Arsenic Stress in Plants: An Inside Story - Crop Improvement: New Approaches and Modern Techniques - page 381

of plant. The leaf water potential decreases while the relative water content (RWC)
slightly decreases in toxic condition. All these changes, collectively lead to the
changes in important physiological activities of plant like photosynthesis, transpira-
tion, stomatal conductance etc.
Arsenic causes a reduction in photosynthetic rate (Miteva and Merakchiyska
2002 ). The reduced photosynthetic rate can be due to the many factors like result of
disturbance in assembling of the pigment protein complex and thylakoid membrane.
Arsenic can release protein; lipid and element components of thylakoid membrane
indispensable for photosynthesis activity of proteins especially connected with the
water splitting system and of lipid, glactolipid probably connected with PSII are lib-
erated. Another reason for decreasing the photosynthesis rate could be disturbance
in the pigment apparatus. The photosynthesis pigments are some of the most impor-
tant internal factor which in certain cases is able to limit the photosynthesis rate. It
is believed that they are target of toxic As effect (Miteva and Merakchiyska 2002 ).
There is considerable decrease of chlorophyll and carotenoid contents on exposure
to arsenic (Stoeva et al. 2005 ). Also photosynthetic reactions are closely related
with stomata behavior, due to closure of stomata in stress condition, diminish or
cessation of CO 2 uptake, by which photosynthetic rate also reduced (Biehler et al.
1996 ). The insufficient water supply in tissue may also induce photo inhibition,
which influence rate of electron transport. The transpiration processes also reduced
on exposure to arsenic probably is a result of the disturbed uptake and transport of
water, caused by the changes in root system. (Stoeva et al. 2005 ). As(V) affect cell
wall, primary and secondary metabolism, abscisic acid metabolism and germination
of the seedlings whereas, As(III) mainly affects hormonal and signaling processes
(Chakrabarty et al. 2009 ). Anarchy in mitotic and labeling index, mitotic arrays of
microtubules, increased percentage of metaphase and DNA fragmentation are also
observed in roots of Pisum sativum exposed arsenic (Faria et al. 2010 ).
BiochemicalChangesinPlantsExposedtoArsenic
At biochemical level plants shows many changes during exposure to arsenic. Some
of the constituents are decreased due to toxicity of arsenic and some are increased to
combat from detrimental effects of toxicant. Arsenate acts as a phosphate analogue
and is transported across the plasma membrane via phosphate co-transport system
(Ullrich-Eberius et al. 1989 ). Once inside the cytoplasm it competes with phosphate
from several vital reactions like in the glycolysis (metabolism of glucose) the con-
version of 1, 3 bisphosphoglycerate to 3 phosphoglycerate, ATP molecule is formed
by using inorganic phosphate but in the toxicity of arsenic, arsenate competes with
phosphate and give l-arseno-3 phosphoglycerate which hydrolyses spontaneously
to 3 phosphoglycerate without forming ATP so deprives the cell from the energy
sources. Also arsenate can replace phosphate group from DNA causing disorganize
the structure of nucleic acid which effects directly to gene function. Similarly arse-
nite is a well known thiol reagent that combines rapidly with dithiol groups on pro-