare) grown in hydroponics. Plants were
harvested every day from full expansion (0 dae) to 5 dae.
Values are the mean of 4 replicatespSE. SLAL calculated
using total dry weight (filled squares) ; SLAL calculated
using extracted dry weight (open squares) ; dry weight
(filled circles) ; leaf area (open circles).
Plants were harvested on the basis of development (2
dae of successive leaves on the main stem). The
0.5
0.4
0.3
SLAP (cm2 mg–1 d. wt)
0.5
Whole plant (SLAP)
SLAL (cm2 mg–1 d. wt)
(a)
Leaf area (cm2)

0.5
Structural d. wt (mg)
ally, and total of all fully expanded leaves on a plant.
Goodness of fit of the points to a straight line was
assessed using the coefficient of determination (r#)
(Zar, 1996). A comparison of the two correlation
coefficients was carried out after a Fisher’s z
transformation of r (Zar, 1996). There was no
significant difference between the two correlation
coefficients (r for all fully expanded leaves individually was 0.978 and for the total of all fully
expanded leaves was 0.989). Comparisons of v were
carried out using a modified t-test and results are
shown with standard errors (Ricker, 1984). A
comparison of the elevations (as opposed to the
intercepts) of the regressions (i.e. a comparison of
the vertical positions of the lines on the graphs) was
carried out using a t-test (Zar 1996).
0.4
0.3
0.2
0.1
0.0
1
2
3
4
Plant age
(number of fully expanded
leaves on the main stem)
0.2
0.1
0.0
1
2
3
Main stem leaf
4
Fig. 1. The effect of leaf position on the specific leaf area
of individual leaf blades (SLAL, cm# mg−") of barley grown
in hydroponics. Plants were harvested 2 d after expansion
of successive main stem leaves. Values are the meanpSE
of 4 replicates. SLAL calculated using total dry weight
(filled squares) ; SLAL calculated using extracted dry
weight (open squares).
Fig. 3. The effect of plant age (as determined by number
of main stem leaves) on the specific leaf area of whole
plants (SLAP, cm# mg−") of barley (Hordeum vulgare)
grown in hydroponics. Plants were harvested 2 d after
expansion of successive main stem leaves. Values are the
mean of 4 replicatespSE. SLAP calculated using total dry
weight (filled squares) ; SLAP calculated using extracted
dry weight (open squares).
corresponding chronological ages were, 2 d after
expansion of leaf 1, 11 d ; leaf 2, 15 d ; leaf 3, 19 d and
leaf 4, 23 d after sowing.
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S. Gunn et al.
48
Table 2. Results from a two way ANOVA of the effect
of leaf position and shading on specific leaf area of
individual leaves (SLAL), structural dry weight and
leaf area
6
5
Ln leaf area
4
3
Source of
variation
SLAL
Structural
d. wt
Leaf area
Shade
Leaf
Shadeileaf
***
**
**
**
***
ns
ns
***
ns
2
***, P 0.001 ; **, P 0.01 ; ns, no significant difference.
1
Transfer to low light
0
–1
0
1
2
3
4
5
Ln leaf dry weight
6
7
Fig. 4. The allometric relationship between leaf area and
leaf weight for barley (Hordeum vulgare) grown in
hydroponics. Individual leaves are shown by open symbols
and the dotted line (leaf 1 on the main stem (circle) ; leaf 2
on the main stem (square) ; leaf 3 on the main stem
(triangle, apex uppermost) ; leaf 4 on the main stem
(triangle, apex down) ; tiller leaves (diamond)), and the
sum of the individual leaves are indicated by solid circles
and the solid line.
When plants were harvested 2 dae of successive
leaves SLAP fell with plant age when expressed
on a total (P
0.001) or structural weight basis
(P 0.001, Fig. 3).
Can SLAP be predicted from the SLAL ?
There was no significant difference between the
allometric coefficient, calculated by geometric mean
regression, for all fully expanded leaves individually
(v l 0.90p0.01) and that calculated for the total of
all fully expanded leaves (v l 0.93p0.03) whereas
both were significantly lower than 1 (P 0.001 ; Fig.
4). There was no significant difference between the
elevations.
On the day of full expansion of leaf 3 on the main
stem, plants were transferred from high to low light
(400 to 90 µmol m−# s−" at plant height). SLAP
is increased by shading when this is expressed on
a total (P
0.001) or structural weight basis
(P 0.001, Table 1) over the next 6 d.
An ANOVA (Table 2) of the results from shaded
and unshaded plants harvested 2 d after full
expansion of the fourth leaf on the main stem
showed that there was a significant interaction
between leaf and shading such that SLAL of leaf
blades 3 and 4 on the main stem, but not leaf blade
2, was increased by shading when this is expressed
on a total (results not shown) or structural weight
basis (Fig. 5a). Shading decreased dry weight (both
total and structural) but not leaf area (Table 2,
Fig. 5b,c).

