Abstract
Leaves have evolved to effectively harvest light, and, in parallel, to balance photosynthetic CO2 assimilation with water losses. At times, leaves must operate under light limiting conditions while at other instances (temporally distant or even within seconds), the same leaves must modulate light capture to avoid photoinhibition and achieve a uniform internal light gradient. The light-harvesting capacity and the photosynthetic performance of a given leaf are both determined by the organization and the properties of its structural elements, with some of these having evolved as adaptations to stressful environments. In this respect, the present review focuses on the optical roles of particular leaf structural elements (the light capture module) while integrating their involvement in other important functional modules. Superficial leaf tissues (epidermis including cuticle) and structures (epidermal appendages such as trichomes) play a crucial role against light interception. The epidermis, together with the cuticle, behaves as a reflector, as a selective UV filter and, in some cases, each epidermal cell acts as a lens focusing light to the interior. Non glandular trichomes reflect a considerable part of the solar radiation and absorb mainly in the UV spectral band. Mesophyll photosynthetic tissues and biominerals are involved in the efficient propagation of light within the mesophyll. Bundle sheath extensions and sclereids transfer light to internal layers of the mesophyll, particularly important in thick and compact leaves or in leaves with a flutter habit. All of the aforementioned structural elements have been typically optimized during evolution for multiple functions, thus offering adaptive advantages in challenging environments. Hence, each particular leaf design incorporates suitable optical traits advantageously and cost-effectively with the other fundamental functions of the leaf.
Highlights
In order for plants to achieve a positive balance of energy and carbon, four key leaf modules, mostly located in plant leaves, i.e., the light capture module, the water– nutrient flow module, the gas exchange module, and the defense– protection module have to collaborate [1]
In this review we examine the following: (1) the optical role of specific leaf structural elements within the frame of the light capture module and (2) the potential involvement of these elements in the function of the other three modules, as well as their probable combination with other leaf structural traits
calcium oxalate (CaOx) crystals contained in vascular bundle sheaths and other tissues such as sclerenchyma, collenchyma, or parenchyma, could scatter light comprising a key component in the homogenization of the light gradient profile along the depth within the mesophyll [103]
Summary
In order for plants to achieve a positive balance of energy and carbon, four key leaf modules, mostly located in plant leaves, i.e., the light capture module, the water– nutrient flow module, the gas exchange module, and the defense (against biotic stresses)– protection (against abiotic stresses) module have to collaborate [1]. The strong variability of the structural elements comprising the light capture module (Figure 1) has led to a large diversity in leaf designs during plant evolution, despite functional elements, such as photosynthetic metabolism, having remained remarkably conserved throughout phylogeny [5,6] This tremendous diversity of leaf anatomical and physical properties manifests both the necessity for adaptability to different environments and the strong influence of these properties on photosynthesis itself. Regarding the light capture module, the properties and the organization of the structural elements of the lamina determine the light-harvesting capacity and the photosynthetic potential of the leaf [6] This organization creates a leaf design adapted to a particular growth environment that combines the suitable optical traits with the functions of the other three modules. In this review we examine the following: (1) the optical role of specific leaf structural elements within the frame of the light capture module and (2) the potential involvement of these elements in the function of the other three modules, as well as their probable combination with other leaf structural traits
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