Photosynthetic sensitivity to drought varies among populations of Quercus ilex along a rainfall gradient.

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 (S_L), mesophyll (MC_L) and biochemical limitations (B_L) to net photosynthesis (A_max) in three Quercus ilex (L.) populations from sites differing in annual rainfall. In the absence of water stress, stomatal conductance (g_s), maximum carboxylation capacity (V_cmax), photosynthetic electron transport rate (J_max) and Amax were similar among populations. However, as leaf predawn water potential (Ψ_l,pd) declined, the population from the wettest site showed steeper declines in g_s, V_cmax, J_max and A_max than those from the drier sites. Consequently, S_L, MC_L and B_L increased most steeply in response to decreasing Ψ_l,pd in the population from the wettest site. The higher sensitivity of A_max to drought was primarily the result of stronger stomatal regulation of water loss. Among-population differences were not observed when g_s was used instead of Ψ_l,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. [ABSTRACT FROM AUTHOR]