Shoot water relations of mature black spruce families displaying a genotype x environment interaction in growth rate. II. Temporal trends and response to varying soil water conditions
Pressure-volume curves and shoot water potentials were determined for black spruce (Picea mariana (Mill.) BSP) trees from four full-sib families at the Petawawa Research Forest, Ontario, Canada. Trees were sampled from a dry site in 1992 and from the dry site and a wet site in 1993. Modulus of elasticity (e ), osmotic potential at turgor loss point (Y tlp) and relative water at turgor loss point (RWC tlp) all decreased during the growing season. Osmotic potential at saturation (Y sat) and turgor displayed no general temporal trend. Across a range of environmental conditions, Female 59 progeny had equal or lower Y sat, and higher or similar e , mean turgor pressure (P x) and predawn turgor pressure (P pd) compared with Female 63 progeny. Osmotic potential at saturation decreased as water stress increased from mild to moderate and increased as water stress increased from moderate to severe. Stable genetic differences in Y sat were maintained by the same rate of osmotic adjustment from low to moderate water stress. Modulus of elasticity and RWC tlp decreased with decreasing water availability, whereas Y tlp showed no response. The combined effects of Y sat and e resulted in no change in P pd as water stress increased from low to moderate values, but turgor declined sharply as water stress increased from moderate to high values. The authors conclude that drought tolerance traits strongly influence the growth of these black spruce families across sites of varying water availability.