Behavior of boron and boron isotopes during uptake by Atriplex canescens

Abstract

This research was conducted to determine the potential for using plants to obtain samples of boron isotopes from groundwater, sod moisture and fracture water. In essence, this work sought to ascertain whether plants can function as in-situ samplers for boron as an environmental isotope. At present, very little is known about the behavior of boron isotopes in plants so this study was designed to reveal any isotopic fractionation that might occur during plant uptake by a specific species under carefully controlled conditions. The relationship between the boron isotope ratios sequestered in the leaves of the species Atriplex canescens and the growth conditions of the plant were investigated using a semi-hydroponic greenhouse experiment. Nutrient boron concentration and solution pH were selected as experimental variables as these parameters span large ranges in nature. In addition, the mechanism through which plants take up anionic nutrients suggests that boron isotope fractionation could occur. The experimental setup was a randomized factorial block design and the plants were provided six different nutrient solutions with pH values ranging from 7.5 to 9.5 and boron concentrations varying from 0.1 mg/L to 10.0 mg/L. Boron concentration in the plant's leaf and stem samples followed expected patterns, with the highest boron amount in the leaves of the plants fed nutrient solution with 10.0 mg/L B. The stern samples of plants fed 0.1 mg/L B contained the least boron. The ratio of boron in plants fed 10.0 mg/L vs. 0.1 mg/L B was far less than the 100:1 ratio of boron in the nutrients, which implies that a component of uptake is actively controlled by the plant. Negative thermal ionization mass spectrometry was employed to analyze the minute amounts of boron extracted from digests of the plant tissues. Statistical tests were utilized to determine that, contrary to the hypothesis, no significant isotopic fractionation occurred during uptake at any treatment pH level. The results of this research indicate that the species Atriplex canescens can provide samples of boron isotopes which closely represent the isotopic signature of the plant's water source.