Low temperature during winter elicits differential responses among populations of the Mediterranean evergreen cork oak (Quercus suber)

Populations of cork oak (Quercus suber L.) were assessed for seasonal and inter-population variability in, and temperature responses of, the ratio between light-induced variable and maximum fluorescence of chlorophyll, F,,IF,, considered a surrogate for the maximum photochemical efficiency of photosystem II (PSII). Seedlings from 10 populations throughout the distribution range of Q. suber in the Mediterranean basin were grown in a common garden in central Spain. The F-v/F-m ratio of dark-adapted leaves was measured at dawn every month for 2 years. Air temperature was recorded at a nearby climatic station. During the summer, when maximum air temperatures reached 40 degrees C, there were no significant differences in F-v/F-m among populations, but significant differences were seen during the winter. In colder months, F-v/F-m ranged in all populations between 0.5-0.6 and 0.2-0.3 in 2001 and 2002, respectively. The variance explained by the population effect was greatest during winter months, especially in 2002, reaching a peak value of 10% when minimum air temperature was below - 10 degrees C. Populations originating from warmer sites showed the largest decline in F-v/F-m between the end of 2001 and the beginning of 2002. Thus, a negative linear relationship was established between mean annual temperature at the population source and population mean F-v/F-m recorded in the coldest month in 2002 and normalized by the F-v/F-m spring measurement.