The impact of light quality and quantity on root-to-shoot ratio and root carbon reserves in aspen seedling stock

The available evidence suggests that for trembling aspen (Populus tremuloides) seedlings, a high root-to-shoot ratio (R:S) and high root total non-structural carbohydrate (NSC) concentration are the best predictors of future growth and outplanting success in harsh reclamation environments. To facilitate more consistent production of aspen seedlings with these characteristics requires the identification of the environmental conditions most responsible for directing resources to growth or to reserve storage. During nursery production, aspen seedlings are often sheltered from environmental stressors in greenhouses and exposed to variation in light quality and quantity, compared to outside conditions; the common result is that greenhouse grown seedlings exhibit greater allocation to shoot growth rather than to root structures and reserves. It is currently unclear whether this imbalance in R: S is the result of stronger environmental controls within a greenhouse on factors such as relative humidity, wind and temperature or rather on differences in the quantity and quality of light. In this study, we sought to determine what influence differences in both light quality and quantity, along with sheltering, has on the development of desired characteristics in aspen seedlings. To do so, we grew aspen seedlings in the field under one of six light treatments [unsheltered, full-sun (100 % full-sun); sheltered, full-sun (similar to 90 %): sheltered, shaded (similar to 40 %): sheltered, shaded blue (similar to 40 %); sheltered, shaded red (similar to 40 %); and sheltered, shaded low red/far-red ratio (similar to 40 %)] to identify the combination of sheltering, light quality and quantity with the greatest influence on R: S and root NSC. The largest increase in desired characteristics was a peak in R: S and in root NSC for seedlings grown under unsheltered, full-sun conditions. These results suggest that growing seedlings outside, with pre-exposure to more stressful conditions prior to bud hardening, is the driver for the desired characteristics in species with indeterminate growth strategies such as aspen seedling stock.