Spring patterns of freezing resistance and photosynthesis of two leaf phenotypes of Hedera helix

1. The relationship between stomatal conductance (gs) and leaf water potential (Ψl) is key tothe understanding of plant function under changing climate. The variability among tree speciesgave rise to selection towards either of two contrasting water management types: isohydric oranisohydric. This study explores the variability of gs to Ψl across tree species.2. Curves of gs(Ψl) were collected from the scientific literature for 70 woody plant species.The data set is comprised of angiosperm and gymnosperm species from all major forest biomes.The hypothesis that curves from different tree species diverge between isohydric andanisohydric behaviours was tested.3. Species-specific curves formed a continuum, rather than dichotomy between isohydric andanisohydric, as confirmed by distribution models. Alternatively, the water potential at 50% ofthe maximum gs (Ψgs50) was used to quantitatively compare between species. A major differenceemerged among xylem anatomy classes whereby ring-porous species had higher absolutegs at Ψl > 2 MPa than diffuse-porous and coniferous species. A positive, linear correlationwas shown between Ψgs50 and Ψl at 50% loss of xylem conductivity.4. The results suggest that stomatal sensitivity to leaf water potential strongly relates to xylemcharacteristics. The use of Ψgs50 offers a quantitative alternative to the current, yet biased,distinction between isohydric and anisohydric species.