Fluoride inhibits root water transport and affects leaf expansion and gas exchange in aspen (Populus tremuloides) seedlings

The effects of sodium fluoride (0.3, 5 and 10 mM NaF) on root hydraulic conductivity, and gas exchange processes were examined in aspen (Populus tremuloides Michx.) seedlings grown in solution culture. A long-term exposure of roots to NaF significantly decreased root hydraulic conductivity (L-p) and stomatal conductance (g(s)). Root absorbed NaF significantly affected electrolyte leakage in leaf tissues and substantially restricted leaf expansion. NaF did not significantly affect leaf chlorophyll contents but decreased net photosynthesis (P-n). A short-term exposure of excised roots to 5 mM NaF and KF significantly decreased root water flow (Q(v)) with a concomitant decline in root respiration and reduced g(s) when applied through intact roots or excised stems. The same molar concentration of NaCl also decreased Q(v) and g(s) in intact seedlings, but to a lesser extent than NaF or KF, and did not significantly affect root respiration. The results suggest that fluoride metabolically inhibited Q(v) or L-p, probably by affecting water channel activity. We suggest that the metabolic inhibition of L-p by root-absorbed fluoride affected gas exchange and leaf expansion in aspen seedlings.