The Optical Properties of Leaf Structural Elements and Their Contribution to Photosynthetic Performance and Photoprotection

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.

The Response of Tomato Fruit Cuticle Membranes Against Heat and Light

“…Figure 7A shows the changes in the total transmittance UV-Vis spectra of tomato CMs during growth and ripening. The shape of the curves resembles the pattern of a selective optical filter with a sharp transition from a low transmission region in the UV to a highly transparent one in the visible range (Krauss et al, 1997;Karabourniotis et al, 2021). The cut-off wavelength progressively displaces during fruit development from about 350 nm for the immature and mature green CMs to 525 m for red ripe ones.…”

“…Most of the research on the interaction of UV-Vis radiation with plant CMs has been concentrated on leaves (see reviews by Kolb and Pfündel, 2005;Pfündel et al, 2006;Karabourniotis et al, 2021). However, much less work has been focused on fruits (Ward and Nussinovitch, 1996;Krauss et al, 1997;Kolb et al, 2003;Solovchenko and Merzlyak, 2003).…”

“…However, much less work has been focused on fruits (Ward and Nussinovitch, 1996;Krauss et al, 1997;Kolb et al, 2003;Solovchenko and Merzlyak, 2003). Excessive exposure, particularly to the higher energy UV component, induces photochemical reactions that may cause damage to proteins and the modification of the enzymatic activity that could have a negative effect on quality parameters such as texture, color and organoleptic and nutritional values of produce (Hollósy, 2002;Karabourniotis et al, 2021). However, under moderate UV exposure, the biosynthesis of flavonoids is stimulated (Awad et al, 2000;Luthria et al, 2006) to the point that UV-A irradiation can be proposed as farming and postharvest treatments to improve the antioxidant level of tomato fruits (Luthria et al, 2006;Dyshlyuk et al, 2020).…”

Radiationless mechanism of UV deactivation by cuticle phenolics in plants

“…Around 300–400 nm reflectance increased to its maximum, 10–35% depending on the species, within visible light. Epicuticular wax disposition and crystallisation, as well as the presence and density of indument, are chiefly responsible for light reflection and could be behind the observed differences among species 13 , 26 . It should be noted the low reflectance observed in grapefruit cuticles despite the massive epicuticular wax accumulation present 27 .…”

Links between leaf anatomy and leaf mass per area of herbaceous species across slope aspects in an eastern Tibetan subalpine meadow

“…Leaf anatomical properties vary in response to environmental variables, such as solar irradiation (Chabot et al, 1979 ; Clements, 1905 ; Givnish, 1988 ; Hanson, 1917 ; Poorter et al, 2019 ), air temperature (Chabot & Chabot, 1977 ; Gratani et al, 2018 ; Zhou et al, 2019 ), precipitation (Binks et al, 2016 ; Cunningham et al, 1999 ; Fletcher et al, 2018 ; Turner, 1994 ), elevation (He et al, 2018 ; Körner et al, 1986 ; Sun et al, 2016 ), and soil fertility (Beadle, 1966 ; Cunningham et al, 1999 ; Tsujii et al, 2017 ). For example, a thick epidermis under arid environments (Körner & Kèorner, 1999 ; Liu et al, 2020 ; Sun et al, 2016 ) is considered an adaptive trait to protect leaf tissues from strong ultraviolet irradiation (Karabourniotis et al, 2021 ; Ma et al, 2012 ) and/or to reduce evapotranspiration from leaf surfaces (Kröber et al, 2015 ). A thick epidermis is also related to high mechanical strength (Onoda et al, 2015 ), and can result in a longer leaf lifespan (Onoda et al, 2011 ).…”