Plasticity in water-use efficiency of Picea sitchensis, P-glauca and theirnatural hybrids

Plasticity in water-use efficiency (WUE) was examined in populations of Pic ea glauca (Moench) Voss and P. sitchensis (Bong.) Carr. and their natural h ybrids from an ecocline along the Skeena Valley, British Columbia, which ru ns from the dry continental interior (P. glauca) to the wet maritime Pacifi c coast (P. sitchensis). Seedlings were grown in a growth chamber and kept well-watered or repeatedly droughted for 3 months. Mean population photosyn thetic WUE and total tissue delta C-13 values were strongly correlated with in and across treatments (r=0.95). There were also strong correlations (r=0 .60-0.80) between individual seedling total tissue delta C-13 and dry mass, and delta C-13 and net photosynthesis (A), indicating, that variation in A was primarily responsible for differences in delta C-13. When kept well-wa tered, P. sitchensis and the hybrids had higher delta C-13 (-27.8 parts per thousand and -27.5 parts per thousand, respectively) and higher dry mass ( 2.17 g and 1.99 a, respectively) than P. glauca (-28.2 parts per thousand a nd 1.68 g). Species ranking was reversed by repeated drought, with P. glauc a and the hybrids having higher delta C-13 (-25.6 parts per thousand and -2 5.5 parts per thousand, respectively) and dry mass (1.10 and 1.08 g, respec tively) than P. sitchensis (-26.4 parts per thousand and 0.98 g). P. glauca had a smaller decrease in dry mass (35%) and a bigger increase in delta C- 13 (by 2.7 parts per thousand) than P. sitchensis (55% and 1.4 parts per th ousand, respectively), with the hybrids in between (45% and 2.0 parts per t housand, respectively). Drought also had a greater effect on A in P. sitche nsis (36% reduction) than in P. glauca (14% reduction) or the hybrids (24% reduction). Thus P. glauca and, to a lesser extent, hybrid populations, per formed better and were more plastic than P. sitchensis in response to water deficit. Under the well-watered treatment, the hybrids behaved more like P . sitchensis in growth and WUE. These patterns are consistent with the seas onal variation in moisture availability that occurs along the introgression zone.