DescriptionTrees (2001) 15:341–345
DOI 10.1007/s004680100110
O R I G I N A L A RT I C L E
S. Sisó · J. J. Camarero · Eustaquio Gil-Pelegrín
Relationship between hydraulic resistance and leaf morphology
in broadleaf Quercus species: a new interpretation of leaf lobation
Received: 31 August 2000 / Accepted: 31 May 2001 / Published online: 6 July 2001
© Springer-Verlag 2001
Abstract We investigated the relationship between leaf
shape and leaf hydraulic resistance in a set of broadleaf
Quercus tree species (Q. cerris, Q. frainetto, Q. petraea,
Q. pyrenaica, Q. robur, Q. rubra, Q. velutina). Seedlings
of all the studied species were grown under uniform environmental conditions. A new high-pressure flowmeter
was designed to measure leaf-blade hydraulic resistance.
Leaf shape was characterised by the complexity of leaf
outline which was regarded as an estimate of leaf lobation. This was done using the box-counting fractal dimension of the leaf silhouette. Leaf hydraulic resistance
was negatively related to leaf lobation. It is suggested
that the lower hydraulic resistance in deeply lobed leaves
may constitute a mechanism for improving water balance under dry atmospheric conditions.
Keywords Quercus · Broadleaf trees · High-pressure
flowmeter · Leaf hydraulic resistance · Fractal dimension
Introduction
Diversity in leaf shape and size among broadleaf trees is
surprisingly high. This is clearly shown by leaf lobation
in the genus Quercus (Fagaceae), which includes tree
species dominant in many temperate forests in the Northern Hemisphere (Krüssmann 1986). Moreover, leaf form
is related to environmental variability, mainly to climatic
conditions (Brenner 1902; MacArthur 1972; Givnish
1979; Hamerlynck and Knapp 1994). However, as Vogel
(1970) noted, “direct evidence” of environment/leaf
shape relationship is “scanty”.
S. Sisó · J.J. Camarero · E. Gil-Pelegrín (✉)
Unidad de Recursos Forestales,
Servicio de Investigación Agroalimentaria (S. I. A.),
Gobierno de Aragón, Apdo. 727, 50080 Zaragoza, Spain
e-mail: egilp@aragob.es
Tel.: +34-976-716373, Fax: +34-976-716353
J.J. Camarero
Dept. d’Ecologia, Fac. de Biologia, Universitat de Barcelona,
Avda. Diagonal, 645. 08028 Barcelona, Spain
Some authors have suggested complementary concepts in order to explain leaf morphology as an adaptive
trait. Firstly, lobed leaves (e.g. “sun leaves”) are more effective convection-heat dissipaters than straight-edged
ones (e.g. “shade leaves”) (Vogel 1968, 1970). Secondly,
leaf lobes and teeth are areas of active early-season photosynthesis in immature leaves of temperate trees
(Baker-Brosh and Peet 1997). Thirdly, leaf lobation influences the interception of direct solar radiation (Niklas
1989).
Additional factors contributing to leaf shape variation
can be found in the influence of hydraulic architecture
on water relationships in woody plants (Yang and Tyree
1993; Tyree et al. 1999). When analysing the wholeshoot hydraulic resistance in deciduous Quercus species,
Tyree et al. (1993) concluded that leaves were responsible for 80–90% of the resistance. This high contribution
of the leaves to the whole shoot hydraulic resistance led
us to explore the influence of leaf shape on the hydraulic
pattern of the shoot.
In fact, a previous study about the whole hydraulic resistance in shoots of Quercus species (Sisó et al. 2001)
suggested an influence of leaf shape on leaf hydraulic resistance. Our main objective is to study this relationship
in depth for a wide spectrum of Quercus species.
We hypothesise that a deeper lobation should be associated with a lower leaf hydraulic resistance, which directly affects the plant’s response to water stress. A lower hydraulic resistance would allow a better water supply
to mesophyll cells in spite of high water-potential differences between the leaf and the atmosphere.
In order to test this hypothesis, we studied the relationship between leaf shape, specifically leaf lobation,
and shoot hydraulic resistance, for a set of broadleaf deciduous Quercus tree species. The selected species were
chosen to represent a wide spectrum of leaf form (e.g.
lobed vs straight edged), but showing at the same time a
high degree of phylogenetic affinity.
342
files with a constant size (512×512 pixels). The FDb was computed
using Fractal Dimension Calculator software (Bourke 1993).
