8.85 ± 1.31
0.37 ±0.09 (45)
0.12±0.02 (15) 0.17±0.06 (2)
5.22 ±1.33 (59)
"Calculated values related to total Na concentration are given in parentheses. Data of each row within each parameters followed by the same letter are not significantly different (P<0.05)
»(Hajibland and Joudmand 2009)
Fractionation of Na in the leaves showed that proportional Na in cell sap (mainly vacuole) was higher in IC, and by contrast, the proportional Na in residual fraction (comprised mainly from cell wall) was lower in this cultivar (Table 16.6). Allocation of more Na to the cell sap may result in facilitating control of water balance of leaf cells and causes an improvement of cell expansion and production of broader leaves (Hajiboland and Joudmand 2009) . Finally, salinity stress is known to induce CAM photosynthesis in the facultative CAM species, such as Mesembryanthemum crystallinum L., (Aizoaceae) and Sedum album (Crassulaceae) (Cushman and Bohnert 2002). Crassulacean acid metabolism (CAM) is a metabolic adaptation of plants to environments with limited availability of water because of restricting CO2 uptake to the dark hours and reduction of tran-spirational water loss.
The presence of Na in the environment and its uptake by plants can reduce the amount of K required to meet the plants basic metabolic requirements. K functions in plants can be summarized as both biophysical (non-K-specific role as an osmoti-cum in the vacuole) and biochemical (specific and nonspecific roles in the cytoplasm). The need of monovalent cations in some plant species can also be filled by Na, thus reducing the required critical level of tissue K. In natrophilic species such as sugar beet with a high ability for substitution of K by Na, in old leaves nearly all the K can be replaced by Na that made K available for specific functions in mer-istematic and expanding tissues. Sodium alleviates K-deficiency symptoms and decreases the critical foliar K concentration at which K-deficiency symptoms appeared (Subbarao et al. 2000).
rice and sugarcane and some cyperaceous plants (Marschner 1995). The beneficial effects of Si are particularly distinct in plants exposed to abiotic and biotic stresses. Epstein and Bloom (2005) have recently established a new definition for essential elements in higher plants. According to these authors, an element is essential that fulfills either one or both of the following criteria: (1) the element is part of a molecule which is an intrinsic component of the structure or metabolism of the plant, and (2) the plant can be so severely deficient in the element that it exhibits abnormalities in growth, development, or reproduction, i.e., "performance", compared to plants with lower deficiency. Accordingly, Si will be an essential element for higher plants, which is to be generally accepted in the near future. Over last two decades, extensive studies have been performed aiming at understanding of the possible mechanism(s) for Si-enhanced tolerance of higher plants to both abiotic and biotic stresses (Liang et al. 2007). More recently, rapid progress has been also made in Si uptake and transport in higher plants. The uptake of Si was found to be the result of two different transport mechanisms. A low affinity transporter (Lsi1) found on the lateral roots of rice plants is responsible for the uptake of silicic acid from the external solution to the root cortical cells (Ma and Yamaji 2006). The transporter has been localized on the distal cells of exodermis and endodermis. A second transporter has also been identified in rice which is responsible for xylem loading of Si (Mitani and Ma 2005).
In this section we review current knowledge on the roles of Si in conferring tolerance to plants against abiotic stresses. Because of a well-documented role of Si in the plants resistance against biotic stress factors such as pathogens, we also give a brief overview on this effect of Si.
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