The cold acclimatization of root water uptake in temperate tree species

Plants are able to acclimatize to cold climates by developing physiological strategies to withstand periods of cold but nonfreezing temperatures during the growing season. In this study we investigated, if a long-term acclimation to low temperatures enables seedlings of temperate tree species to reduce the negative effects of low root temperatures on root water uptake and transport that has been observed in previous experimental studies. We investigated 7 common European tree species that differ largely in their natural elevational distribution ranges. To acclimatize the plants to different temperatures, newly germinated seedlings were raised at two different air temperatures (warm 22°C day/18°C night; cold, 12°C day/8°C night) for several months, and then were exposed to three different root temperatures (15, 7 and 2°C) in hydroponic systems while maintaining the same warm aboveground temperatures (20-25 °C) for a two-day period. We used stable isotope labelling with ²H-H₂O source water to quantify the water uptake and transport from roots to leaves by the amount of ²H-label in leaf water after labelling. The species-specific sensitivity of root water uptake to low root temperatures was indentified by the relative change of ²H labels in leaf water at low root temperatures relative to 15°C. We found cold acclimation treatment did not improve the cold sensitivity of root water uptake, except for the two species with the lowest elevational distribution limits. In contrast, the majority of the investigated species showed a generally enhanced capacity of water uptake and transport after cold acclimation, regardless of the applied root temperatures. This results suggested a limited ability for physiological adjustemnts to overcome the cold limitation of water transport in roots, that is generally associated with decraesing efficiency of water diffusion across plasma membranes. The tendency towards an overall enhanced capacity for water movements in cold acclimatized seedlings might be due to the modified root anatomy and morphology that would help to improve root hydraulic conductance of trees in cold environments.