Multilayer canopy systems: plant energy dissipation and photoprotection mechanisms along the vertical profile - Fagus sylvatica L. as a model

Acclimation of leaf photosynthesis to different light environments is a fundamental factor that influences photosynthetic capacity and, consequently, productivity. Light interception and nitrogen content (N) are main factors driving carbon (C) assimilation at the leaf scale, and many hypotheses and models on the acclimation of leaf photosynthetic capacity have been formulated. Most of these studies focus on shade and sun leaves, but do not take into account the vertical irradiance gradient as a continuum under which plasticity of leaf structure and photosynthetic capacity can be expressed. On the other hand, the metabolic functionality of leaves along the vertical profile of treelike plants may reflect the different environmental conditions under which other plant species grow and develop in the multilayer canopy of agroforestry systems. Indeed, under optimal nutrient and water supply, in mixed-cropping systems or in Multiple Cropping Systems (MCS) the main competition factor becomes the light exposure and interception by leaves. In this perspective, the study of leaf response to the variable light conditions of the different layers along the vertical profile of a deep canopy, such as that of a beech forest, can represent a valuable model. In Southern Europe the beech forest canopy shows high plasticity to cope with heat and drought, due to a high adaptability of the leaf photosynthetic capacity to micro-climate changes over days or weeks and to climate fluctuations during the season. It is conceivable that the beech leaves have developed highly specialized mechanisms to regulate the photosynthetic capacity and the light use efficiency at the canopy level under different environmental conditions. We studied the different structure and metabolic functionality of leaves along the vertical profile of a beech canopy, in order to clarify the morpho-physiological and molecular mechanisms underlying the adaptability of the different canopy layers to variable light regimes. With this aim, determinations of leaf structure (leaf mass per unit area - LMA, thickness and density), N distribution, gas exchange and chlorophyll fluorescence, photosynthetic pigments and antioxidant status were performed along three canopy layers of a beech (Fagus sylvatica L.) forest near Collelongo (Abruzzo region, Central Italy), where a permanent experimental facility (Selva Piana) was installed since 1991. Results revealed a progressive increase of the maximal carboxylation rate (Vcmax), light driven electron transport rate (Jmax) and CO2 assimilation rate from the bottom to the upper canopy layers, associated to a gradient in LMA and N content per unit of leaf area. The light response curves of the actual efficiency of PSII photochemistry (?PSII) and non-photochemical quenching (NPQ), suggested a different strategy in the radiative and non-radiative energy dissipation capacity depending on the canopy layer position along the vertical profile. This leaf differential response corresponded to qualiquantitative variations in photosynthetic pigment composition. In particular, the highest light exposure determined an enhancement of the Chl a/Chl b ratio and ?-carotene content. The xanthophyll (V+A+Z) pool, closely related to the leaf energy dissipation and photo-protective capacity, progressively increased with the increasing height of the canopy layer (bottom, intermediate, upper). Modifications of the leaf antioxidative system gave insights on the nature and action of defence/adaptation plant mechanisms to the multilayer canopy environments. Applications for managing of MCS are discussed.