Research Article
Salinity Tolerance in Cotton (Gossypium hirsutum L.) Genotypes
Iran Cotton Research Institute, Iran
Ratnakar J. Thengane
Department of Botany, Pune University 411007, India
Cotton is a dual-purpose crop, widely used for fiber and oil throughout the world. It is placed in the moderately salt-tolerant group of plant species with a salinity threshold level 7.7 dS m1. Its growth and seed yield being severely reduced at high salinity levels and different salts affect the cotton growth to a variable extent. Salinity of agricultural lands and irrigation water is a major environmental constraint to crop productivity in many arid and semiarid regions of the world. It is especially prevalent in irrigated agriculture and in marginal lands associated with poor drainage on high water tables. Estimates for the extent of salinity damage vary from 25 to 50% of the worlds irrigated land (Postel, 1989). The stresses imposed by salinity are mainly due to ion compositions and concentrations in rhizosphere and also in plant tissues (Volkamar et al., 1998). Salinity occurs mainly as a result of either an accelerated redistribution of salts in the soil profile due to high water tables in most areas, or the use of insufficient irrigation water to leach salts out of the soil (Ashraf, 1994). Further selection of some genotypes that could be tolerance for growing in salinity lands is necessary for the establishment of plants at the early stage of growth. Saline environments affect plant growth in different ways, including a decrease in water uptake, an accumulation of ions to toxic levels and a reduction of nutrient availability (Ashraf, 1994). Ca2+ and K+ ameliorate the adverse effects of salinity on plants (Volkamar et al., 1998). Salinity impairs the uptake of Ca2+ by plants, possibly by displacing it from the cell membrane or in some way affecting membrane function (Lauchli, 1990). Gorham (1993) claimed that all plants discriminate to some extent between Na+ and K+. Sodium can be substituted for K+ for uptake and it is believed that similar mechanisms of uptake may operate for both ions (Schroeder et al., 1994). High levels of K+ in young expanding tissue are associated with salt tolerance in many plant species (Khatum and Flowers, 1995). Closely allied to salt exclusion and its relationship to salt tolerance is the regulation of ion selectivity, in particular the role of Ca2+/Na+ and K+/Na+ discrimination in salt tolerance (Maas and Grieve, 1989). Porcelli et al. (1995) reported that when soil salinity and Sodium Adsorption Ratios (SAR) increased K+/Na+ and Ca2+/Na+ ratios in plants and also K+/Na+ and Ca2+/Na+ selectivity decreased. Ahmad et al. (2002) reported that salinity increases Na+ and Cl and decreases K+, Ca2+ and Mg2+ in leaves of cotton on the other hand reduces uptake of Nitrogen and Phosphor in cotton. They also have been used K+/Na+ as a successful selection criterion for salt tolerance in cotton.
An analysis of principal components often reveals relationships that were not previously suspected and thereby allows interpretations that would not ordinarily result. Analyses of principal components are more of a means to an end rather than an end in them, because they frequently serve as intermediate steps in much larger investigations. (Johnson and Wichern, 2002). The essential purpose of factor analysis is to describe, if possible, the covariance relationships among many variables in terms of a few underlying, but unobservable, random quantities called factors (Johnson and Wichern, 2002). Means for determining the relationship between traits in a stress environment, factor analysis can be efficient. The main applications of factor-analytic techniques are to define a few factors (less than the number of studied traits) (Dillon and Goldstein, 1984). Factor analysis has been used to determine structural factors related to growth traits and yield components in some crop.
In order to obtain selection criteria based on stress and non-stress environment, some selection criteria, including Geometric Mean Productivity (GMP), Stress Intensity (SI), (Fischer and Murrer, 1978) Stress Tolerance Index (STI), (Fernandez, 1993) and tolerance index (TOL) (Rosielle and Hamblin, 1981) were defined. The range of SI estimates is between zero and one and the larger value of SI indicates the more severe stress intensity. A larger value of TOL represents relatively more sensitivity to stress, thus a smaller value of TOL is favoured. The higher value of GMP and STI for a genotype indicates its stress tolerance and yield potential. The objectives of this study one hand were to determine the traits and stress tolerance indices that were more related to salinity in Gossypium hirsutum cultivars and their F1 in early stages of growth and the other hand to identify salt-tolerant lines and their F1 in early stages of growth.
