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

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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 (ΨMD) and predawn (ΨPD) 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 ΨPD 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 (ΨMD—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.

This study received financial support from the French government in the framework of the IdEX Bordeaux University “Investments for the Future” program/GPR Bordeaux Plant Sciences. Financial support from PHENOME (ANR-11-INBS-0012) is also acknowledged. ST was supported by an IdEx postdoctoral fellowship from the University of Bordeaux. NGM received support from the Agreenskills + fellowship program, which has received funding from the EU’s Seventh Framework Program under grant agreement No. FP7-26719 (Agreenskills + contract). TEG received funding from the Spanish Ministry of Science (grant number: PID2019‐107817RB‐I00).

Peer reviewed