Growth and Some Physiological Parameters of Four Sugar Beet (Beta vulgaris L.) Cultivars as Affected by Salinity

INTRODUCTION

Salinity is an important limiting factor for agricultural production by causing two distinct types of stress in plants: hydric stress, caused by the greater difficulty of water absorption and ionic stress, associated to the sodium ion effect on the diverse cellular functions, decreased nutrient absorption, enzyme activities, photosynthesis and metabolism (Zhu, 2001). Salt stress causes inhibition of growth and development, reduction in photosynthesis, respiration and protein synthesis and disturbs nucleic acid metabolism (Levine et al., 1990).

Changes in these parameters depend on the severity and duration of stress (Lakshmi et al., 1996) and on plant species (Dubey, 1994).

Under salt stress, plants have evolved complex mechanisms allowing for adaptation to osmotic and ionic stress caused by high salinity. These mechanisms include osmotic adjustment by accumulation of compatible solutes such as glycinebetaine, prolines and polyols (Bohnert et al., 1999) and lowering the toxic concentration of ions in the cytoplasm by restriction of Na⁺ influx or its sequestration into the vacuole and/or its extrusion (Hajibagheri et al., 1987; Binzel et al., 1988).

The adaptation to salinity stress is accompanied by alternations in the levels of numerous metabolites, proteins and mRNA (Serrano, 1996). Various genes, expression of which is activated in response to salt stress, have been identified (Kawasaki et al., 2001). Some of these genes encode for protective proteins such as osmotin (Zhu et al., 1995), Late Embryogenesis Abundant (LEA) proteins (Espelund et al., 1992) and ion transporters (Blumwald, 2000) others code for enzymes that participate in metabolic processes specifically triggered by salinity stress (Gong et al., 2001).

The aim of the present study is to assess four sugar beet varieties for their salt tolerance and to give more information on the significance of soluble sugar accumulation under salt stress, also growth and gas exchanges were studied.

MATERIALS AND METHODS

Sugar beet (Beta vulgaris L.) seeds were prepared from Agricultural Research Center, Tehran, Iran in June. Sugar beet seeds were sterilized in 5% (w/v) sodium hypochlorite (15 min) and washed five times with sterile distilled water. Seeds germinated in pots containing sand in a growth chamber under a 24°C temperature and at a relative humidity of 70%. Germinated seeds were translated to pots in growth chamber with 17 h light periods and 300 μmol quanta m^-2 sec^-1 light intensity, day/night temperatures of 25/18°C and irrigated with Hoagland`s solution. After 10 days, the seedlings were transplanted in the saline nutrient solutions containing 0, 25 and 50 mM sodium chloride, with pH 6.5 and fresh nutrient solution replaced the old one every week. The plants were grown under controlled environment (17 h light periods, 300 μmol quanta m^-2 sec^-1light intensity, day/night temperatures of 25/18°C) in a greenhouse. After 21 days of experimental duration, for each physiological analysis from each treatment, four plants were harvested. Photosynthetic rate (P_n), respiration rate and CO₂ compensation concentration (Γ) were determined from intact plants, employing an infrared gas (CO₂) analyzer (IRGA) as described by Khavari-Nejad (1980, 1986). Growth analyses were carried out using the equations of Watson (1952) and Evans and Hughes (1962). Saccharides (soluble sugars and starch) were measured using phenol sulfuric acid method according to Hellubust and Craigie (1978).

The research was conducted using completely randomized design with four replications. Data were analyzed statistically using SAS software.

RESULTS AND DISCUSSION

In 25 and 50 mM NaCl, P_n was significantly increased in ET5, but for other cultivars changes were not significant. For the four cultivars, NaCl caused reduction of respiration rate, however, with increasing NaCl, Γ only in ET5 was significantly decreased and in other cultivars changes were not significant (Table 1).

For the two sugar beet cultivars, ET5 and C3-3, 25 mM NaCl caused a significant increase for all growth parameters involved (Table 2). However, in 50 mM NaCl, changes were observed not to be significant as compared to those of controls. Reduction in growth parameters were observed at 25 and 50 mM NaCl for cultivars of 41RT and 19669-T. In 25 mM NaCl treatment, LWCA in ET5, C3-3 and 41RT were significantly increased in comparison with other treatments (Table 2), however, in 50 mM NaCl LWCA only in ET5 was increased.

