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

Abstract

Fluorescence transient (FT) 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: (1) morphology in sun leaves as long term acclimation to high light; (2) short term responses to high light, i.e. during the course of the day; (3) evolution of responses during the growing season and the ageing process. Sun leaves differ from shade leaves by their lower trapping capacity (expressed by the ratio F_V/F_M) and higher capacity to reduce end acceptors beyond PSI (expressed by phase I–P of the FT). These features were assessed at pre-dawn, when the entire plant is fully dark-adapted. Short term responses (differences between midday and pre-dawn assessments) occur especially in sun leaves, indicating that the photosynthetic machinery reacts to the high excitation pressure by increasing photochemical and non-photochemical de-excitation processes. Other responses concern the inactivation of the oxygen evolving system and a stability loss of the tripartite system reaction centre (RC) – harvesting light complex – core antenna, expressed by the onset of K and L-bands at midday. Sun leaves are well acclimated to high light from a structural and functional point of view, however, the irradiance at midday provokes conditions of instability in the photosynthetic machinery, possibly through reactive oxygen species (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 which present a permanent (chronic) condition of photoinhibition. Differences between sun and shade leaves decrease over time.

Keywords:

• Electron transport
• Fagus sylvatica L.
• net photosynthesis
• O-J-I-P fluorescence
• oxygen evolving system
• photoinhibition
• photoprotection
• sun and shade leaves
• transient analysis

Acknowledgements

The experimental facilities at Curno, where this work was carried out, were established and funded by the General Directorate for Environmental Quality of the Lombardy Region in the framework of the programme “Assessment of ozone air pollution on forest vegetation in the transalpine region of Lombardy and Canton Ticino”, in partnership with the Regional Agency for Services to Agriculture and Forests – E.R.S.A.F. (project co-ordinator), the Lombardy Foundation for the Environment (F.L.A.) and the Regional Agency for Environment Protection (A.R.P.A.). The authors are grateful to the E.R.S.A.F. personnel for their valuable assistance at the Curno forest nursery.