Leaf Turgor Loss Does Not Coincide With Cell Plasmolysis in Drought-Tolerant Chaparral Species

The water potential at which leaf cells lose turgor (Ψ_TLP) is a useful predictor of whole-plant drought tolerance and biome wetness. However, many plants can achieve water potential values below Ψ_TLP and recover, raising questions about the physiological processes that occur below Ψ_TLP. We established a controlled greenhouse experiment to induce turgor loss on six shrub species from a Mediterranean-type ecosystem in Southern California and characterised physiological and leaf-structural adjustments to Ψ_TLP. We documented seasonal adjustments in Ψ_TLP, both with and without applied drought. Stomatal closure always occurred below Ψ_TLP, and the margin between the two phenomena increased with lower Ψ_TLP. Drought tolerance was strongly correlated with heat tolerance. Most histological responses to Ψ_TLP involved shrinkage of both spongy mesophyll cells and intercellular air spaces, leading to reduced leaf thickness, but not plasmolysis. Overall, our results indicate a propensity to reach Ψ values far below Ψ_TLP and maintain function for extended periods of time in Southern California shrubs. Whereas species in many ecosystems fall below Ψ_TLP for brief periods of time, the erratic nature of precipitation patterns makes Southern California an outlier in the range of operational plant water potentials.