Differential seed size and maternal effects in Quercus douglasii affects drought stress in seedlings

Background/Question/Methods

For plants, water stress response is of critical importance in Mediterranean-type climates.  In Quercus douglasii (blue oak), native to California foothill woodlands, survival of seedlings to adult stages depends on the critical transition time between seed emergence and seedling establishment. This phase is highly dependent on available moisture, among other factors.  Seedlings from large seeds generally have more initial resources, and may be better able to withstand drought conditions than those from smaller seeds.  However, because seed size in trees is a trade-off between fecundity and maternal investment in offspring, seed size may be variable in a population.  We tested the hypothesis that seedlings with a larger initial seed size are less susceptible to drought conditons by measuring water stress response in seedlings from ten different blue oak parent trees.  Water stress was measured by comparing the photochemical efficiency of photosystem II via dark-adapted fluorescence measurements in 10 seedlings from 10 parent trees, half grown under dry-down and half as a control group in a controlled greenhouse environment. 

Results/Conclusions

We found significant differences in seed size and volume by mother tree, as well as seedling height and number of leaves on Q. douglasii.  In the seedlings subjected to drought, we found a significant negative relationship between seed size and water stress, while in the controls, no relationship was detected. Our results indicate that maternal investment in seedling resources may become increasingly important for first year drought survival, especially with drier predicted conditions of climate change in California’s foothill woodland environments.