Photosynthetic response of white spruce families to drought stress

In the context of climate change, an increased frequency of drought stresses might occur at a regional scale in boreal forests. To assess photosynthetic responses to drought treatment, seedlings of 12 open-pollinated families of white spruce (Picea glauca (Moench) Voss) differing in their growth performance were grown in a controlled environment. Gas exchange and chlorophyll fluorescence parameters as well as shoot xylem water potential (WP) were measured for 21 successive days after watering was stopped. Net photosynthesis decreased as stomatal conductance decreased, Net photosynthesis was not affected by drought until WP reached -2.0 MPa when stomata were closed. Initial fluorescence (F-0) and basic fluorescence after induction (F-0') were not affected by drought. A progressive decrease in maximal (F-m) and variable fluorescences (F-v), maximum photosystem II (PS II) efficiency (F-v/F-m), effective quantum yield of PS II (Phi(II)), photochemical efficiency of open PS II (Phi(p)), and photochemical quenching (qP) was observed at WP < -1.0 MPa, whereas non-photochemical quenching (qN) remained high throughout the drought treatment. White spruce families with inferior growth performance showed higher values of F-m, F-v, F-v/F-m, Phi(p), and qN at WP < -2.0 MPa. The results indicated that chlorophyll fluorescence variables can be used as drought markers in relation to present or predicted climate conditions. These could be used for selecting planting stock adapted to drought periods or dry environments. These markers showed that slow-growing genotypes are better adapted to drought conditions than intermediate or fast-growing genotypes in present and predicted drought conditions.