Photoinhibition in seedlings of Fraxinus and Fagus under natural light conditions: implications for forest regeneration?

Ash (Fraxinus excelsior L.) and beech (Fagus sylvatica L.) seedlings were grown in the field under three levels of natural light: (1) open, (2) gap and (3) shade. Light acclimation of photosynthesis was characterized by means of modulated chlorophyll a fluorescence of intact leaves and growth parameters were measured at the end of the growing season. Measurements of maximum photochemical efficiency (Fv/Fm) of dark-adapted leaves at intervals through the day showed that ash had a higher Fv/Fm than beech in open and gap plots but not in shade plots. This indicated a larger build-up of photoinhibition in beech under gap and open conditions. Steady-state light response curves of the operating efficiency of PSII (F'q/F'm), the electron transport rate (ETR) and the photochemical efficiency factor (F'q/F'v) showed greater variability across light treatments in ash than in beech. Both species exhibited similar responses of non-photochemical quenching (NPQ) to light. When the data were normalized to the mean maximum irradiance in the growth environment, all photochemical parameters showed a reduction in variation across treatments, indicating that light acclimation in the two species occurred primarily through adjustments in rates of photochemistry. Adjustments in thermal heat dissipation were small in both species. This pattern was stronger in ash, suggesting a greater degree of phenotypic plasticity in photosynthetic capacity in this earlier successional species. Contrary to our expectations, the build-up of photoinhibition in beech did not appear to have a negative effect on total biomass accumulation relative to ash.