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Thresholds for leaf damage due to dehydration: declines of hydraulic function, stomatal conductance and cellular integrity precede those for photochemistry

Trueba, Santiago, Pan, Ruihua, Scoffoni, Christine, John, Grace P., Davis, Stephen D., Sack, Lawren
Thenew phytologist 2019 v.223 no.1 pp. 134-149
Angiospermae, ecosystems, hydraulic systems, leaf water potential, leaves, photochemistry, photosystem II, rehydration, stomatal conductance, stomatal movement, turgor, water content, xylem
Given increasing water deficits across numerous ecosystems world‐wide, 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 (Fᵥ/Fₘ) in the leaves of 10 diverse angiosperm species, and tested when these occurred relative to turgor loss, declines of stomatal conductance gₛ, and hydraulic conductance Kₗₑₐf, including xylem and outside xylem pathways for the same study plants. We resolved the sequences of relative water content and leaf water potential Ψₗₑₐf thresholds of functional impairment. On average, losses of leaf rehydration capacity occurred at dehydration beyond 50% declines of gₛ, Kₗₑₐf and turgor loss point. Losses of Fᵥ/Fₘ occurred after much stronger dehydration and were not recovered with leaf rehydration. Across species, tissue dehydration thresholds were intercorrelated, suggesting trait co‐selection. Thresholds for each type of functional decline were much less variable across species in terms of relative water content than Ψₗₑₐf. 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.