Differential photosynthetic and survival responses to soil drought in two evergreen Nothofagus species

We asked if differences in distribution between Nothofagus nitida and N. dombeyi were associated with differences in drought tolerance. Survival, gas exchange and chlorophyll fluorescence were measured on seedlings subjected to a gradual drought. At a predawn leaf water potential (Ψm) of -2.7 MPa, survival of N. nitida was 50%, compared to 100% in N. dombeyi. Under well-watered conditions, the two species displayed similar stomatal conductance (gw) and transpiration (E), but net photosynthesis (A) and instantaneous water-use efficiency (WUEi) were slightly higher in N. nitida. A, E and gw declined in N. nitida along the gradual drought but increased in N. dombeyi at a Ψm between -1.5 and -2.5 MPa, and declined then drastically at a Ψm below < - 2.5 MPa. As N. dombeyi was able to maintain A at higher levels despite declining gw, this species displayed significantly increased WUEi at Ψm below -2.5 MPa. Photochemical efficiency of PSII in the light (ΔF/Fm') and photochemical quenching (qP) were always lower in N. nitida and along with the photochemical efficiency in the dark (Fv/Fm) they declined in both species. Non-photochemical quenching (NPQ) increased slowly in N. dombeyi along with the gradual drought, whilst it decreased in N. nitida. These results show that differences in drought tolerance are in agreement with sorting of Nothofagus species along moisture gradients in south-central Chile.6 page(s