Evolution and Function of Leaf Venation Architecture: A Review

Abstract

The leaves of extant terrestrial plants show highly diverse and elaborate patterns of leaf venation. One fundamental feature of many leaf venation patterns, especially in the case of angiosperm leaves, is the presence of anastomoses. Anastomosing veins distinguish a network topologically from a simple dendritic (tree-like) pattern which represents the primitive venation architecture. The high degree of interspeci®c variation of entire venation patterns as well as phenotypic plasticity of some venation properties, such as venation density, indicate the high selective pressure acting on this branching system. Few investigations deal with functional properties of the leaf venation system. The interrelationships between topological or geometric properties of the various leaf venation patterns and functional aspects are far from being well understood. In this review we summarize current knowledge of interrelationships between the form and function of leaf venation and the evolution of leaf venation patterns. Since the functional aspects of architectural features of dierent leaf venation patterns are considered, the review also refers to the topic of individual and intraspeci®c variation. One basic function of leaf venation is represented by its contribution to the mechanical behaviour of a leaf. Venation geometry and density in¯uences mechanical stability and may aect, for example, susceptibility to herbivory. Transport of water and carbohydrates is the other basic function of this system and the transport properties are also in¯uenced by the venation architecture. These various functional aspects can be interpreted in an ecophysiological context.

Hydraulic Analysis of Water Flow through Leaves of Sugar Maple and Red Oak

“…The importance of the vein system in determining R leaf , as found in this study, suggests potentially important functional consequences at the whole-leaf and plant level for the diversity of leaf venation architecture in angiosperms (Roth-Nebelsick et al, 2001), i.e. in the density and topology of major and minor veins, in the geometry, number and size of vascular bundles in the veins, as well as in the numbers and sizes of the xylem conduits inside.…”

Leaf plasticity in successive selection cycles of 'Saracura' maize in response to periodic soil flooding

“…It was observed that from C5 to C18 there was a smaller distance between the vascular bundles and, as a consequence, smaller internerval space and higher venal density (Roth-Nebelsick et al, 2001). The distance between vascular bundles can be related to the more efficient transportation and distribution of carbohydrates and water to the mesophyll cells.…”

“…Such characteristic, present in C4-type plants, conveys higher ability in photosynthate translocation and higher water distribution in environments with water restriction and high temperatures (Sage, 2004). Just as a greater phloem thickness could increase the photosynthetic flow, a reduced space between the vascular bundles in the selection cycles of 'Saracura' maize could also increase this flow, because it would promote higher photosynthate distribution (Roth-Nebelsick et al, 2001) in addition to a higher water distribution.…”

Optimal vein density in artificial and real leaves

“…Indeed, maximum rates of photosynthesis scale linearly with the transport capacity of the xylem (2)(3)(4). A key attribute of vascular design within leaves is to ensure that no portion of the leaf is under-supplied and thus unable to support the water losses associated with CO 2 uptake (5)(6)(7)(8). However, studies of leaf venation have largely treated leaves as two-dimensional structures (8).…”

“…However, the internal constraints imposed on evaporative transport by the architecture of the leaf hydraulic system have received relatively little attention (5,6). Water utilizes two successive pathways as it flows from the veins of a leaf to the outside air, passing from the liquid to the vapor state.…”