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

ABSTRACT 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.