Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 143

decreased cell moisture content or increased oxidative stress, resulting in peroxidation
of membrane lipids. Other authors have reported similar chloroplast structural and
functional changes in the leaves of Ni-treated plants; such changes ultimately produce
chlorosis and leaf necrosis (Khalid and Tinsley 1980 ; Ewais 1997 ; Sheoran et al.
1990 ; Piccini and Malavolta 1992 ).
Nickel may also reduce the photosynthetic rate by inhibiting components of the
light and dark reactions. Nickel is known to disrupt electron transport during the
light reaction (Krupa and Baszynski 1995 ; Malkin and Niyogi 2000 ). Photosystem
II (PSII) is the primary site of Ni-induced electron transport chain (ETC) inhibition,
and in that regard is similar to how other metals act (Mohanty et al. 1989 ; Krupa and
Baszynski 1995 ). Veeranjaneyulu and Das ( 1982 ) showed that the predominant sites
of Ni deposition are in lamellar regions of chloroplasts that contain PSII. In addition,
Ni also reduces cyt. b 6 -f and b 559 , ferredoxin, and plastocyanin in thylakoid mem-
branes. Moreover, Ni may inhibit photosynthesis by affecting key enzymes involved
in the operation and regulation of the Calvin cycle (dark reaction). Key enzymes of
the dark reaction that are affected by metals, other than Ni, include the following:
rubisco, 3-PGA kinase, F-1,6-bisphosphatase, aldolase, and phosphoglyceraldehyde
dehydrogenase (both NAD- and NADP-dependent) (Sheoran et al. 1990 ). Inhibition
of these dark reaction enzymes leads to the accumulation of light-reaction products,
such as ATP and NADPH. This, in turn, produces a high pH gradient across the
thylakoid membrane, which blocks PS-II activity (Krupa and Baszynski 1995 ).
In summary, metal effects on metabolic processes during the light and dark reac-
tions may cause direct inhibition of photosynthesis. These metabolic alterations
inhibit plant growth and disrupt morphogenesis. Notwithstanding, the disruption of
photosynthesis by Ni cannot be attributed to any single factor, and appears to derive
from a combination of factors that disrupt chloroplast structure and function, chlo-
rophyll content and photosynthetic protein complex functioning.

9.5 Water Relations

Plant water relations are among the most important parameters that are affected by
environmental stresses (Kramer and Boyer 1995 ). However, whether they are
affected by heavy metal stresses is unclear, as there are contrasting reports in the
literature on the effects of metal stress on plant water relations (Barcelo and
Poschenrieder 1990 ; Menon et al. 2005 ; Vernay et al. 2007 ). Furthermore, little
research has been conducted on the effects of heavy metals on plant water relations,
and most such work has addressed other stressors, such as drought, salinity and
temperature. Still, many authors believe that there is evidence to support the view
that metal stress does affect imbalances in plant water relations (Sharma and Sharma
1987 ; Barcelo and Poschenrieder 1990 ; Chatterjee and Chatterjee 2000 ).
Unfortunately, this topic is fraught with considerable complexity and different opin-
ions exist that vary with the type of metal, the concentration of the metal studied,
plant species or genotypes tested, exposure time, etc. (Kastori et al. 1992 )