Extending the osmometer method for assessing drought tolerance in herbaceous species

Abstract

Community-scale surveys of plant drought tolerance are essential for understanding semi-arid ecosystems and community responses to climate change. Thus, there is a need for an accurate and rapid methodology for assessing drought tolerance strategies across plant functional types. The osmometer method for predicting leaf osmotic potential at full turgor (πo), a key metric of leaf-level drought tolerance, has resulted in a 50-fold increase in the measurement speed of this trait; however, the applicability of this method has only been tested in woody species and crops. Here, we assess the osmometer method for use in herbaceous grassland species and test whether πo is an appropriate plant trait for understanding drought strategies of herbaceous species as well as species distributions along climate gradients. Our model for predicting leaf turgor loss point (πTLP) from πo (πΤLΡ = 0.80πo-0.845) is nearly identical to the model previously presented for woody species. Additionally, πo was highly correlated with π-^ for graminoid species (πtlp = 0.944πo-0.611; r2 = 0.96), a plant functional group previously flagged for having the potential to cause erroneous measurements when using an osmometer. We report that πo, measured with an osmometer, is well correlated with other traits linked to drought tolerance (namely, leaf dry matter content and leaf vulnerability to hydraulic failure) as well as climate extremes linked to water availability. The validation of the osmometer method in an herb-dominated ecosystem paves the way for rapid community-scale surveys of drought tolerance across plant functional groups, which could improve trait-based predictions of ecosystem responses to climate change.

Drought survival is positively associated with high turgor loss points in temperate perennial grassland species

“…Midday leaf water potentials under drought (Ψ MD ) and turgor loss points ( π tlp ) varied significantly across the 41 focal species (Table S2). π tlp ranged from −2.30 ± 0.12 MPa to −1.49 ± 0.02 MPa ( M ± SE ) in our study and were well within published data for herbaceous species under irrigated conditions, that is, −3.2 MPa to −0.43 MPa (Figure ; Bartlett, Scoffoni, & Sack, ; Griffin‐Nolan et al, ). Forbs had significantly higher Ψ MD and π tlp than grasses (Figure a,b; Table S2).…”

“…Turgor loss point measured with traditional P – V curve methods ( π tlp‐ P – V ) was significantly related to osmotic water potential at full turgor assessed with an osmometer ( π o‐osmo ) across all 14 grassland species, and separately within the forbs and grasses measured in our study. Additionally, relations are shown for C 3 and C 4 grasses, grasses and forbs, and herbaceous and woody species combined from this and previous studies (Bartlett, Scoffoni, Ardy, et al, ; Bartlett, Scoffoni, & Sack, ; Griffin‐Nolan et al, ; Gotsch et al, ; Ocheltree et al, ; Farrell et al, ; Májeková et al, ). Relations were significant within each of the species groups (all p < .05, see legend for r 2 values) and slopes did not differ between grasses versus forbs (this study nor combined), C 3 versus C 4 , nor herbaceous versus woody species (all p > .1).…”

“…Turgor loss point ( π tlp ), the leaf water potential at which the turgor pressure of leaf cells equals zero, has long been considered a crucial parameter in plant water relations (Cheung, Tyree, & Dainty, ). It varies widely among plant species and has more recently been suggested to be a useful proxy of species drought resistance (Bartlett, Scoffoni, & Sack, ; Griffin‐Nolan et al, ). A low (more negative) π tlp can allow the leaf to remain turgid despite decreasing leaf water potential (Ψ leaf ) and thereby maintain photosynthesis, water transport, transpiration and growth, conferring high drought resistance as a mechanism of desiccation tolerance.…”

“…For example, in C 4 perennial grasses, species with high leaf resistance to hydraulic failure exhibited low π tlp , consistent with woody species (Ocheltree et al, ), but species from drier habitats had higher π tlp than those from wetter habitats, which is opposite to the trend in woody species (re‐analysed from Liu & Osborne, ). In most studies focusing on herbaceous species, π tlp was unrelated to their distributional association with the dryness of habitats (Farrell, Szota, & Arndt, ; Griffin‐Nolan et al, ; Májeková et al, ; Ocheltree et al, ) or biomes (re‐analysed only for herbaceous species from Bartlett, Scoffoni, & Sack, ). In a recent study, π tlp was instead positively correlated with the wet extremes of species distributions (Griffin‐Nolan et al, ), which was attributed to competition from more acquisitive (high‐ π tlp ) species limiting the distribution of low‐ π tlp species into wetter habitats.…”

“…In most studies focusing on herbaceous species, π tlp was unrelated to their distributional association with the dryness of habitats (Farrell, Szota, & Arndt, ; Griffin‐Nolan et al, ; Májeková et al, ; Ocheltree et al, ) or biomes (re‐analysed only for herbaceous species from Bartlett, Scoffoni, & Sack, ). In a recent study, π tlp was instead positively correlated with the wet extremes of species distributions (Griffin‐Nolan et al, ), which was attributed to competition from more acquisitive (high‐ π tlp ) species limiting the distribution of low‐ π tlp species into wetter habitats. Overall, in herbaceous species, the role of π tlp for whole‐plant drought resistance and distribution remains elusive, and its potential as a predictor of species drought resistance is still unclear.…”