Petiole-Lamina Transition Zone: A Functionally Crucial but Often Overlooked Leaf Trait

Abstract

Although both the petiole and lamina of foliage leaves have been thoroughly studied, the transition zone between them has often been overlooked. We aimed to identify objectively measurable morphological and anatomical criteria for a generally valid definition of the petiole–lamina transition zone by comparing foliage leaves with various body plans (monocotyledons vs. dicotyledons) and spatial arrangements of petiole and lamina (two-dimensional vs. three-dimensional configurations). Cross-sectional geometry and tissue arrangement of petioles and transition zones were investigated via serial thin-sections and µCT. The changes in the cross-sectional geometries from the petiole to the transition zone and the course of the vascular bundles in the transition zone apparently depend on the spatial arrangement, while the arrangement of the vascular bundles in the petioles depends on the body plan. We found an exponential acropetal increase in the cross-sectional area and axial and polar second moments of area to be the defining characteristic of all transition zones studied, regardless of body plan or spatial arrangement. In conclusion, a variety of terms is used in the literature for describing the region between petiole and lamina. We prefer the term “petiole–lamina transition zone” to underline its three-dimensional nature and the integration of multiple gradients of geometry, shape, and size.

Twist-to-Bend Ratios and Safety Factors of Petioles Having Various Geometries, Sizes and Shapes

“…(hereafter C. bicolor ), Hemigraphis alternata (L.) Hallier f. (hereafter H. alternata ), and Pilea peperomioides Diels (hereafter P. peperomioides ) were cultivated in the greenhouse of the Botanic Garden (University of Freiburg, Germany). These four species were selected based on the same criteria as those described by Langer et al (2021) : (1) two species of each body plan (monocotyledonous and dicotyledonous) having a foliage leaf with either a 2D or 3D spatial configuration of petiole and lamina, (3) herbaceous, (4) perennial, and (5) easy to cultivate to provide sufficient material for experimentation. One random leaf of each of the 25 plants studied per species was investigated ( Figure 1 ).…”

“…This is crucial as leaves and their petioles show a wide variety of morphological and anatomical characteristics. One aspect is the spatial configuration of the lamina and petiole, described by Langer et al (2021) as three-dimensional (3D) for peltate leaves and two-dimensional (2D) for leaves in which the petiole is attached to the lamina base. Moreover, marked differences are found between the cross-sectional geometries of petioles, which can be circular, elliptical or U-profiled ( Vogel, 1992 ; Ennos et al, 2000 ; Pasini and Mirjalili, 2006 ).…”

Acclimation to wind loads and/or contact stimuli? A biomechanical study of peltate leaves of Pilea peperomioides

“…In literature, the biomechanical role of the petiole is well described, especially in terms of its support for the lamina and yielding under wind loads. However, little is known about the biomechanical behaviour of the short transition zone between the rod-shaped or U-profiled petiole and the (nearly) planar lamina ( Langer et al , 2021b ). As described above, petiole, transition zone, and lamina of foliage leaves respond to wind or to touch by passing animals by simultaneously bending downwind and twisting away.…”

“…As far as the general geometry of petioles is concerned, we mainly find circular, elliptical, and adaxially grooved cross-sections ( Vogel, 1992 ; Ennos et al , 2000 ; Pasini and Mirjalili, 2006 ; Langer et al , 2021b ). With the exception of a perfectly circular cross-section, the torsion constant K is usually smaller than J .…”

Data_Sheet_1.PDF

“…(hereafter H. alternata), and Pilea peperomioides Diels (hereafter P. peperomioides) were cultivated in the greenhouse of the Botanic Garden (University of Freiburg, Germany). These four species were selected based on the same criteria as those described by Langer et al (2021): (1) two species of each body plan (monocotyledonous and dicotyledonous) having a foliage leaf with either a 2D or 3D spatial configuration of petiole and lamina, (3) herbaceous, (4) perennial, and (5) easy to cultivate to provide sufficient material for experimentation. One random leaf of each of the 25 plants studied per species was investigated (Figure 1).…”