Seven diverse cotton (Gossypium hirsutum L.) genotypes were crossed in a half diallel fashion and then their 21 F1 progenies along with their parents were evaluated in sand cultures under normal and saline environments in a Randomized Complete Block Design (RCBD) with three replications at the greenhouse of Research Farm of Botany Department of Pune University. (73°, 51 E longitude, 18°, 31 N latitude and altitude 559 m) during January to February 2005. Sterilized seeds were germinated in Petri dishes at 25±2°C for 4 days. Eight uniform seedlings were transplanted into plots that separated in two sandboxes filled with washed and sterilized river sand, covered with polythene beads and after establishment, five plants were maintained for evaluation. Temperatures during the experiment were averaged 30.21/9.6°C (day/night) and relative humidity was 39.43-86.04% and the photoperiod was 14 h. Plants were given deionized water up to 10 days after transplanting and saline and non-saline (control) grown plants were irrigated thereafter every 2 days with half-strength Hoglands nutrient solution (Hoagland and Arnon, 1950) with NaCl (EC = 23.8 dS m1) and without it (EC = 0.94 dS m1) and pH = 7. Electrical conductivity of the saline treatment was increased to the desired level by incremental addition of the salt over 10 day period to avoid osmotic shock to the seedlings. Plants in both environments were irrigated until saturated, with the excess solution allowed to drain under sandboxes. Plants in both environments were harvested 50 days after planting at 7-8 leaf stages. The characteristics were shoot dry weight (SDW) in g/plot, plant height (cm), root length (cm), shoot dry weight Ca2+, K+ and Na+ contents (mg g1), Ca2+ /Na+ and K+/Na+. The harvested plants were washed with distilled water and then they were dried in an oven for 72 h at 80°C to a constant weight. Plant samples were ground by mill and then dried in a furnace at 500°C for 2 h in order of ion extraction. After that, plant samples were added to 5 mL of 2 M HCL for digestion and then digested solutions were filtered and diluted by distilled water. The final volume of each sample was 100 mL. Sodium and K+ levels of each sample were measured by flame photometry and Ca2+ was measured by atomic absorption spectrophotometry (Isaac and Kerber, 1971). Selection criteria indices, including geometric mean productivity as GMP = √ (Yp)x(Ys), (Fischer and Murrer, 1978) stress tolerance index as STI = (Yp)x(Ys)/Yp2, (Rosielle and Hamblin, 1981) and tolerance index TOL = Yp-Ys (Fernandez, 1993) were calculated for yield of shoot dry weight in non-saline (Yp) and saline (Ys) environments. TOL was also calculated for Ca2+, K+, Na+, Ca2+/Na+ and K+/Na+. Stress intensity as (SI = 1-(Ys/Yp)), (Fischer and Murrer, 1978) was estimated for all studied characteristics (Ys and Yp are the means of shoot dry weight in saline and non-saline environments, respectively). All studied traits were combined analysis based on two environments (Gomez and Gomez, 1984).
Combined analysis based on salinity levels revealed significant environment effects for all studied traits except for root length (Table 1). Root length mean in saline environment was more than non- saline environment. Significant effects of genotypes and their interactions with salinity levels were observed for all studied traits except root length and RL/SH, which indicated that genotypes have had diverse behavior in two salinity levels. Stress-intensity estimates for Shoot Dry Weight (SDW), root length, plant height, shoot dry weight Ca2+, K+ and Na+ contents, Ca2+/Na+ and K+/Na+ were 0.19, -0.056, 0.31, 0.27, -1.15, 0.05, -0.31, 0.55, 0.39, -0.22 and -0.52, respectively (Table 1). On the basis of SI, it was concluded that root length, Na+, Ca2+, Ca2+/Na+ and K+/Na+ were increased and shoot dry weight was decrease more than other rest traits by salinity (Table 1). The mean of Na+ in a saline environment was two times more than the mean of Na+ in a non-saline environment; therefore, although SI was not calculable for it and other traits but negative index of SI is indicator of increasing of traits in salinity environment (Table 1). However, Porcelli et al. (1995) has reported that with increasing salinity, plant height, root length, Ca2+/Na+ and K+/Na+ were decreased and Na+ was increased.
In a non-stress environment, no significant correlations were observed between shoot dry weight and all traits except Ca2+/Na+ (0.604**) and SDW/SH (0.51**). The means of shoot dry weight in a non-saline environment varied between 1.040 and 2.023 g (Table 2) and the three top crosses and one top parental cultivar for shoot dry weight were Kiokorovax Koker, Sindosex Koker and SahelxKiokorova and Belizovar, respectively in a non-saline environment. The means of root length in a non-saline environment varied between 7.167 and 12.167 cm (Table 2) and the three top crosses and two top parental cultivars for root length were KiokorovaxBelizovar, SahelxTabladila, TabladilaxSindose, Belizovar and Koker in a non-saline environment, respectively. The means of plant height in a non-saline environment varied between 19.20 and 32.30 cm (Table 2) and the three top crosses and one top parental cultivar for plant height were KiokorovaxSindose, SiokraxBelizovar, SiokraxKoker and Kiokorova in a non-saline environment, respectively (Table 2).
