Drought frequency and intensity are expected to increase in the Mediterranean as a consequence of global
climate change. To understand how photosynthetic capacity responds to long-term water stress, we measured seasonal
patterns of stomatal (SL), mesophyll (MCL) and biochemical limitations (BL) to net photosynthesis (Amax) in three Quercus
ilex (L.) populations from sites differing in annual rainfall. In the absence of water stress, stomatal conductance (gs),
maximum carboxylation capacity (Vcmax), photosynthetic electron transport rate (Jmax) and Amax were similar among
populations. However, as leaf predawn water potential (Yl,pd) declined, the population from the wettest site showed steeper
declines in gs, Vcmax, Jmax and Amax than those from the drier sites. Consequently, SL, MCL and BL increased most steeply in
response to decreasingYl,pd in the population from the wettest site. The higher sensitivity ofAmax to drought was primarily the
result of stronger stomatal regulation of water loss. Among-population differences were not observed when gs was used
instead ofYl,pd as a drought stress indicator. Given that higher growth rates, stature and leaf area index were observed at the
wettest site, we speculate that hydraulic architecture may explain the greater drought sensitivity of this population.
Collectively, these results highlight the importance of considering among-population differences in photosynthetic
responses to seasonal drought in large scale process-based models of forest ecosystem function.