98
I. Fiorello et al.
in the basal region [7]. This value is common for aquatic plants [8], but it has never
been reported for terrestrial species, and the mechanism behind such high breaking
strain is still unclear (i.e., the reorientation of microfibrils seems to be not responsible)
[7]. Furthermore, observations of G. aparine in the field suggested marked differences
between the lower and upper parts of the stem [7], even if the variations of mechanical
properties along stem parts have not yet quantified. In this paper, we present a preliminary
study on the mechanical properties of G. aparine stems at the level of basal, middle and
apical regions, based on tensile tests.
2 Materials and Methods
2.1 Biological Materials
G. aparine plants were purchased from a nursery and moved into a growth chamber
(temperature 25 °C, 60% humidity, 16 h–8 h light-dark cycle). Pictures of the natural
stem sections were obtained with a Hirox KH-7700 digital microscope.
2.2 Tensile Tests
We carried out tensile tests on the different samples (4 cm) collected from the basal,
middle and apical parts of the G. aparine stems. The basal part was defined as the region
closed 0–2 cm to the base, while the middle and the apical parts were defined as the
regions closed 4–6 cm and 0–3 cm to the apex, respectively (Fig. 2a). Tensile tests were
performed using a UTM (Universal Testing Machine, Zwick/Roell Z005). Both ends
of the stem parts were clamped in the jaws of the UTM and stretched at 10 mm/min
until the breakage of the stem occurred. We analyzed only samples where the breakage
occurred far from the gripping parts. The distance between the clamps was 2 cm. To
prevent surface slipping, a sandwich of neoprene rubber and a piece of sandpaper were
used, similarly to [7]. During tests, the force (F [N]) vs. displacement (D [mm]) curve
was recorded using TestXpert II software. To extract the mechanical properties, stress
(σ [N/mm 2 ]) and strain (ε) where computed by:
ε = D/L
[9, 10]
σ = F/A
[9, 10]
where L [mm] is the original length and A [mm 2 ] is the cross-sectional area.
The cross-sectional areas were calculated by image analysis subdividing them into
polygons. The main mechanical parameters were extracted from the resulting stressstrain curve (see Fig. 2a). Particularly, Young’s modulus (E), or stiffness of the stem
material, was calculated as the fitted-slope of the initial linear elastic part of the curve
[9, 10]. Furthermore, the breaking strain (ε max ) and the tensile strength (σ max ) were also
calculated from the curve as the maximum strain and the maximum stress, respectively
[9, 10].
I. Fiorello et al.
in the basal region [7]. This value is common for aquatic plants [8], but it has never
been reported for terrestrial species, and the mechanism behind such high breaking
strain is still unclear (i.e., the reorientation of microfibrils seems to be not responsible)
[7]. Furthermore, observations of G. aparine in the field suggested marked differences
between the lower and upper parts of the stem [7], even if the variations of mechanical
properties along stem parts have not yet quantified. In this paper, we present a preliminary
study on the mechanical properties of G. aparine stems at the level of basal, middle and
apical regions, based on tensile tests.
2 Materials and Methods
2.1 Biological Materials
G. aparine plants were purchased from a nursery and moved into a growth chamber
(temperature 25 °C, 60% humidity, 16 h–8 h light-dark cycle). Pictures of the natural
stem sections were obtained with a Hirox KH-7700 digital microscope.
2.2 Tensile Tests
We carried out tensile tests on the different samples (4 cm) collected from the basal,
middle and apical parts of the G. aparine stems. The basal part was defined as the region
closed 0–2 cm to the base, while the middle and the apical parts were defined as the
regions closed 4–6 cm and 0–3 cm to the apex, respectively (Fig. 2a). Tensile tests were
performed using a UTM (Universal Testing Machine, Zwick/Roell Z005). Both ends
of the stem parts were clamped in the jaws of the UTM and stretched at 10 mm/min
until the breakage of the stem occurred. We analyzed only samples where the breakage
occurred far from the gripping parts. The distance between the clamps was 2 cm. To
prevent surface slipping, a sandwich of neoprene rubber and a piece of sandpaper were
used, similarly to [7]. During tests, the force (F [N]) vs. displacement (D [mm]) curve
was recorded using TestXpert II software. To extract the mechanical properties, stress
(σ [N/mm 2 ]) and strain (ε) where computed by:
ε = D/L
[9, 10]
σ = F/A
[9, 10]
where L [mm] is the original length and A [mm 2 ] is the cross-sectional area.
The cross-sectional areas were calculated by image analysis subdividing them into
polygons. The main mechanical parameters were extracted from the resulting stressstrain curve (see Fig. 2a). Particularly, Young’s modulus (E), or stiffness of the stem
material, was calculated as the fitted-slope of the initial linear elastic part of the curve
[9, 10]. Furthermore, the breaking strain (ε max ) and the tensile strength (σ max ) were also
calculated from the curve as the maximum strain and the maximum stress, respectively
[9, 10].