Short and long term photosynthetic adjustments in sun and shade leaves of fagus sylvatica L., investigated by fluorescence transient (FT) analysis

The fluorescence transient analysis (fast kinetics) was carried out on sun and shade leaves of beech seedlings in an experimental field at Curno (North Italy), during the 2008 growing season (June – August). The aim of the research was to determine: (i) structural adjustments in sun leaves as long term acclimation to high light; (ii) short term responses to high light, i.e. during the course of the day; (iii) evolution of the responses during the growing season and ageing processes. Sun leaves differ from shade leaves for their lower trapping capacity (expressed by the ratio FV/FM) and higher capacity to reduce the end acceptors beyond the PSI (expressed by the I-P phase of the fluorescence transient). These features were assessed at pre-dawn, when the entire plant is fully adapted to dark. The short term responses (differences between the midday and pre-dawn assessments) are produced especially in sun leaves, and indicate that the photosynthetic machinery reacts to the high excitation pressure by increasing the photochemical and the non-photochemical de-excitation processes. Other responses concern the inactivation of the oxygen evolving system and a loss of stability of the tripartite system RC - HLC – core antenna (expressed by the onset of a K and L-bands at midday). Sun leaves are well acclimated to high light from a structural and functional point of view; the irradiance at midday provokes conditions of instability in the photosynthetic machinery, with ROS production at both PSII and PSI sides. The temporal patterns, across the growing season, indicate the progression of senescence processes, that are more pronounced in sun leaves with a permanent condition of photoinhibition. Differences between sun and shade leaves decrease over time.