This work is a study of the inter-relationship between parameters that principally affect metal up-take in the plant. The relationships between the concentration of metal in the growth medium, Cs, the concentration of metal absorbed by the plant, Cp, and the total biomass achieved, M, all of which are factors relevant to the efficiency of phytoremediation of the plant, have been investigated via the macro-physiological response of Brassica juncea seedlings to Ni(II) stress. The factorial growth experiments treated the Ni(II) concentration in the agar gel and the diurnal light quanta (DLQ) as independently variable parameters. Observations included the evidence of light enhancement of Ni toxicity at the root as well as at the whole plant level, the shoot mass index as a possible indicator of shoot metal sequestration in B. juncea, the logarithmic variation of Cp with Cs and the power-law dependence of M on Cp. The sum total of these observations indicate that for the metal accumulator B. juncea with regard to its capacity to accumulate Ni, the overall metabolic nature of the plant is important; neither rapid biomass increase nor a high metal concentration capability favor the removal of high metal mass from the medium, but rather the plant with the moderate photosynthetically driven biomass growth and moderate metal concentrations demonstrated the ability to remove the maximum mass of metal from the medium. The implications of these observations in the context of the perceived need in phytoremediation engineering to maximize Cp and M simultaneously in the same plant, are discussed.
The ability of certain plants to absorb large quantities of heavy metals from contaminated soil and water, holds significant promise for the deployment of "green technologies" such as phytoremediation towards environmental clean-up [1]. While the technology presents several distinct advantages over conventional remediation methods [2], much still needs to be learnt about the physico-chemical and biological factors that guide the movement and speciation of heavy metals within the rhizosphere and their subsequent up-take, translocation and detoxification by the plant [3]. Of the several technologies that constitute phytoremediation, phytoextraction appears to be optimal for the clean-up of those types of soil where it can be applied, because it is only in this that the toxic metal, absorbed by the root and sent to aerial shoots via xylem transport, can be entirely removed by the subsequent harvest of the shoots [1][2][3][4][5]. While diverse and complex processes specific to the geochemistry of the metallic compound present in the soil and the biochemistry and physiology of metal up-take by the plant, govern the process of phytoextraction at the molecular level [6,3], these factors ultimately affect the efficiency of phytoextraction at the whole plant level [7,8], via the mass M metal of heavy metal extracted from the soil per crop. M metal is the product of M the mass of dry plant tissue produced per crop and C p the concentration of metal in the plant tissue.
The objective of phytoextraction engineering is to maximize M metal for the crop which in the ideal case is served by the simultaneous high values of both M and C p .
Phytoremediating plant species generally fall into the categories of high M but relatively low C p plants, such as maize (Zea mays), or the so-called metal hyper-accumulators, e.g.
Thlaspi caerulescens (Brassicaceae), that are typically characterized by high C p but low M [5]. One aim of plant molecular biologists is to identify novel genes important for phytoremediation including regulatory networks and tissue-specific transporters, and subsequently to manipulate the expression of these genes in high biomass species [3,5,9]. However, the question of the exact trade-off between metal toxicity and plant biomass development as dictated by the whole plant physiological response in quantitative terms, requires more detailed studies. It is a fundamental question that dictates the outcome of any phytoremediation scheme whether using naturally selected species or genetically modified ones. The accuracy of predictive models of phytoextraction efficiency depends on such studies and thereby potentially, the acceptance of this emerging technology by regulatory bodies and enterprises.
The calculation of M metal (eqn. 1) forms the basis of estimates designed to find out how many successive crops would be required to bring about the desired reduction in the soil concentration of the metal [10,7]. Two key assumptions are generally made:
(a) that C p , or its analogue the bio-concentration factor, BCF 1 , remains constant as the soil metal concentration, C s , changes (BCF is the ratio of C p to C s ) and (b) that M is unaffected by C p [10,7] The objective of the present work is to examine the validity of both these assumptions and in doing so to arrive at an empirically quantitative understanding of the relationship between the metal concentration in the medium, C s , its concentration in the plant C p and the biomass M at the whole plant level. The physiological stress impact of increased metal concentration (C p ) within the plant is likely to affect M as well as the fact that the soil-plant system represents two mutually interacting phases in which the metal composition in one phase is likely to affect the other by means of the system’s thermodynamics. Moreover the exact nature of the relationships between C p and C s and between C p and M are likely to depend on the metabolic status of the plant and its species.
We base our study on the detailed examination of several macro-physiological stress indices such as the temporal root length, plant total mass (M) and shoot mass index vis à vis the measured absorbed concentration of the metal (C p ) in the 2 week old seedlings of the metal accumulator Brassica Juncea. This plant is known to phytoextract Ni(II) [11][12][13] and is a quick-growing relatively large biomass plant that has often been cited as a potential phytoremediant [14,15]. Bausch&Lomb L1-1000 light meter to be about the same and translated to the diurnal light quanta (DLQ) of 4.32 X 10 6 and 1.20 X 10 7 µmoles photons.m -2 d -1 respectively.
Incremental root lengths for each seedling were measured at 8h intervals. The average cumulative root lengths and their errors (determined propagatively) were calculated for each plate from these incremental lengths.
After 2 weeks growing time, all seedlings from each plate were combined to make one composite sample per plate, dried at 60 0 C f
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