Materials and methods
Plant material
In order to study a wide range of leaf morphologies, from deeply
lobed to nearly straight ones, various Quercus species were selected, namely: Q. cerris, Q. frainetto, Q. petraea, Q. pyrenaica, Q.
robur, Q. rubra and Q. velutina. This selection was based on our
previous experience and supported by the descriptions given in
Krüssman (1986). Seeds of the species found in the Iberian Peninsula (Q. petraea, Q. pyrenaica) were provided by the El Serranillo
Forest Breeding Station (Ministry of Environment, Guadalajara,
Spain), while the remainder were supplied by Sandeman Seeds
(The Croft, Sutton, UK). Seedlings of each species were grown in
cylindrical pots in the shadehouse of the Forest Research Unit,
Agriculture Research Service (Zaragoza, Spain). Climatic conditions in the shadehouse were monitored with a thermohydrograph
(JRI MINIDISQUE 165–00, Jules Richard, Argenteuil, France).
The mean maximum and minimum temperatures during the growing season (March–August) were 24.9°C and 10.4°C, while the
relative air humidity varied between 92% and 58%.
The shadehouse was covered by a shade frame to limit light intensity to a maximum of 40% of the total external radiation. The
light regime was quite regular throughout the growing period because only about 5% of the days were cloudy. Plants were watered
twice a day (early morning and evening) during the entire growing
period, by using micro-sprinklers fixed 1.5 m above the plants.
The cylindrical pots were filled with nutrient-free sand. Nutrient supply was ensured by using a slow-liberation fertiliser
(Osmocote Plus, 5–6 months liberation at 21°C, Sierra Chemical,
Milpitas, Calif.) as described by Naidu and DeLucia (1997). The
fertiliser (3 g l–1 substrate) was added at the start of the experiment to the top 10cm of sand. At the end of the second growth cycle (July), 6–7 seedlings per species were randomly harvested for
hydraulic and leaf morphology measurements.
In all species, the second cycle of leaf growth was similar to the
first one except for Q. cerris. The second growth cycle was selected
for all leaf measurements except for Q. cerris, because of its great
variability of leaf lobation between the first and second growth cycles. Therefore, Q. cerris seedlings were grouped according to their
ontogenic development: the first (Q. cerris with even straighteredged leaves – Q. cerris entire leaves) and the second growth cycle
(Q. cerris with even more lobed leaves – Q. cerris lobed leaves).
The new high-pressure flowmeter
The leaf hydraulic resistance was estimated according to the methodology of Tyree et al. (1993, 1995), which is based on the perfusion of water at a given pressure into the base of a whole shoot
while, simultaneously, recording the flow in a quasi-steady state
mode (Nardini and Tyree 1999). However, the system we used for
recording the flow differed from that described by these authors.
The apparatus shown in Fig. 1 is similar to a syringe pump (see
also Sisó et al. 2001). In this kind of device, the measurement of
plunger displacement (X) and the syringe cross-section (S) allows
the dispensed volume to be calculated (V=X S). If the time elapsed
(t) is recorded simultaneously, a flow rate (F) can be calculated
(F=V t-1). When the resistance to flow is high enough, an almost
constant pressure (P) within the syringe is reached as a result of
the compressive force (E) exerted by the plunger in the syringe.
In this device, the syringe was made of a borosilicate glass tube
(0.125 m long, 14 mm internal diameter, 19 mm external diameter;
GT in Fig. 1) housed in a clear metacrilate block (MB). Its inner
plunger (IP), of stainless steel and equipped with a silicone gasket,
was connected to the piston rod of a pneumatic microcylinder
(Model 1200-Pneumax, Lurano, Italy). The force exerted by the
pneumatic microcylinder (PM) was adjusted by using a pressure
regulator connected to an air tank. A three-way stopcock (TS) was
incorporated in the circuit of the device to control the gas inflow.
The plunger displacement was recorded by using a built-in electronic calliper with a resolution up to 10 µm in a dynamic mode
(Mitutoyo, Digimatic SD-M 572). This calliper (EC) was connected to a computer by an interface (Mitutoyo, Digimatic DMX-2).
Specific software was designed to convert displacement data into
flow values and show them along with time (flow rates every 5 s).
A linear regression was fitted for flow values before and after leaf
removal. As the slope was not significantly different from zero in
all cases (P
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