Soluble sugar also was affected by NaCl treatment, with a greater increase as the NaCl concentration was increased. It was the highest in cultivars ET5 and C3-3. And in cultivars 41RT and 19669-T changes of soluble sugar were not significant as compared with control. For insoluble sugar, it appeared that it was the less affected parameter in comparison with others in all cultivars (Table 3).

Sugar beet is a glycophytic member of the Chenopodiaceae. It is sensitive to elevated salinity at the germination and early seedling phase of development (Durrant et al., 1974; Ghoulam and Fares, 2001). Established plants showed a high osmotic adjustment (Katerji et al., 1997) and accumulation of glycinebetaine, proline and inorganic ions under salt stress (Hanson and Wyse, 1982; Heuer and Plaut, 1989; Gzik, 1996).

In the present study, the presence of 25 mM NaCl in the nutrient solution, increased growth generally in ET5 cultivar. However, in other cultivars, with increasing NaCl, growth either did not significantly changes or decreased. McCue and Hanson (1992) reported that leaf expansion in beet cultivar Great Western D-2 declined steadily as NaCl concentration was raised. Also, the growth of habituated sugar beet Altissima callus was inhibited by

Table 1:	Effects of NaCl on photosynthetic rate (P_n), respiration rate and CO₂ compensation concentration

Means (±SE) of four replications, Numbers followed by the same letter(s) are not significantly different (p>0.05)

Table 2:	Effects of NaCl on growth parameters

Means (±SE) of four replications, Numbers followed by the same letter(s) are not significantly different (p>0.05)

Table 3:	Effects of NaCl on saccharides (soluble sugar and insoluble sugar) concentration

Means (±SE) of four replications, Numbers followed by the same letter(s) are not significantly different (p>0.05)

NaCl concentrations higher than 30 mM (Hagege et al., 1990). Similar results were reported for other species such as Atriplex prostrate, where leaf area, dry mass of leaves and roots were significantly reduced by increasing salinity but the number of nodes was not affected by salt treatment (Wang et al., 1997).

As shown, the sugar beet cultivars 41RT and 19669-T showed the greatest reductions of growth parameters under salt stress (Table 2). Thus, they could be judged as the less tolerant and ET5 and C3-3 as the more tolerant, cultivars.

Salt treatment induced a reduction in LWCA in 19669-T (Table 2). The decrease in LWCA indicated a less turgor that resulted in limited water availability for cell extension process. LWCA in 25 and 50 mM NaCl in ET5 cultivars, was significantly enhanced.

Under salt stress, the tested cultivars accumulated more soluble sugar in leaves of tolerant cultivars (ET5 and C3-3) than that of sensitive cultivars (41RT and 19669-T). This accumulation of soluble sugars could play an important role in osmotic adjustment, in stressed sugar beet plants.

With increasing NaCl in the root growing medium, photosynthetic rate was significantly increased in ET5 cultivar in comparison with other cultivars. Also in 25 and 50 mM NaCl in ET5, respiration rate and CO₂ compensation concentration were significantly increased. However, changes in the rates of gas exchanges in other cultivars were not significant.

Results obtained in the present research revealed a more accumulation of soluble sugars in the ET5 cultivars treated with NaCl than other cultivars. Accordingly, it may be concluded that high soluble sugars play an important role in turgor maintenance.

Also, with increasing NaCl in solution LWCA was significantly enhanced in ET5 cultivars. As described earlier (Tester and Davenport, 2003) an ability to grow in saline conditions has been attributed to an ability to close stomata. In fact both glycophytes and halophytes tend to show reduced stomatal conductance in high NaCl conditions (Ball, 1988; Robinson et al., 1997; James et al., 2002).

It is concluded that Beta vulgaris cv. ET5 plants are much more tolerant to 50 mM NaCl than other cultivars of Beta vulgaris.

CONCLUSION

Although use of ions for osmotic adjustment may be energetically more favorable than biosynthesis of organic osmolyte under osmotic stresses, many plants accumulate organic osmolytes to tolerate osmotic stresses. These osmolytes include proline, betaine, polyols, sugar alcohols and soluble sugars (Chinnusamy et al., 2005). In present study has shown sugar beet plants (cv. ET5) have an ability to change the osmotic potential under saline condition and soluble sugars play a main role in the regulation of osmotic potential, also, their Leaf Water Content Area (LWCA) is enhanced in comparison with other cultivars.

ACKNOWLEDGMENTS

This research was funded by a grant from the Tarbiat Moallem University and the seeds were kindly provided from Sugar Beet Seed Institute.