Chlorophyll fluorescence of leaves of a willow (Salix sp.) stand grown in the field in northern Sweden was measured on several occasions during the growing season of 1987. For leaves that received mostly full daylight, the Fv/Fe ratio declined roughly 15% in the afternoon on cloudless days in July (Fp is the fluorescence at the peak of the induction curve obtained at the prevailing air temperature after 45 min of dark adaptation, and Fv is variable fluorescence,/~v = Fp-Fo, where Fo is minimal fluorescence). There was no decrease in the Fv/Fp ratio on cloudy days, while the effect was intermediate on changeable days. In view of this light dependence, together with the fact that the decline in the Fv/F~ ratio was paralleled with an equal decline in the corresponding fluorescence ratio Fv/FM at 77K, and a similar decline in the maximum quantum yield of 02 evolution, it is suggested that the decline in the Fv/Fp ratio represents a damage in photosystem II attributable to photoinhibition. Recovery of the Fv/Fp ratio in dim light following a decline on a cloudless day took 7-16 h to go to completion; the Fv/Fp ratio was fully restored the following morning. When all active leaves of a peripheral shoot were compared, the Fv/Fp ratio in the afternoon of a day of bright light varied greatly from leaf to leaf, though the majority of leaves showed a decline. This variation was matched by a pronounced variation in intercepted photon flux density. When leaves developed in the shade were exposed to full sunlight by trimming of the stand an increased sensitivity to photoinhibition was observed as compared to peripheral leaves. The present study indicates that periph-Abbreviations and symbols: Fo=minimum fluorescence; Fp= fluorescence at the peak of the induction curve obtained at normal ambient temperatures; Fv ~ variable fluorescence; FM = maximum fluorescence obtained at 77K; PPFD=photosynthetic photon flux density
eral willow shoots experienced in the order of 10-20% photoinhibition during an appreciable part of their life. This occurred even though the environmental conditions were within the optimal range of photosynthesis and growth.