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.
Highlights
Shape and Size arrangements of petiole and lamina in terms of peltate leaves (3D-configuration) or leaves for which the petiole is marginally attached to the lamina (2D-configuration)
Lamina transition zone have assigned a variety of terms to this area, depending on their scientific discipline and the underlying scientific question
The transition zone is interesting because, by superimposing various gradients, it creates a damage-resistant transition between the petiole and the lamina, which often differs considerably in geometry, shape, and size
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The planar leaf blade (=lamina), which is often connected to the stem by a rod-shaped leaf stalk (=petiole), captures the light needed to produce energy-rich organic molecules. On this basis, the loss of leaf blades, e.g., as caused by drought stress, frost, diseases, or mechanical damage, poses an existential threat to the plant. One can assume that a high selective pressure exists on the development of a damage-resistant transition between the lamina and petiole during evolution
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