Environmental Engineering Reference
In-Depth Information
•
−
•
as O
2
, particularly in chloroplasts and mitochondria (Mittler
2002
; Masood et al.
2006
). Plants possess a number of antioxidant enzymes such
as superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione
reductase (GR) for protection against the damaging effects of ROS (Asada
1992
;
Prochazkova and Wilhelmova
2007
), but ROS-linked salinity stress can cause
membrane disorganization, metabolic toxicity and attenuated nutrients (Frommer
et al.
1999
; Zhu
2000
; Costa et al.
2005
) These initial effects can then induce more
catastrophic events in plants. Excessive salt stress can eventually cause photoinhi-
bition and photodamage of PSII (Krause and Weis
1991
; Belkhodja et al.
1994
).
(ii) Strong salt stress in salt-sensitive species can severely reduce the potential of
electron transport in PSII (Jungklang et al.
2003
). (iii) Salinity can increase or
decrease uptake of some nutrients (e.g. Fe, Mn, Cu, Zn, K, etc.) depending on
the plant species (Vıllora et al.
2000
; Turhan and Eris
2005
; Wang and Han
2007
;
Achakzai et al.
2010
; Tunçtürk et al.
2011
; Greenway and Munns
1980
; Martinez
et al.
1987
; Cornillon and Palloix
1997
; Alpaslan et al.
1998
). The increase in these
metals can enhance complexation with the PSI and PSII functional groups, lead-
ing to ROS production. High Na
+
content is generally responsible for alteration of
the nutrient balance, which can cause specific ion toxicity in addition to disturb-
ing the osmotic regulation (Greenway and Munns
1980
). (iv) Due to the complex
formation between metals and PSII functional groups, electron excitation at low
irradiance can induce effective generation of H
2
O
2
and ROS. This can be justified
by the in vivo observation of ROS generation inside PSII membranes. Salt stress
may thus damage the photosynthetic activity of PSII even at low irradiance (Pandey
et al.
2009
). (v) Complexation of trace metal ions with functional groups bound
to PSII under salinity conditions can enhance electron release and, as a conse-
quence, ROS production (see chapter
“
Complexation of Dissolved Organic Matter
With Trace Metal Ions in Natural Waters
”
). Such effects are able to photodamage
PSII in
Chlamydomonas reinhardtii
, barley leaves (
Hordeum vulgare
), sorghum
(
Sorghum bicolor
), rye (
Secale cereal
), and
Spirulina platensis
(Neale and Melis
1989
; Sharma and Hall
1991
; Hertwig et al.
1992
; Lu and Zhang
1999
).
Chl content in salt-tolerant plants would either remain the same or be sig-
nificantly enhanced with increasing salinity (Qiu and Lu
2003
; Brugnoli and
Björkman
1992
), and accumulation of compatible solutes (e.g. proline, betaine,
polyols, sugar alcohols, and soluble sugars) in many plants can increase the toler-
ance of PSI and PSII to salt stress (Chen and Murata
2002
; Fulda et al.
1999
; Zhu
2002
; Reed and Stewart
1988
). The increase of Na
+
and Cl
−
ions in both leaves
and roots is accompanied with an increase in proline and soluble sugars which
could play a role in salt tolerance (Melgar et al.
2008
; Ahmed et al.
2008
).
While functioning in an otherwise similar way as non-tolerant plants, salt-tolerant
plant species may supply relatively low amounts of salt ions to leaves through roots.
The consequence may be the occurrence of relatively low contents of H
2
O
2
. If the
latter be present in moderate amount, it would mostly be used in photosynthesis and
would not produce dangerous levels of HO
•
. Therefore, the plant may maintain nor-
mal photosynthesis in the presence of high salt levels. Salt tolerance in canola is asso-
ciated with the ability to reduce uptake and/or transport of saline ions (Bybordi
2010
).
, H
2
O
2
and HO
