Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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Z . Dajic
ological traits associated with maintaining water relations and photosynthesis (e.g.
different pathways of carboxylation, such as C 4
, intermediate C 3
-CAM and CAM) (Dajic
et al., 1997a). Additionally, various metabolic changes, such as the maintenance of ion
and molecular homeostasis (e.g. synthesis of compatible solutes necessary for osmotic
adjustment), detoxification of harmful elements and growth recovery, which depends
mainly on various signaling molecules, occur under exposure to salt/drought stress
(Xiong and Zhu, 2002).
Increasing salinity in the growth medium decreases content of chlorophyll
and the net photosynthetic rate, which is expressed more conspicuously in salt-sensitive
plants, such as alfalfa (Khavarinejad and Chaparzadeh, 1998) and canola (Qasim et
al., 2003). Under salinity treatment, two wheat cultivars expressed two phases of photosynthetic
inhibition: in the first phase, photosynthetic reduction was gradual, whereas
in the second phase it was rapid and accompanied by a decline of the energy conversion
efficiency in photosystem II, strongly related to adverse effects of salinity (Muranaka
et al., 2002). Reduction of net CO 2
assimilation with salinity in tomato and sunflower
was related to decrease in stomatal conductance and stomatal density (Romeroaranda
et al., 2001; Rivelli et al., 2002b). The decrease was due to reduced CO 2
assimilation
associated with a decline in stomatal conductance, water use efficiency and Rubisco
activity, as well as slower electron transport of photosystem II under severe salt stress.
In many halophytic species regulation of the water regime is associated with
the type of CO 2
fixation. Certain halophytes, originating from the tropics and subtropics,
utilize the CAM (Crassulacean Acid Metabolism) pathway of carboxylation. Water
availability is the major selective factor for evolution of the CAM pathway in plants,
where nocturnal CO 2
fixation saves loss of water by transpiration and increases wateruse
efficiency (Larcher, 1995). Induction of the CAM pathway in the common ice plant
(Mesembryanthemum crystallinum) under stress conditions is dependent on its biochemical
machinery, which enables an increase in PEP-carboxylase and other CAM
enzyme activities (Michalowski et al., 1989, Thomas et al., 1992), as well as enzymes
involved in synthesis of compatible solutes, particularly pinitol (Vernon and Bohnert,
1992). The change from C 3
-photosynthesis to CAM in M. crystallinum is elicited by
salt stress and drought (Winter and Lüttge, 1979), and the kinetics of CAM induction
depends on the strength of the stress and the developmental stage of the plant (Cushman
et al., 1990a). Moreover, the stress-induced switch from C 3
to CAM may be linked with
the ABA-induced activity of vacuolar ATPase in adult plants, while vacuolar Na +
compartmentation is regulated through ABA-independent pathways in M. crystallinum
(Barkla et al., 1999). The perennial cactus Cereus validus, having constitutive CAM,
exhibits adaptations at the whole-plant level which differ from those of the annual
CAM-inducible common ice plant, for example regulation of turgor and gas exchange,
and metabolic adjustment at the cellular level and molecular level (Lüttge, 1993). Evaluation
of signal transduction events involved in the induction of CAM in the common
ice plant revealed that transcript abundance of Ppc1, a gene encoding the CAM-specific
isoform of phosphoenol pyruvate carboxylase, rapidly increased during osmotic
stress (Taybi and Cushman, 1999).