Seasonal Responses of Hydraulic Function and Carbon Dynamics in Spruce Seedlings to Continuous Drought

Abstract

Drought is expected to increase in the frequency and duration associated with climate change. Although hydraulic function and carbon (C) storage have been widely recognized as key components to plant survival under a single drought, the physiological responses to continuous drought remain largely unknown, particularly for high northern temperate and boreal forests which are sensitive to water stress. In this study, we quantified the survival, growth, gas exchange, water relations, and nonstructural carbohydrates (NSCs) in 3-year-old Jezo spruce (Picea jezoensis) seedlings responding to continuous drought stress. Seedlings were maintained in drought conditions for 392 days, covering two growing and one dormant winter season. Seedlings subjected to drought showed a significant decrease in net photosynthesis rate (Anet) and stomatal conductance (gs) in both growing seasons, and biomass in the second growing season. The seedling mortality continuously increased to 35.6% at the experimental end. Notably, responses of C storage and leaf water potential to drought varied greatly depending on seasons. Living seedlings exposed to drought and control treatments had similar NSC concentrations in both growing seasons. However, seedlings with concentrations of both the soluble sugars and starch less than 1% in root died in the winter dormant season. In the second growing season, compared with the control treatment, droughted seedlings had significantly lower leaf water potential and stem wood-specific hydraulic conductivity (Kw). Meanwhile, the leaf predawn water potential did not recover overnight. These suggest that C starvation might be an important reason for seedlings that died in the winter dormant season, while in the growing season drought may limit seedling survival and growth through inducing hydraulic failure. Such seasonal dependence in hydraulic dysfunction and C depletion may lead to higher mortality in spruce forests facing extended drought duration expected in the future.

Abscisic Acid and Cytokinins Are Not Involved in the Regulation of Stomatal Conductance of Scots Pine Saplings during Post-Drought Recovery

“…High drought sensitivity of stomatal conductance and delayed and/or insufficient recovery of stomatal conductance together cause plants to experience a prolonged period of diminished gas exchange and assimilation, thus deteriorating plant carbon balance. Insufficiency of assimilates is unlikely to directly cause plant death during drought, but it would make plants more vulnerable to the adverse effects of the upcoming winter [ 15 ] and compromise plant growth and drought tolerance in the next growing season, which is especially important for conifers given their small fraction of stem parenchyma tissue and low stem carbohydrate storage [ 8 ]. Together, these factors make rate and completeness of gas exchange recovery the most important factor determining plant productivity and mortality in the expected future conditions with increasing frequency and severity of adverse climatic events.…”