Ghosts of dry seasons past: Legacy of severe drought enhances mangrove salinity tolerance through coordinated cellular osmotic and elastic adjustments

Abstract

The incidence and severity of global mangrove mortality due to drought is increasing. Yet, little is understood of the capacity of mangroves to show long‐term acclimation of leaf water relations to severe drought. We tested for differences between mid‐dry season leaf water relations in two cooccurring mangroves, Aegiceras corniculatum and Rhizophora stylosa before a severe drought (a heatwave combined with low rainfall) and after its relief by the wet season. Consistent with ecological stress memory, the legacy of severe drought enhanced salinity tolerance in the subsequent dry season through coordinated adjustments that reduced the leaf water potential at the turgor loss point and increased cell wall rigidity. These adjustments enabled maintenance of turgor and relative water content with increasing salinity. As most canopy growth occurs during the wet season, acclimation to the ‘memory’ of higher salinity in the previous dry season enables greater leaf function with minimal adjustments, as long‐lived leaves progress from wet through dry seasons. However, declining turgor safety margins ‐ the difference between soil water potential and leaf water potential at turgor loss ‐ implied increasing limitation to water use with increasing salinity. Thus, plasticity in leaf water relations contributes fundamentally to mangrove function under varying salinity regimes.

Plasticity in branch water relations and stem hydraulic vulnerability enhances hydraulic safety in mangroves growing along a salinity gradient

“…Two species that differ in optimum salinities and salt management properties were selected for study: A. corniculatum , a salt‐secreting species with optimal growth salinities of 10%–20% seawater (Ball, 1988b), and R. stylosa , a nonsalt secreting species (Ball, 1988a) with optimal growth salinities of 25%–50% seawater (Clough, 1984). However, despite A. corniculatum being a salt‐secretor and R. stylosa being a nonsalt secretor, there is no evidence for fundamental differences in bulk water relations between species with different salt management strategies (Beckett et al, 2023). Plant material was collected mid‐dry season from trees growing naturally along the banks of the Daintree River, Daintree National Park, Far North Queensland, Australia (16.1700° S, 145.4185° E).…”

“…Therefore, the difference between ψ estuary , and ψ TLP in leaves defines the turgor safety margin (TSM); the range of ψ over which the plant can operate while maintaining turgor from root water. In the mangroves Aegiceras corniculatum and Rhizophora stylosa , Beckett et al (2023) found that despite lowering ψ TLP in response to increasing salinity, plasticity in ψ TLP was less than proportionate with the increase in ψ estuary , reducing the TSM with increasing salinity. Because of the importance of turgor for growth and carbon gain (Fricke, 2017; Peters et al, 2021), reduction in the TSM constrains turgor‐dependent function, and increases the risk of cell injury with dehydration below ψ TLP .…”