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

The water potential at which leaf cells lose turgor (Ψ) is a useful predictor of whole-plant drought tolerance and biome wetness. However, many plants can achieve water potential values below Ψ and recover, raising questions about the physiological processes that occur below Ψ. 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. We documented seasonal adjustments in Ψ, both with and without applied drought. Stomatal closure always occurred below Ψ, and the margin between the two phenomena increased with lower Ψ. Drought tolerance was strongly correlated with heat tolerance. Most histological responses to Ψ 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 and maintain function for extended periods of time in Southern California shrubs. Whereas species in many ecosystems fall below Ψ for brief periods of time, the erratic nature of precipitation patterns makes Southern California an outlier in the range of operational plant water potentials.