Physiological adjustments of temperate tree species and herbs in response to low root temperatures.

Hydraulic constraints induced by low root temperature might be a major cause for the low temperature limit of plants. However, to date most of our knowledge on the physiological effects of low root temperatures is derived from short-term lab experiments, with very limited information on potential adjustments to continuous low temperature stress. In this study, we quantified the cold sensitivity of root water uptake and transport to leaves in seedlings of different functional plant types (conifers, broadleaved trees and annual herbs) by 2H-H2O labeling after exposure to three constant root temperatures (15 °C, 7 °C and 2 °C) but the same higher aboveground temperatures (between 20 and 25 °C). We investigated changes in the cold sensitivity of roots after 0, 10 and 20 days prolonged exposure to the respective root temperatures. Plant water uptake and transport was decreased by lowered root temperature in all species, with a stronger effect at 2 °C compared with 7 °C. The water uptake and transport capacity of tree species gradually declined over the 20-day treatment, while the two investigated herbs exhibited immediately strong decreases that were kept at the same low level throughout the entire experiment time. The speed of the water uptake reduction across the 20 days observation period differed among the tree species and was faster in species that reach their natural upper distribution limits at lower elevations compared with species that occur at subalpine regions. The restricted root water uptake and transport was accompanied by reductions in leaf water potential, stomatal conductance and growth. Overall, our study showed increasingly reduced capacity for water uptake and transport across functional plant groups at continuous cold root conditions. This result might indicate accumulative negative effects on cell membrane permeability for water in roots, or a controlled reduction of root water conductivity of temperate trees in preparation for winter dormancy.