ABSTRACT
The assessment of the effect of tree age on leaves is usually limited by the difficulty of sampling sun leaves from tall ageing trees. In this study, we investigated tree age-related effects on sun leaves in a chronosequence of beech (Fagus sylvatica L.) stands. The effects of stand age on leaf mass to area ratio (LMA), chlorophyll (Chl), epidermal polyphenols (EPhen), nitrogen and carbon contents in sun leaves were investigated in 17 even-aged stands distributed into six age classes (14-175 years old). Chl and EPhen were assessed in vivo with SPAD and Dualex portable leaf-clips respectively. Leaves were sampled by shooting and sun leaves were identified based on criteria obtained from a vertical profile of the ratio abaxial vs adaxial EPhen across the canopy. Sun leaves were characterised by a high and similar adaxial and abaxial EPhen contents, high LMA value and low mass-based Chl content. These sun leaf characteristics, together with leaf nitrogen and carbon contents, were not significantly affected by stand age. Along the chronosequence, beech trees invested a stable fraction of leaf mass into nitrogen, carbon, Chl and EPhen with decreasing leaf size, i.e. dry mass and area.
ABSTRACT
Seasonal patterns of dry mass invested in chlorophyll and epidermal phenolic compounds (EPhen) were investigated in vivo using optical methods, in leaves of 2-year-old oaks (Quercus petraea Matt. (Liebl.)) grown under semi-controlled conditions. The plasticity of the seasonal pattern was investigated by applying stem girdling treatment. In control young expanding leaves, leaf dry mass per area, dry mass investment in chlorophyll and abaxial EPhen content increased. In late May, at leaf maturity, these variables reached a plateau, and adaxial and abaxial EPhen contents became similar. Thereafter, as leaves aged, dry mass investment in chlorophyll gradually decreased, whereas it remained steady for EPhen. Girdling treatment impacted this seasonal pattern differently depending on the phenological stage. Treatment effects and their reversion revealed in vivo EPhen turnover. Finally, optical signatures of immature and mature leaf phenological stages with contrasting nitrogen and carbon economy were proposed, based on the relationship between the chlorophyll to EPhen ratio and the leaf nitrogen to carbon ratio.
ABSTRACT
A quantitative and comprehensive knowledge of leaf fluorescence is required for the interpretation of fluorescence signals at the canopy level and also for the modelling of leaf and canopy fluorescence. In this work we present full range fluorescence excitation and emission spectra of intact leaves, expressed in units of apparent spectral fluorescence yield, from both the adaxial and the abaxial sides of the leaves, and for both front-side and back-side geometries. Emission spectra were measured for incident radiations in the blue and the green spectral range. The red/far-red fluorescence ratio depended on the measurement geometry and on the excitation wavelength. Excitation spectra were measured for emissions at 687 and 760 nm. When the abaxial side was illuminated, the measured spectra always had a larger intensity compared to adaxial side that is explained by the higher scattering of the spongy tissues. At 760 nm, the spectra had the same shape for front-side and back-side geometry, indicating that scattering predominated. At 687 nm, the shape of the spectra was very different for front-side and back-side geometry due to re-absorption of red fluorescence within the leaf. The comparison of excitation spectra measured from the adaxial or the abaxial side revealed differences in carotenoid absorption.
