The rates of starch depletion and hydraulic failure both play a role in drought-induced seedling mortality

Key message

The elapsed times to deplete starch concentrations and to reach a null hydraulic safety margin were related to tree seedling mortality under experimental drought. Starch concentration showed an accelerated decline across all species during the early stages of dehydration, while the concentrations of soluble sugars and total nonstructural carbohydrates remained stable. Concomitant carbohydrate depletion and hydraulic failure drive seedling mortality under drought.

Context

Current upsurges of drought events are provoking impacts on tree physiology, resulting in forest mortality. Hydraulic dysfunction and nonstructural carbohydrate (NSC) depletion have been posited as the main mechanisms leading to plant mortality under drought.

Aims

This study explores the dynamics of the two mortality-inducing processes during drought stress using an experimental approach with 12 evergreen tree species.

Methods

Seedlings were subjected to drought until 100% mortality was observed. Midday (Ψ_MD) and predawn (Ψ_PD) water potentials, xylem pressure leading to a 50% loss of hydraulic conductivity (Ψ₅₀), along with NSC concentrations in different organs (leaves, stems, and roots) were measured regularly during drought.

Results

Total NSC concentrations and soluble sugar pools did not decline during drought. However, starch pools showed strong reductions early during drought stress as Ψ_PD decreased, and the time leading to starch depletion emerged as a strong mortality predictor. Ψ₅₀ alone did not provide an accurate estimate of mortality, while the elapsed time to reach a null hydraulic safety margin (Ψ_MD—Ψ₅₀ = 0) was related to seedling mortality.

Conclusion

Adopting a dynamic approach by estimating the times to consume both starch reserves and hydraulic safety margins is highly relevant to improve predictions of tree mortality under the current context of increasing global drought.