We have investigated the variation in SLAL within a
single plant to test if SLAP could be related to SLAL
and vice versa. We set out to answer a number of
questions.
Does SLAL vary with leaf position when leaves are
harvested at a given developmental stage ?
When leaves of barley are harvested at the same
physiological age the SLAL varies with leaf position
by as much as 38%. This is in contrast to wheat
Table 1. The effect of shade on whole plant SLA (SLAP, cm# mg−") of
barley (Hordeum vulgare) plants grown in hydroponics and harvested 2 d
after full expansion of leaf blade 4 on the main stem
2 dae leaf 4
SLAP
(Total d. wt basis)
SLAP
(Structural d. wt basis)
Control
Shade
Control
Shade
0.288p0.005
0.414p0.012
0.318p0.006
0.434p0.019
Plants were transferred from 400 to 90 µmol m−# s−" at plant height on the day
of full expansion of the third leaf on the main stem. SLAp was calculated on the
basis of total dry weight or structural dry weight. Values are the means pSE of
4 replicates.
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SLAL
(cm2 mg–1 structural d. wt)
SLA of leaf and plant
0.5
Table 3. A comparison of (A) SLAP of plants
harvested 2 dae leaf 4 on the main stem (SLAP l
0.318, Fig. 3) with SLAL of leaves 1, 2, 3, 4 on the
main stem (see text for assumptions about SLAL)
(a)
0.4
0.3
0.2
(A)
(B)
0.1
0.0
Structural d. wt (mg)
Leaf 1
Leaf 2
Leaf 3
Leaf 4
j26
j38
j6
j16
j28
k11
j44
([[SLAPkSLAL]\SLAL]i100) and (B) SLAL of unshaded
plants with shaded plants ([[SLALunshadedkSLAL
shaded] SLAL unshaded]\i100) (results from Fig. 5).
100
(b)
80
60
40
20
0
30
Leaf area (cm2)
49
(c)
20
10
0
2
3
4
Main stem leaf
Fig. 5. The effect of shade on (a) specific leaf area of
individual leaves on the main stem (SLAL, cm# mg−"), (b)
structural dry weight (mg) and (c) leaf area (cm#) of barley
(Hordeum vulgare) plants grown in hydroponics and
harvested 2 d after full expansion of leaf blade 4 on the
main stem. Plants were transferred from 400 to 90 µmol
m−# s−" at plant height on the day of full expansion of the
third leaf on the main stem. Values are the means of 4
replicatespSE. Control plants (filled bars) ; shaded plants
(open bars).
where, after full expansion, SLAL for different leaves
varied by only c. 5% (Rawson et al., 1987). It also
contrasts with results of Poorter & de Jong (1999)
who studied SLAL for 70 species in the field. They
found on average a difference of only 4% between
the youngest fully expanded leaf and the oldest still
viable leaf.
Does the SLAL of a leaf change with age after it has
fully expanded ?
SLAL of leaf 3 of barley decreases as the leaf ages,
after full expansion, by as much as 20%. Similar
results have been found for primary leaves of barley
(Sicher et al., 1984). In wheat, however, the SLA of
some leaves increased 16–20 d after tip emergence
(Rawson et al., 1987), beyond the timescale of the
experiment reported here.
Can SLAP be predicted from SLAL ?
In order to calculate how well SLAP would be
predicted by SLAL of any individual leaf we shall
assume that SLAL of leaves 1, 2 and 3 on the main
stem of plants harvested 2 dae of leaf 4 on the main
stem are the same as for leaf 1 harvested 2 dae, leaf
2 harvested 2 dae and leaf 3 harvested 2 dae (Fig. 1)
and compare these values for SLAP of plants
harvested 2 dae of leaf 4 on the main stem (Fig. 3).
The results show that SLAP underpredicts SLAL by
as much as 11% and overpredicts by as much as 38%
(Table 3). These results are comparable with changes
in SLAL due to shading : in barley SLAL increased
by a maximum of 44% (Table 3) whereas in soybean
SLAL of the first fully expanded trifoliate leaf
increased by c. 50% after 12 d of shading (Pons &
Pearcy, 1994). Therefore we cannot draw any
conclusions about individual leaf structure from
SLAP. However when SLA is plotted allometrically
(Fig. 4) as the natural logarithum of leaf weight
versus leaf area the points of individual leaves and of
whole plants lie on the same line. Since the slope of
the line 1, larger leaves have smaller SLAL than
smaller leaves. Hence only if data are expressed
allometrically can predictions about SLAL be made
from SLAP and vice versa.
When plants are transferred from high to low light,
what aspect of SLA changes ?
