Original article A comparison of photosynthetic responses to water stress in seedlings from 3 oak species:

Summary — Photosynthetic responses of oak seedlings (Quercus petraea, Q rubra and Q cerris) to drought were investigated using gas-exchange and chlorophyll fluorescence. Decreases in predawn leaf water potential (ψw) led to pronounced reductions in both stomatal conductance (gw) and net CO2 assimilation rate (A). In contrast, the maximal photochemical efficiency of photosystem II (PS II) measured predawn (Fv/m) remained unaffected until complete cessation of CO2 assimilation. Re-sponses of PS II photochemical efficiency (ΔF/Fm’) to increasing photon flux density (PFD) were de-termined for leaves of both control and water-stressed seedlings. Drought resulted in a stronger re-duction of ΔF/Fm. at a given PFD in Q rubra and Q petraea, but not in Q cerris, and led to an overreduction of the primary electron acceptor pool (decrease in photochemical quenching, qp). Such behavior could explain the observed increase in sensitivity to photoinhibition when these 2 species were water-stressed. In contrast, drought did not promote such an increase in the suscepti-bility of Q cerris leaves to photoinhibition