Alleviation of Cu and Pb rhizotoxicities in cowpea (Vigna unguiculata) as related to ion activities at root-cell plasma membrane surface

Authors: KOPITTKE, PETER - Queensland University - Australia; Kinraide, Thomas; WANG, PENG - Chinese Academy Of Sciences; BLAMEY, F. PAX C. - Queensland University - Australia; REICHMAN, SUZIE - Royal Melbourne Institute Of Technology University; MENZIES, NEAL - Queensland University - Australia

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2011
Publication Date: 5/12/2011
Citation: Kopittke, P.M., Kinraide, T.B., Wang, P., Blamey, F., Reichman, S.M., Menzies, N.W. 2011. Alleviation of Cu and Pb rhizotoxicities in cowpea (Vigna unguiculata) as related to ion activities at root-cell plasma membrane surface. Journal of Environmental Science and Technology. 45:4966-4973.

Interpretive Summary: The acquisition of nutrients and the avoidance of toxicants by plants is an important problem in agricultural production and human health. The bioavailability of nutrients and toxicants is determined by plant characteristics and by the environment. For example, copper, both a nutrient and a toxicant, and lead, a toxicant, are accumulated in some plants more than in others, and in ‘hard’ waters (high concentration of calcium and magnesium) these metals are much less available (and less intoxicating) than in ‘soft’ waters. Toxicologists have adopted the Biotic Ligand Model (BLM) to interpret and predict toxicant bioavailability. The model states that ameliorative ions (e.g., Ca and Mg) alleviate toxicity by competing with toxicants (e.g., Cu and Pb) for cell-surface binding site. The BLM is only a partial explanation and it ignores the great importance of cell electrical phenomena. The contribution of the present study is the extension of our understanding of plant and environmental determinants of nutrient and toxicant acquisition. In particular, the study demonstrates that cell-surface electrical potentials influence the uptake or avoidance of charged solutes in two distinct ways. This knowledge is essential for improved management practices and plant breeding and selection for production in nutrient-poor, phosphate retentive, acidic soils.

Technical Abstract: Cations, such as Ca and Mg, are generally thought to alleviate toxicities of trace metals through site-specific competition (as incorporated in the biotic ligand model, BLM). Short term (48 h) experiments were conducted using cowpea (Vigna unguiculata L. Walp.) seedlings in simple nutrient solutions to examine the alleviation of Cu and Pb toxicities by Al, Ca, H, Mg, and Na. For Cu, the cations depolarized the plasma membrane (PM) and reduced the negativity of '0o (electrical potential at the outer surface of the PM), and thereby decreased {Cu2+}0o (activity of Cu2+ at the outer surface of the PM). For Pb, root elongation was generally better correlated to the activity of Pb2+ in the bulk solution than to {Pb2+}0o. However, we propose that the addition of cations (and the associated decrease in negativity of '0o) resulted in a decrease in {Pb2+}0o but a simultaneous increase in the rate of Pb uptake (due to an increase in the negativity of Em,surf, the difference in potential between the inner and outer surfaces of the PM) thus offsetting the decrease in {Pb2+}0o. In addition, Ca was found to alleviate Pb toxicity through a specific effect. Thus, we propose that Cu toxicity in cowpea roots is influenced primarily by {Cu2+}0o, whilst Pb toxicity is influenced by {Pb2+}0o, Em,surf, and {Ca2+}0o (further work is required, particularly regarding Pb). Although our data do not preclude site-specific competition (as incorporated in the BLM), we suggest that electrostatic effects have an important role in determining toxicity.