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 carbon isotopic composition (delta(13)C) in white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings representing 10 controlled crosses were planted in the field in individual buried sand-filled cylinders. The soil water content in the cylinders was measured using time domain reflectometry over two growing seasons and seedling water use determined by water balance. Two watering treatments were imposed: irrigation and dry land. There was significant (1.6-2.0 parts per thousand) genetic variation in needle delta(13)C. Ranking of crosses in terms of delta(13)C was generally maintained over watering treatments and there was not a significant genetic versus environmental interaction. There was a positive correlation between delta(13)C and both intrinsic and long-term WUE (more positive delta(13)C with increased WUE) and between delta(13)C and productivity, suggesting a correlation due to variation in photosynthetic capacity. Root to shoot ratios did not increase in water-stressed plants, indicating that responses to drought were primarily at the level of gas exchange, rather than through morphological changes. Our results indicate that it should be possible to use delta(13)C as a surrogate for WUE and to select white spruce genotypes for high WUE without compromising yield.