Biogeochemistry of fluoride in a plant-solution system

Area of Science:

• Environmental Chemistry
• Plant Physiology
• Agricultural Science

Background:

• Fluoride (F-) pollutants pose risks to plants and animals.
• The impact of complexing agents on fluoride (F-) bioavailability is not well understood.
• Understanding F- uptake mechanisms is crucial for managing plant contamination.

Purpose of the Study:

• To investigate the effect of humic acid (HA) and N-hydroxyethylenthylenediaminetriacetic acid (HEDTA) on F- bioavailability in rice.
• To determine the influence of calcium (Ca) concentration on F- availability and uptake.
• To elucidate the uptake mechanism of fluoride in rice plants.

Main Methods:

• Rice (Oryza sativa L.) was grown in solution culture with varying concentrations of F- and either HA or HEDTA.
• A second study examined the effect of increased Ca levels on F- availability and uptake.
• Fluoride concentrations in plant tissues (leaves, stems, seeds) and solution were analyzed using tissue analysis and energy-dispersive X-ray spectroscopy.

Main Results:

• Total biomass was highest with HEDTA and low F- concentrations (< 1 mM).
• Leaf and stem F- concentrations increased with rising solution F-, but seed F- accumulation was minimal.
• F- competed with HA for Ca, inhibiting Ca-HA flocculent formation; added Ca led to CaF2 precipitation on roots.

Conclusions:

• Fluoride uptake in rice appears to occur primarily as undissociated hydrofluoric acid (HF0), with limited F- uptake.
• Complexing agents like HEDTA can influence plant biomass, but F- accumulation in vegetative tissues remains a concern.
• Calcium plays a critical role in modulating F- bioavailability and its precipitation as CaF2, affecting root uptake.