Environmental Engineering Reference
In-Depth Information
3.2.5
Water Relations
Maintenance of plant water relations is critical under environmental stresses such
as drought and salinity (Krämer and Boyer
1995
). However, the impact of heavy
metal stress is still unclear, as there are controversial reports available in the litera-
ture (Barceló and Poschenrieder
1990
; Menon et al.
2005
; Vernay et al.
2007
). Still
there are many reports available in the support of negative impact of heavy metal
stress on imbalances in plant-water relations (Sharma and Sharma
1987
; Barceló
and Poschenrieder
1990
; Chatterjee and Chatterjee
2000
).
Plant water relations may undergo changes under heavy metal stress, which may
be due to the accumulation of compatible solutes/osmotica. For example, proline
accumulates in plants under metal stress (El-Enany and Issa
2001
; Lin and Kao
2006
). Soluble sugars and other free amino acids also accumulate in metal-stressed
plants (Baccouch et al.
1998
). It is now well established that organic osmotica can
significantly reduce water and solute potentials in metal-stressed plants which help
plants to maintain cell turgor.
3.3
Anatomical Characteristics
3.3.1
Root Anatomy
The uptake of heavy metals by a plant may trigger a series of anatomical altera-
tions with potential functional consequences in the plant in addition to other mor-
pho-physiological changes (Fig.
5.1
). One of the most prominent among these is
a marked decrease in cell size of root tissues. This may be due to a decrease in
the elasticity of cell walls of the root as was previously reported by Barceló et al.
(
1986
). Heavy metal-induced reduction in the cell size includes all root tissue,
whether these are parenchymatous tissues in the root cortex and/or pith or sclerified
tissues in sclerenchyma or xylem vessels, thereby resulting in a shrinkage of root
diameter (Kasim
2006
).
A significant decrease in xylem vessels, in particular, metaxylem vessels may
significantly limit the movement of water and mineral nutrients from root to aerial
parts of the plant. Consequently, the plant might need to decrease water loss through
transpiration as was indicated by Greger and Johansson (
1992
) in sugar beet. This
is certainly of great ecological significance, particularly under a variety of abiotic
stresses. Gowayed and Almaghrabi (
2013
) also reported a reduction as a result of
heavy metal stress in root anatomical traits such as root diameter, central cylinder
diameter, cortex thickness, cross section area of root and cross section area of cen-
tral cylinder.
In view of a number of studies it is evident that reduction in root growth may be
due to a decrease in cell division that leads to increase the thickness of cell wall,
and/or a disorder in the activity and contents of phytohormones like auxins in the
roots exposed to heavy metals (Sharma and Dietz
2006
; Farzadfar and Zarinkamar
