Possible involvement of some secondary metabolites in salt tolerance of sugarcane
Introduction
Increased salinity affects primary carbon metabolism, plant growth and development by ion-toxicity, induced nutritional deficiency, water deficits and oxidative stress (Sairam and Tyagi, 2004; Flowers, 2004). Moreover, it modulates the levels of secondary metabolites, which are physiologically important particularly in stress tolerance. Some of these metabolites have light absorptive properties, harvest light for photosynthesis and protect the cells from damaging effects of high energy radiation, while others promote defensive action against herbivores and pathogens (Harborne and Williams, 2000; Taiz and Zeiger, 2002). Most secondary metabolites are synthesized from the intermediates of primary carbon metabolism via phenylpropanoid, shikimate, mevalonate or MEP pathways (Buchanan et al., 2000). Among those, chlorophylls, carotenoids (CAR) and phenolics are commonly studied metabolites in plant kingdom. Enhanced synthesis of determined secondary metabolites under stressful conditions is believed to protect the cellular structures from oxidative damage (Buchanan et al., 2000), in addition to osmotic advantage to the plants (Chalker-Scott, 1999; Winkel-Shirley, 2002; Close and McArthor, 2002).
Concerning direct involvement in photochemical reactions of leaves, total chlorophyll (CHL) and CAR are the most important secondary metabolites. Decrease in CHL content is associated to enhanced expression of chlorophyllase activity under stress (Majumdar et al., 1991). The CAR (carotenes and xanthophylls), besides acting as accessory light harvesting pigments, show antioxidant properties (de Pascale et al., 2001). They protect the photosystems by: (a) reacting with lipid peroxidation products to terminate chain reactions; (b) scavenging singlet oxygen and dissipating the energy as heat; (c) reacting with triplet-excited chlorophyll molecules to prevent formation of singlet oxygen; or (d) dissipating excess of excitation energy through the xanthophyll cycle (Rmiki et al., 1999). Steady levels of CAR were tightly correlated with salt tolerance of mungbean (Wahid et al., 2004). Less reduction in CHL and steady levels of CAR are salinity tolerance strategies of sugarcane (Kanhaiya, 1996).
The soluble phenolics (PHE) produced by the phenylpropanoid or shikimate pathways are powerful antioxidant in plant tissues under stress (Dixon and Paiva, 1995; Sgherri et al., 2004). They are chemically heterogeneous compounds, mainly flavonoids, lignins and tannins, which play a variety of roles, e.g. defense against herbivores and pathogens, mechanical support, attract pollinators, absorb high energy radiations and reduce the growth of nearby competing plants (Harborne and Williams, 2000; Taiz and Zeiger, 2002). Recently the role of phenolics has been reviewed because of their great involvement in the oxidative stress tolerance instead of herbivory (Close and McArthor, 2002).
Among flavonoids, the anthocyanins (ANT) are highly water soluble pigments derived from flavonoid precursors via the shikimate pathway that accumulate in the vacuole (Chalker-Scott, 1999). Their accumulation has been seen in various plants including Morus alba (Ramanjulu et al., 1993), Arabidopsis (Mita et al., 1997) and Hedera helix leaves when grown in the presence of sugars (Murray et al., 1994). ANT production is beneficial to the plants in terms of protection of shade-adapted chloroplasts from brief exposure to high solar radiation (Gould et al., 2000; Lee and Gould, 2002), protection of leaves from photo-oxidative damage during senescence (Feild et al., 2001), and reduced damage to photosynthetic systems by absorbing UV-B (Burger and Edwards, 1996). Since the ANT are osmotically active, their enhanced expression may increase hardiness through increased osmotic control (Chalker-Scott, 1999). Kaliamoorthy and Rao (1994) reported up to 40% accumulation of ANT in maize as a salinity stress response. Furthermore, ANT accumulate under UV-B (Mendez et al., 1999), drought (Balakumar et al., 1993), low temperature (Krol et al., 1995), nutrient deficiency (Rajendran et al., 1992) and exposure to ozone (Foot et al., 1996). Flavones (FLA), another category of flavonoids, are formed by oxidation of flavanones (Justesen et al., 1997; Buchanan et al., 2000). The enzymes of parsley have been extensively studied for the biosynthesis of FLA (Winkel-Shirley, 2002). Their biosynthesis is induced by fungal attack or at later developmental stages of plants (Justesen et al., 1997). Like ANT, the FLA glycosides also accumulate in vacuole and possibly H+-antiporters are involved in their sequestration into this cellular compartment (Frangne et al., 2002). However, modulations in their levels under abiotic stresses require investigation.
Sugarcane (Saccharum officinarum L.) is a major source of sucrose and therefore ubiquitous in cultivation. Although it is ranked as moderately salt-sensitive (Francoise and Maas, 1999), there are differences in salt resistance (Wahid et al., 1997). Primary metabolism of sugarcane has been well studied under normal or saline conditions; however, there is a lack of information on the biosynthesis and role of secondary metabolites in sugarcane under salinity, although they accumulate in low levels under normal conditions (França et al., 2001). It is assumed that accumulation of secondary metabolites enhances the sugarcane capacity for salt tolerance. Therefore, the aim of this study was to determine time course changes in the levels of some secondary metabolites and their physiological implications in salt tolerance of sugarcane.
Section snippets
Materials and methods
Sugarcane clones CP-4333 and HSF-240 were previously tested and confirmed tolerant and sensitive to NaCl salinity with average EC50 of 12.5 and 6.8 dS m−1, respectively, at the formative stage (Wahid et al., 1997; Ghazanfar, 2004). Twenty single-noded sets were planted in field plots (measuring 1.5 m×4 m×0.3 m deep) lined with double layer of polythene before refilling with loam soil, and 12 uniform sprouts were finally retained. The analysis of soil revealed the following properties: sand 39%, silt
Growth, photosynthesis and ionic characteristics
Although shoot length decreased in both clones under salinity, it showed maximum reduction in HSF-240 (43%) and minimum reduction in CP-4333 (21%) at final sampling time (Table 1). These changes led to significant ( ) differences among treatments, harvests and clones. There was no significant ( ) reduction in shoot dry weight, although CP-4333 performed relatively better (30% d.w.) than HSF-240 (48% d.w.) under salinity stress at final sampling time (Table 1). Comparisons among salt
Discussion
Glycophytes exposed to saline conditions usually show enhanced levels of ions under which they are grown. Excess of toxic ions modulates the levels of both primary and secondary metabolites as assessed from apparent growth and tissue analysis (Sairam and Tyagi, 2004; Winkel-Shirley, 2002; Flowers, 2004). Sugarcane clones under this study were affected by salinity as displayed by elongation and dry weight of shoots and expansion of leaves. The growth inhibition was associated to the reduction in
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