Genetic Variation in Response to Drought of Interior Spruce (Picea glauca (Moench) Voss X P. engelmannii Parry ex Engelm.)
Interior spruce (Picea glauca (Moench) Voss × Picea engelmannii Parry) somatic seedlings from six families, with two clones per family, were measured for gas-exchange processes in relation to repeated drought cycles. Within-and between-family variation in gas-exchange response to drought was determined. Somatic seedlings, from all genetic sources, had a decrease in needle conductance (gwv) and net assimilation rate (A) as maximum shoot water potential (Ψmax) declined. Variation was not consistent between clones, from all families or within each family, for wv and A response to drought, although when it occurred differences were primarily detected when Ψmax was between −0.5 and −1.0 MPa. As drought severity increased (Ψmax <& minus;1.0 MPa), there was very little variation in the gas-exchange response. Recovery of gas-exchange parameters from drought stress was dependent on the degree of water stress to which seedlings were exposed just before watering; recovery gwv and A decreased as Ψmax before rewatering declined from −0.5 to −2.5 MPa. Rapid recovery of gas-exchange parameters to drought was found in low to moderate drought treatments (Ψmax−1.5 MPa before rewatering). When more severe drought stress conditions were applied (Ψmax < −2.0 MPa), a 14 day period was required to detect any genetic variation, between clones from all families or within each family, in the recovery of gas-exchange parameters.