Light Quanta Modulated Characteristics of NI Uptake by Brassica Juncea Seedlings: The Interdependence of Plant Metal Concentration and Biomass

Abstract

The relationships between the concentration of metal in the growth medium, C_s, the concentration of metal absorbed by the plant, C_p, and the total biomass achieved, M, all of which are factors relevant to the efficiency of metal uptake and tolerance by the plant, have been investigated via the physiological response of Brassica juncea seedlings to Ni stress. The factorial growth experiments treated the Ni concentration in agar medium and the diurnal light quanta as independently variable parameters. Observations included the evidence of light enhancement of Ni toxicity in the root, as well as at the whole-plant level. The shoot mass index possibly is an indicator of the amount of shoot metal sequestration in B. juncea, as are the logarithmic variation of C_p with C_s and the power-law dependence of M on C_p. The sum total of these observations indicates that, for the Ni accumulating plant B. juncea, the overall metabolic allocation to either growth or metal tolerance of the plant is important. Neither a rapid biomass increase nor a high metal absorbed concentration favored the removal of high metal mass from the medium. Rather, the plants with a moderate rate of biomass growth and a moderate absorbed metal concentration demonstrated the ability to remove the maximum mass of metal from the medium. The implication of these results as related to the extant model of phyoextraction efficiency is discussed.

KEY WORDS:

• Brassica juncea
• Ni
• phytoextraction
• light quanta
• power-law

ACKNOWLEDGEMENTS

The authors thank Dr. M. Mahmoudian for help with plant growth, Dr. H. DeYoe for growth-chamber usage, Drs. A. Murphy and W. Peer for useful discussions, and E. Hurtado for technical help with the AAS. Funding for this work was provided by the National Science Foundation (USA) through the CRUI –NSF Program 1091, grant 0330815, and the US Department of Agriculture grant 2003–04117 through the USDA-HSI program.