Grassland plants show no relationship between leaf drought tolerance and soil moisture affinity, but rapidly adjust to changes in soil moisture

Abstract

Assessing drought tolerance and the ability of plants to adjust to changes in available water resources is crucial for understanding current and future distributions of plant species. While turgor loss point (πtlp) has been recognized as a direct determinant of drought tolerance in woody plants, information on it for grassland species is largely missing. We first validated a rapid method to estimate πtlp for grassland species, using osmometry measurements (πtlp-osm) of osmotic potential at full hydration (πo-osm), originally proposed for woody species. We confirmed that πo-osm was tightly positively related to πtlp measured by the classic pressure–volume curve (πtlp-pv). Cell wall elasticity was not important in the maintenance of turgor and neither specific leaf area nor leaf dry matter content influenced πtlp. We then studied the relationship between πtlp-osm measured under controlled conditions and species’ soil moisture affinity (Ellenberg indicator values) in 40 diverse grassland species from the full range of soil moisture conditions found in temperate grasslands. On a subset of 12 species, we studied the adjustment in πtlp-osm to drought stress and recovery from drought. We hypothesized that dry grassland species are better adapted to drier conditions by having comparatively higher leaf drought tolerance (more negative πtlp) and a better ability to adjust to acute drought stress. We found that πtlp-osm was unrelated to species’ soil moisture affinity. However, all species developed higher leaf drought tolerance by decreasing πtlp-osm after seven weeks of drought stress. After only one week of recovery, all species reduced the leaf drought tolerance back to the πtlp-osm level comparable with non-stressed plants. Our results considerably extend the relationship between πo-osm and πtlp, originally defined for woody species, and thus propose a yet unexploited direct method for assessing leaf drought tolerance via turgor loss point in herbaceous species. However, the lack of relationship between πtlp and soil moisture affinity suggests that, unlike in woody plants, leaf-level drought tolerance in grassland plants does not fully translate into whole-plant drought resistance, suggesting an importance of other drought resistance strategies. A plain language summary is available for this article.