Environmental Engineering Reference
In-Depth Information
rate may be due to the noxious effects of nickel on other metabolic processes rather
than the regulation of stomata (Papazogloua et al.
2007
). For example, increased
nickel concentration resulted in photosynthetic reduction that ultimately damaged
the chloroplast structure (Bethkey and Drew
1992
), reduced functioning of chlo-
roplast (Molas
2002
; Boisvert et al.
2007
) or, caused breakdown of photosynthetic
pigments, reduction in the synthesis of chlorophyll (Seregin and Kozhevnikova
2006
), induced changes in electron transport system (Singh et al.
1989
) and in-
hibited enzymes activities (Seregin and Ivanov
2001
). Chloroplast size may also
reduce significantly along with, in addition to granum and thylakoid membranes
disorganization and deformation and also changes in the lipid composition present
in chloroplast membranes (Molas
1997
).
A mechanism that could also be involved in inhibition of photosynthesis could
be reduced formation of components of light and dark reactions. During light reac-
tion, nickel is known to impair the electron transport system (Krupa and Baszynski
1995
). In many plants, PSII is the key site of Ni-induced ETC disruption (Mohanty
et al.
1989
; Krupa and Baszynski
1995
; Maksymiec
1997
). Veeranjaneyulu and Das
(
1982
) showed that the dominant sites of nickel deposition are lamella regions of
chloroplast. Furthermore, Cyt. b
6
f and b
559
, ferredoxin as well as plastocyanin in
thylakoid membranes also reduce by the increased Ni concentration. Due to these
changes, efficiency of electron transport may be decreased (Veeranjaneyulu and
Das
1982
). Secondly, photosynthesis may be decreased by metal-induced changes
in the activities of key enzymes involved in the regulation of Calvin cycle (dark
reaction) (Sheoran et al.
1990
; Krupa and Baszynski
1995
). The deleterious effects
of toxic metals on metabolic processes may also result in direct inhibition of pho-
tosynthesis.
3.2.2
Chlorophyll Contents
Exposure to heavy metals can significantly affect the concentrations of photosynthet-
ic pigments as has been reported in a number of plant species (McIlveen and Negu-
santi
1994
; Balaguer et al.
1998
; Gajewska et al.
2006
; Seregin and Kozhevnikova
2006
). A decrease in concentration of photosynthetic pigments mainly chlorophyll
(
a
,
b
, total), xanthophylls and carotenoids have been reported by several authors,
such as Krupa et al. (
1993
), Pandey and Sharma (
2002
), Gajewska et al. (
2006
) and
Ahmad et al. (
2007
).
Heavy metals may also hamper the uptake of other essential nutrients like Fe,
Mg, Mn, Zn and Cu, and causing their deficiency in a plant body (Krupa and
Baszynski
1995
; Maksymiec
1997
). Deficiency of essential nutrients, in particu-
lar Mg, Mn and Fe, can directly lead to reduced chlorophyll synthesis (Gajewska
et al.
2006
; Shukla and Gopal
2009
). In addition, extreme toxicity of heavy metals
can lead to the breakdown of existing chlorophyll in chloroplast (Checkai et al.
1986
; Voss
1993
; Bennett
1993
). Thus, metal-induced decrease in photosynthetic
pigments and/or deterioration of ultra-structure of pigments may result in decreased
photosynthetic rate.
