Water Regime And Photosynthesis Under Salt Stress

The effects of drought and salt stress on plants are tightly related, since the first responses of plant cells under these stress conditions are induced by osmotic shock. Thus, upon exposure to osmotic stress, plants exhibit many common adaptive reactions at the molecular, cellular and whole-plant level (Greenway and Munns, 1980; Yeo, 1998; Bohnert et al., 1995; Zhu et al., 1997). These include morphological and anatomical alterations (life cycle, xeromorphic features, increased root/shoot ratio), and physi ological traits associated with maintaining water relations and photosynthesis (e.g. different pathways of carboxylation, such as C4, intermediate C3-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 photo-synthetic 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 CO2 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 CO2 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 CO2 fixation. Certain halophytes, originating from the tropics and subtrop-ics, 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 CO2 fixation saves loss of water by transpiration and increases water-use 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 C3-photosynthesis to CAM in M. crystallinum is elicited by salt stress and drought (Winter and Luttge, 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 C3 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 (Luttge, 1993). Evaluation of signal transduction events involved in the induction of CAM in the common ice plant revealed that transcript abundance of Ppcl, a gene encoding the CAM-specific isoform of phosphoenol pyruvate carboxylase, rapidly increased during osmotic stress (Taybi and Cushman, 1999).

A significant number of halophytes are C4 species, which are characterized by their higher requirements for sodium ions compared with C3 species (Brownell and Crossland, 1972). In conditions of osmotic stress and high temperatures, C4 plants have an advantage in comparison with C3 plants, because of their ability to carry on photosynthesis when stomata are to a large extent closed, coupled with the absence of photorespiration in the mesophyll cells (Larcher, 1995). Photosynthetic responses to salinity in the halophytic tribe Salsoleae (family Chenopodiaceae) have been reviewed, with particular attention paid to relations between the C4 NAD-ME (malic enzyme) Salsoloid type of carboxylation and the chloroplast structure (Voznesenskaya et al., 1999).

Abscisic acid is well recognized as an important stress hormone. The concentration of ABA increases when water deficits occur, with its de novo synthesis beginning in the roots, in response to sensing an insufficient supply of water (Zhang et al., 1989). In halophytes, which grow in conditions of "physiological drought", due to low water potential in the root medium, the lowest concentrations of ABA were found under salinity concentrations optimum for growth (Clipson et al., 1988). Such case was reported for the highly tolerant halophytic species Suaeda maritima, which exhibited the lowest seasonal range of ABA contents (from 649.4 ng g-1 to 835.6 ng g-1 dry weight) in comparison with several other species, where higher ABA concentrations were correlated with increased sodium content of the shoot (Dajic et al., 1997a).

In glycophytes, salinity leads to the accumulation of ABA (Asch et al., 1995), as in tomato (Chen and Plant, 1999; Yurekli et al., 2001) and wheat (Aldesuquy and Ibrahim, 2002). In bush bean plants exposed to 75 mM NaCl, inhibition of leaf expansion was mediated by ABA rather than by Na+ or Cl- toxicity, and the increase of ABA induced by a salt-pretreatment limited the accumulation of Na+ and Cl- in the leaves, resulting in adaptation to salinity stress (Montero et al., 1997). Besides the significant role of ABA, favorable effects of other hormones in plant responses to salinity, such as cytokinins (Kuiper and Steingrover, 1991) and gibberellins (Kaur et al., 1998; Ashraf et al., 2002) have been documented.

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