Three Key Sub-leaf Modules and the Diversity of Leaf Designs

Abstract

Earth harbors a highly diverse array of plant leaf forms. A well-known pattern linking diverse leaf forms with their photosynthetic function across species is the global leaf economics spectrum (LES). However, within homogeneous plant functional groups such as tropical woody angiosperms or temperate deciduous woody angiosperms, many species can share a similar position in the LES but differ in other vital leaf traits, and thus function differently under the given suite of environmental drivers. How diverse leaves differentiate from each other has yet to be fully explained. Here, we propose a new perspective for linking leaf structure and function by arguing that a leaf may be divided into three key sub-modules, the light capture module, the water-nutrient flow module and the gas exchange module. Each module consists of a set of leaf tissues corresponding to a certain resource acquisition function, and the combination and configuration of different modules may differ depending on overall leaf functioning in a given environment. This modularized-leaf perspective differs from the whole-leaf perspective used in leaf economics theory and may serve as a valuable tool for tracing the evolution of leaf form and function. This perspective also implies that the evolutionary direction of various leaf designs is not to optimize a single critical trait, but to optimize the combination of different traits to better adapt to the historical and current environments. Future studies examining how different modules are synchronized for overall leaf functioning should offer critical insights into the diversity of leaf designs worldwide.

Keywords: leaf anatomical structure; leaf diversity; leaf form; leaf function; stomata; venation.

FIGURE 1

Illustration of the concept of modularity of leaf design. Different species can have different internal leaf structures such that the key leaf trait, leaf dry mass per unit area (LMA) alone may not be informative enough for describing the diversity of leaf designs as exemplified by a comparison of two species with similar LMA (77.3 g m^-2 for Archidendron clypearia and 68.9 g m^-2 for Litsea baviensis) (A). A frequently observed mode for the combination of three key sub-leaf modules: from top to bottom, a leaf can be divided into a light capture module, a water-nutrient flow module and a gas exchange module (B). Demonstration of the vast diversity of leaf internal structures. Such diverse leaf designs can be seen as the result of different adjustments to the frequently-observed mode (C).

FIGURE 2

A trait framework based on the three key sub-leaf modules. In the center, the leaf economics spectrum (LES) concept of integrating investment and payback (LMA and leaf longevity, LL) is a fundamental principle, defining the trade-offs of the three modules. By unpacking the trade-off between LMA-LL into four other relations (i.e., interactions between leaves and light-water-nutrients-gas composition), we show the pathway moving beyond traditional leaf economics. Taking this framework as a starting point, we can have a more holistic understanding of global-scale variation in leaf structure and function. Certain trait syndromes are suggested for each module, ranging from cell size, tissue-level density or thickness to the rates of physiological processes.