First-year seedlings respond to a 21-day summer water deficit: morphological and physiological insights of two populations of five Mediterranean Quercus species

In the Mediterranean basin, extreme drought events are expected to become more frequent due to climate change. Consequently, selecting and registering forest reproductive material (FRM) adapted to such conditions is crucial to support effective forest restoration. This study aims to identify whether certain seed sources of Quercus exhibit traits potentially advantageous under ongoing climate change. An initial screening of seedlings from five ecologically diverse species from Puglia (southern Italy) was performed to evaluate early-stage responses to drought conditions simulating extreme Mediterranean summer events. Under semi-controlled conditions in a nursery, we evaluated the effect of water removal for 21 days on the growth, physiological, and metabolomic performance of FRM of two seed sources per five Mediterranean Quercus species. Morphological, physiological (SPAD, chlorophyll fluorescence), and metabolic traits (pigments and malondialdehyde) were measured throughout the experimental period and at its end. All Quercus species and their seed sources survived the water deficit and exhibited positive morphological and physiological responses. The effects of seed source were more pronounced in the “Cerris” section than in the evergreen “Ilex” section, making the former species good candidates for afforestation or restoration in Mediterranean areas and valuable material for further research. The high performance of Q. cerris, a species generally considered less drought-tolerant, highlights the Puglia region as a promising source of southern Mediterranean provenances with enhanced resistance to arid conditions.