Page 223 - Plant-Based Remediation Processes

11 Flax (Linum usitatissimum L.) and Hemp (Cannabis sativa L.)... 215

Majority of published reports on Cd uptake by flax/linseed was dealing with
natural (geogenic) or slightly increased soil Cd concentrations (usually less than
0.5 mg Cd kg –1 soil), which may be categorised as non Cd-contaminated soils
(Cieslinski et al. 1996). Broadley et al. (2001) analysed phylogenetic variation in
heavy metal accumulation (based on published data) and found flax ranked 22nd of
108 totally recorded plant species (wild species þ crops) as related to relative shoot
Cd content, and even 4th of 51 recorded crops/grasses/vegetables. This finding
categorises flax/linseed in the upper border in above-ground Cd accumulation within
up to date studied crops. Nevertheless, according to Baker (1981) classification, flax/
linseed may be classified as related to Cd as “indicator” (i.e. internal Cd concentration
reflects external Cd levels) not accumulator species. High soil Cd concentrations
used in experiments of Bjelkova ´ et al. (2011a, b) were probably never
published before—only Chakravarty and Srivastava (1997a, b) used 50– 2,000 μM
1
Cd (¼ 5.62–224.82 mg Cd L culture medium) in tissue culture medium for linseed.
Such Cd concentrations do not occur in polluted agricultural soils, but they should
answer the question of accumulation potential of flax/linseed for Cd and the effect of
such toxic Cd concentrations on the plant growth and development. Broadley et al.
(2001) also concluded that accumulation potential of particular plant species may be
better demonstrated in a high metal environment than in a low metal environment.
Even 1,000 mg Cd kg 1 soil did not result in severe damage of flax/linseed plants;
nevertheless, it is evident that only a part of artificially added Cd belongs to bioavail-
able soil Cd fraction. In addtition, it is clear that—at Cd soil concentrations of
hundreds of mg Cd kg 1 soil—substantial portion of the metal is blocked in the
roots. To answer the questions of genetic variation in HMs accumulation and namely
HMs distribution in plant organs, the experiments allowing growth/development of
complete mature plants represent an optimum approach. Thus, pot experiments with
soil or representative field trials may give rigorous results on behaviour of plants in
real environment. Nevertheless, hydroponics and laboratory experiments (if well
designed), may bring useful supplementary information. Here we provide several
examples in flax/linseed. Bjelkova ´ et al. (2007)reported that 10 and 100mmolL 1
concentrations of HMs (ten metal elements tested) resulted in lethality in all flax and
linseed cultivars in a standard germination test. Similarly, Soudek et al. (2010)studied
23 flax/linseed cultivars as related to heavy metal toxicity based on seed germination
test (¼inhibition of root elongation). They have found a high diversity in the response
of flax/linseed cvs to particular heavy metal elements. No cultivar exhibited total
tolerance/resistance to a spectrum of toxic heavy metals, but there were specific
interactions cv x metal element. In general, the heavy metal toxicity decreased in
3+ 5+ 2+ 2+ 2+ 6+ 2+
the following order: As As > Cu > Cd > Co > Cr > Ni >
2+
Pb 2+ > Cr 3+ > Zn . Linger et al. (2005) reported that Cd concentrations up to
72 mg kg 1 soil had no negative effect on hemp seeds germination.
Tissue culture experiments in vitro may help in quick screening of HMs tolerant
or accumulating genotypes/lines on HMs-supplemented culture media (neverthe-
less, the recorded data should correspond to pot or field trials in order to reflect real
situation). Chakravarty and Srivastava (1997a, b) first tested the effect of Cd stress
(50–2,000 μM CdCl 2 ) on callus growth and plant regeneration in vitro in three