Inter- and Intraspecific Acclimation of Hydraulic Functions to Low Temperature in Temperate Tree Species

Low temperature is one of the main factors affecting plant growth and propagation, and determining species-specific high elevation and high latitudinal limits of temperate trees. Chilling generally refers to low but nonfreezing temperatures, ranging from 0°C to 15°C, that negatively impact biological processes. While our understanding of the direct cold temperature induced restrictions of cambial and meristematic activity has increased substantially over the last decades, other physiological effects that might also contribute to the cold temperature limit of tree growth have gained much less attention so far. Especially, the potential effects of restricted water uptake and deteriorated hydraulic relations at low root zone temperatures might be additional drivers for the cold limit of tree growth. To explore how low temperatures limit growth by restricting root hydraulic conductance in temperate trees, we applied ²H‐H₂O pulse‐labelling to quantify the water uptake and transport velocity from roots to leaves in seedlings exposed to constant 15°C, 7°C or 2°C root temperature, but identical aboveground temperatures between 20°C and 25°C in greenhouse. This study aimed to specifically explore (1) if hydraulic constrains induced by low root temperatures can be a cause for the species-specific elevational distribution limits of European temperate tree species; (2) if physiological adjustments of root water uptake in response to a short-term low root temperature stress of up to 20 days can be identified in different functional plant types; and (3) if there is a potential for cold acclimation of root water uptake in tree seedlings that acclimated at cold and warm conditions. This study firstly highlighted low temperature‐induced hydraulic constraints contribute to the cold distribution limits of temperate tree species and are a potential physiological cause behind the low temperature limits of plant growth in general. We additionally confirmed the accumulation of cold effects on water permeability of cell membrane in roots, or a controlled reduction of root water conductivity would be a potentially physiological reason causing winter dormancy in temperate trees. We finally revealed that lower montane temperate tree species possess a greater capacity to acclimate to long-term cold acclimation, potentially enabling them to migrate to higher elevations through improved cold sensitivity of root hydraulic conductance while lacking in upper montane species. This study largely concur with the existing concepts of the biological mechanisms responsible for the cold‐temperature limits of temperate trees.