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 m M 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 m M 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.