Environmental Engineering Reference
In-Depth Information
expansion in both roots and leaves in salt-sensitive plants (Bybordi
2010
; Tunçtürk
et al.
2011
; Melgar et al.
2008
; Pandey and Yeo
2008
; Pandey et al.
2009
; Bybordi
et al.
2010a
,
b
; Flowers et al.
1977
; Munns and Termaat
1986
; Zidan et al.
1990
;
Ashraf and Wu
1994
; Neumann et al.
1994
; Evans
1996
; Jungklang et al.
2003
;
Meloni et al.
2003
; Qiu and Lu
2003
; Lee et al.
2004
; Pal et al.
2004
; Suwa et al.
2006
; Ali et al.
2007
; Desingh and Kanagaraj
2007
; Šiler et al.
2007
; Ahmed et al.
2008
). It has been shown that some physiological responses (e.g. chlorophyll and
carotenoids) are initially increased at moderate NaCl levels, but they are generally
decreased by increasing salinity. It has also been observed that cations or metal
ions in all plant parts are typically increased with an increase in salt stress.
The effects of salinity are mostly linked to a decrease in stomatal conductance and/
or to the non-stomatal limitation related to carbon fixation (Bongi and Loreto
1989
;
Brugnoli and Björkman
1992
; Delfine et al.
1998
,
1999
; Centritto et al.
2003
). It is sug-
gested that stomatal limitation prevails at intermediate salinity levels, while the non-
stomatal limitations predominate under severe salt stress conditions (Bongi and Loreto
1989
). The photosynthetic rate, PSII efficiency, root and shoot growth of
Centaurium
erythraea
is increased or remains the same at moderate salt levels (50-200 mM NaCl),
but it is decreased significantly at high salt concentration (400 mM NaCl). Root
growth is more adversely affected by increasing NaCl concentration than shoot growth
(Šiler et al.
2007
). Chlorophyll contents are decreased under elevated salinity condi-
tions for some salt-sensitive plant species, but they are not modified at moderate salt
levels (Jungklang et al.
2003
; Lee et al.
2004
; Šiler et al.
2007
; Delfine et al.
1998
,
1999
; Ashraf et al.
2002
). This suggests that the decline of chlorophyll content depends
on the salinity level, on the time of exposure to salts and on the plant species. Salinity
can rapidly inhibit root growth and subsequently decrease the uptake of water and
essential mineral nutrients from soil (Neumann
1997
). An increase of NaCl concen-
tration in solution can reduce N and NO
3
concentrations in leaves, when plants are
treated with NaCl and NH
4
NO
3
(Bybordi
2010
). An apparent increase in salt tolerance
is observed when N levels, supplied under saline conditions, exceed the optimum ones
observed under non-saline conditions (Bybordi et al.
2010a
; Papadopoulos and Rendig
1983
). This indicates that increased fertilization, especially by N, may improve the del-
eterious effect of salinity (Ravikovitch and Porath
1967
).
A contribution to salt stress in salt-sensitive plants may derive from the fact that
an increase of salinity can enhance the metal ion contents in plant cells, because
metal ions can form complexes with PSII functional groups. As already men-
tioned, such a complexation may cause either a high production of photoinduced
electrons (e
−
) and of superoxide anion (O
2
•
−
•
, which can damage
PSII, or block further photoinduced generation of electrons from PSII itself.
Conversely, the plant growth at moderate levels of NaCl might also be favored
by photoinduced generation of H
2
O
2
from PSII-metal complexes. If moderate, such
H
2
O
2
levels could be favourable to photosynthesis as discussed before (Eq.
3.1
).
The balance is delicate, however, because excessive salt can cause high production
of H
2
O
2
and HO
), H
2
O
2
and HO
•
that can damage the PSII. These proposed mechanisms can be
justified by the observation of several physiological functions caused by salt stress,
such as: (i) salinity stress in plants can produce reactive oxygen species (ROS) such