Growth at low light or switching plants from high to
low light generally causes an increase in SLAP and
SLAL because of an increase in leaf area (Blackman,
1956 ; Evans, 1972 ; Rice & Bazzaz, 1989). However
there does not appear to have been any work which
determines if all leaves on a plant are affected in the
same way by shading of the whole plant. In barley
switched to low light on the day of full expansion of
the third leaf on the main stem, SLAP was increased
compared with controls. However whole plant
shading affected individual SLAL differentially.
Leaves which developed in low light (in this case leaf
4 on the main stem) did indeed have a higher SLAL
than those which developed in high light when both
were harvested at the same physiological age.
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50
S. Gunn et al.
Reducing the light available to the whole plant also
increased the SLA of leaves which were fully
expanded at the time the treatment was imposed
(leaf 3). However the SLAL of the oldest leaf (leaf 2),
which was 6 dae when the treatment was imposed,
was unaffected by shading.
If SLAL changes is this because of changes in leaf
area or leaf weight ?
The variations in SLAL due to leaf ageing and
shading were analysed by parallel investigations of
leaf area and leaf weight. The decrease in SLAL with
leaf age was due to an increase in structural dry
weight ; older leaves were heavier. The increase in
SLAL in both newly expanded and expanding leaves
because of shading was due to the structural weight
of the leaves being lower. This was similar to results
from soybean (Pons & Pearcy, 1994).
Do the answers to the previous questions change if the
results are expressed on the basis of total weight or
structural weight ?
The influence of non-structural carbohydrates on
leaf weight was analysed, to exclude the possibility
that accumulation or mobilization of storage carbohydrate during leaf ageing, or differential accumulation by leaves at the same developmental stage
could influence SLAL. The decrease in SLAL with
increasing leaf position was not due to differences in
accumulation of soluble carbohydrates ; SLAL calculated on total weight or structural weight differed
by only 5–10%. Likewise, the decrease in SLAL
because of leaf ageing was not due to an accumulation
of soluble carbohydrates with time ; structural dry
weight increased.
General conclusions
These simple experiments serve to emphasize that
there are several routes which can lead to a single
value of SLAL ; area may be increased due to a
treatment imposed before leaf emergence and expansion, soluble carbohydrate content may increase
or decrease, structural weight may change after full
leaf emergence. Since SLAL is so plastic it may be
optimistic to try and find meaningful correlations
between SLAP and growth rate or to try and relate
changes in SLAP to changes in the structure of
individual leaves. Care must also be taken in
assessing the effects of treatment on SLA and leaf
structure since each leaf on a single plant may react
to the treatment in a different way depending on its
position on the plant, age, stage of development and
maturity when the treatment was imposed. When
making comparisons of SLAL and leaf structure,
intrablade variability must also be taken into ac-
count. Rawson et al. (1997) found that SLA of wheat
leaves varied from the tip to the base of a leaf and
that the mid-vein had more effect on the SLA of the
base than the tip. SLAP as a component of growth
analysis remains valid in determining whether any
changes in leaf area ratio are due to a greater
investment in leaf weight or leaf area for a given
plant as it grows. But the assumption that it carries
useful information about individual leaves is not
supported. SLAP is determined by the sum of the
processes which determine the surface expansion
and net weight gain of individual leaves. Surface
expansion is considered by Tardieu (1999) and net
weight gain wil be considered here.
Net weight gain by a leaf is a complex of many
processes. To progress, first consider only C, which
constitutes between 38–48% of plant dry weight
(Poorter et al., 1997), and concentrate on the system
about which we know most : the control of export of
C from mature leaves. Export in the phloem is by
Munch pressure flow, and is driven by gradients of
turgor pressure within the phloem between source
and sink (Farrar, 1992). Precisely, export of C is a
function of the loading of sucrose and of other
turgor-generating solutes in the source leaf and of
their removal in sinks, as well as of apoplastic
solutes. Export is therefore not a function of the
source leaf alone. Rather it is a whole plant property,
a conclusion happily converged on by theory
(Minchin et al., 1993) and experiment (Moorby &
Jarman, 1976 ; Minchin et al., 1994). It is clear
therefore that the rate of export of C in the phloem
from mature source leaves is a whole-plant property,
dependent on the nature of the transport system and
on events in other sources and in sinks. It is certainly
not determined wholly by events within the leaf
itself. The principles which underlie this conclusion
will also underlie the control of import to a young,
developing leaf, and indeed any exchange of any
substance between a leaf and the remainder of the
plant. It follows that the net fluxes of C and other
components which together constitute leaf weight
are whole-plant properties. SLAL, and thus SLAP, is
a whole plant property in the sense that its particular
value is determined mechanistically by a set of
processes that involve parts of the plant remote from
the leaf or leaves being considered.

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