Agriculture Reference
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the number of genes from one to smaller or larger number of genes varies and the
action may be influenced by modified genes (Macnair 1993 ; Schat et al. 1996 ).
Tolerance may be constitutive and adaptive i.e depending upon external factor and
both types are interlinked (Macnair 1993 ). For example, an increase in soil toxic-
ity leads to selection pressure, which plays an important role in tolerance (Schat
et al. 1996 ). Metal tolerance can be separate i.e regulated by separated genes for
each metal tolerance or co-tolerance (Pleiotropy), although the multiple tolerance
developed by plant is by growing them on soils with more than one metal (Macnair
1993 ) e.g., Silene vulgar population originating from metalliferous sites in Ireland
and Germany and from non-metalliferous site in Netherlands, two main co-additive
gene control of Zn tolerance in this species (Schat et al. 1996 ). The understand-
ing of cellular level processes has been progressed by metal trafficking proteins,
but the relationship between tolerance to toxic metals or metalloids and element
homeostasis of the entire organism is less known (Clemens 2001 ). Parameters like
yield reduction, shoot and root length or fresh and dry matter describes the toxic-
ity (Berry and Wallace 1981 ; Odjegba 2012 ), but the reversibility of plasmolysis
enzyme activities, chlorophyll contents and other physiological parameters are also
employed (Baker and Walker 1989 ). Several indices have been developed for the
measurement of tolerance and toxicity. The relationship between root growth with
and without a toxic element is expressed by tolerance index (TI). The dose CD50
toxicant causes the reduction, it may be expressed as EC10 to EC50 i.e. 'effective
concentrations (EC) to lower the yield by 10-50 % (Ernst 1997a ; Kooijman 1997 ).
Macnicol and Beckett ( 1985 ) found that critical tissue toxicant concentration can
also be used e.g. in soybean and cabbage soil culture, 10 % toxicity occurs at the
upper critical tissue by the As level between 1 and 4 mgAs kg -1 plant shoot and leaf
dry weight. For soybean, bush beans and pea, the values for the essential micronu-
trient like Zn are higher and the values are 450, 250, and 380-500 mgZnkg -1 for
shoot or leaf dry weight (Macnicol and Beckett 1985 ).
Physiological Response Mechanisms
Modes of action of plants under exposure instead of the term tolerance mechanism
are used in the meaning of 'response mechanism'. Tolerance or toxicity mecha-
nisms are not fully defined as yet (Schat et al. 1996 ), and tolerance mechanism also
includes responses like altered permeability, enhanced metal binding capacity of
the root apoplasm and root exudates. Cellular mechanism includes the synthesis
of phytochelatins, organic acids, proteins and membrane adjusting functions togeth-
er with the synthesis of specific transporters (Hall 2002 ). Still the question arises as
which mechanism provides contribution to the primary and secondary responses.
These mechanisms shows the element and plant species dependency and more than
one mechanism is active simultaneously in a species (Macnair 1993 ). Recently
Fodor ( 2002 ) has reviewed heavy metal responses of higher plants.
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