Thresholds for leaf damage due to dehydration: dec...

Given increasing water deficits across numerous ecosystems worldwide, it is urgent to understand the sequence of failure of leaf function during dehydration.

We assessed dehydration‐induced losses of rehydration capacity and maximum quantum yield of the photosystem II (F_v/F_m) in the leaves of 10 diverse angiosperm species, and tested when these occurred relative to turgor loss, declines of stomatal conductance (g_s), and hydraulic conductance (K_leaf), including xylem and outside xylem pathways for the same study plants. We resolved the sequences of relative water content (RWC) and leaf water potential (Ψ_leaf) thresholds of functional impairment.

On average, losses of leaf rehydration capacity occurred at dehydration beyond 50% declines of g_s, K_leaf and turgor loss point. Losses of F_v/F_m occurred after much stronger dehydration and were not recovered with leaf rehydration. Across species, tissue dehydration thresholds were inter‐correlated, suggesting trait co‐selection. Thresholds for each type of functional decline were much less variable across species in terms of RWC than Ψ_leaf.

The stomatal and leaf hydraulic systems show early functional declines before cell integrity is lost. Substantial damage to the photochemical apparatus occurs at extreme dehydration, after complete stomatal closure, and seems to be irreversible.

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