Similar susceptibility to excess irradiance in sun and shade acclimated saplings of Norway spruce [Picea abies (L.) Karst.] and stone pine (Pinus cembra L.)

We compared the responses of sun and shade acclimated saplings of Picea abies and Pinus cembra to excess photosynthetic photon flux density (PPFD) equivalently exceeding the level for saturating net photosynthetic rate (PN) Exposure for 2 h up to 2000 mu mol(photon) m(-2) s(-1) did not affect radiant energy saturated PN Photoinhibition of photosynthesis was indicated by a small (10 %) reduction of the potential efficiency of photosystem 2 as derived from measurements of chlorophyll fluorescence (F-V/F-M). However, the extent of F-V/F-M reduction and half-time for recovery were similar in sun and shade acclimated saplings of both species. Furthermore, the effect on F-V/F-M was not stronger when the plants were exposed to excess PPFD at 5 degreesC instead of 15 degreesC. Frost-hardening of plants increased slightly their resistance to excess PPFD. Establishment of these conifer saplings usually acclimated to shade in their natural habitat may hardly be endangered by a sudden increase of PPFD, e.g., by gap formation.