STABLE CARBON ISOTOPES AS INDICATORS OF INCREASED WATER-USE EFFICIENCY AND PRODUCTIVITY IN WHITE SPRUCE (PICEA-GLAUCA (MOENCH) VOSS) SEEDLINGS

The relationship among water use efficiency (WUE), productivity and ca rbon isotopic composition (delta(13)C) in white spruce (Picea glauca ( Moench) Voss) seedlings was investigated. Sixteen hundred seedlings re presenting 10 controlled crosses were planted in the field in individu al buried sand-filled cylinders. The soil water content in the cylinde rs was measured using time domain reflectometry over two growing seaso ns and seedling water use determined by water balance. Two watering tr eatments were imposed: irrigation and dry land. There was significant (1.6-2.0 parts per thousand) genetic variation in needle delta(13)C. R anking of crosses in terms of delta(13)C was generally maintained over watering treatments and there was not a significant genetic versus en vironmental interaction. There was a positive correlation between delt a(13)C and both intrinsic and long-term WUE (more positive delta(13)C with increased WUE) and between delta(13)C and productivity, suggestin g a correlation due to variation in photosynthetic capacity. Root to s hoot ratios did not increase in water-stressed plants, indicating that responses to drought were primarily at the level of gas exchange, rat her than through morphological changes. Our results indicate that it s hould be possible to use delta(13)C as a surrogate for WUE and to sele ct white spruce genotypes for high WUE without compromising yield.