Significant positive correlation was observed between shoot dry weight and root length (0.588**) in a stress environment. The means of shoot dry weight in a saline environment varied between 0.905 and 1.767 g (Table 3) and the two top genotypes and two top parental cultivar for plant height were SiokraxKoker, Sindosex Koker and Siokra in a saline environment, respectively (Table 3). No significant correlation was observed between shoot dry weight and root length in a saline environment. The means of root length in a saline environment varied between 7.733 and 14.730 g (Table 3) and the three top genotypes and one top parental cultivar for root length were TabladilaxKoker, SiokraxKoker, SindosexKoker and Siokra in a saline environment, respectively. The means of plant height in a saline environment varied between 17.133 and 21.167 cm (Table 3) and the two top crosses and one top parental cultivar for plant height were Sahel x Sindose, Sahelx Belizovar and Kiokorova a saline environment, respectively (Table 3).
Most of the crosses with high shoot dry weight in a saline environment had one of the two parent including Siokra and Sindose , therefore, the shoot dry weights of parents in saline stress can be used as good criteria for the prediction of shoot dry weight in their crosses.
Table 1: | Analysis of variance for shoot dry weight , root length, plant height, shoot fresh weight, shoot dry weight Na+, K+, Ca2+, K+/Na+ and Ca2+/Na+ contents, SDW/SFW, RL/SH in a non-saline and saline environment |
*, ** Significant at p = 5 and 1% , respectively |
Table 2: | Means of shoot dry weight, root length, plant height, shoot fresh weight and shoot ions, including Na+, K+, Ca2+, K+/Na+ and Ca2+/Na+ , SDW/SFW, RL/SH in a non-saline environment |
On the basis of GMP, STI and TOL indices for the genotypes, it was concluded that Siokra as a parent and Sindosex Koker as a cross had preference to other parents and crosses in shoot dry weight totally (Table 4). Significant positive correlation was observed between fresh weight and shoot dry weight (0.531**) in a stress environment. The means of fresh weight in a saline environment varied between 7.220 and 12.05 g (Table 3) and therefore top genotype and top parental cultivar for fresh weight were TabladilaxKoker and Kiokurova in a saline environment, respectively.
The shoot dry weight Na+ content varied from 17.768 to 31.112 mg g1 and the two top crosses and one top parental cultivar for shoot dry weight Na+ content were SahelxSindose, SahelxBelizovar and Sindose, respectively in a saline environment. On the basis of GMP, STI and TOL indices for Sindose (15.83, 2.73 and -12.97, respectively) as a parent and SahelxSindose (14.587, 2.319 and -13.161, respectively), SahelxBelizovar (16.381, 2.92 and -22.487, respectively) as a crosses had preference to other parents and crosses in shoot dry weight Na+ content totally (Table 3).
The shoot dry weight K+, Ca2+, K+/Na+, Ca2+/Na+ contents and SDW/SFW, RL/SL traits varied between (17.596 and 28.235), (47.441 and 54.979), (0.849 and 1.313), (1.697 and 2.893), (12.747 and 19.059) and (36.537 and 77.004) mg g1, respectively.
Significant positive correlation was estimated shoot dry weight with root length (0.588**), Ca2+/Na+ (0.531*), SDW/SFW (0.545**), RL/SL (0.615**) and significant negative correlation with K+ (-0.452*) in a saline environment. Therefore this trait can be used as good selection criteria for improving shoot dry weight in a saline environment. Some researchers have emphasized the importance of Ca2+, K+, Ca2+/Na+ and K+/Na+ for salinity tolerance (Maas and Grieve, 1989).
On the basis of GMP, STI and TOL indices (21.76, 0.852, -11.461), (42.655, -1.225, -18.847), (1.404, 0.327, 0.605) and (3.146, 0.612, 0.711) of shoot dry weight K+, Ca2+, K+/Na+ and Ca2+/Na+ contents distinguished Sahel as a parent, TabladilaxBelizovar, KiokorovaxSiokra and BelizovarxKoker as a crosses, respectively (Table 4).
High concentrations of K+ in young expanding tissue are associated with salt tolerance in many plant species, (Khatum and Flowers, 1995) therefore, genotypes with low contents of K+, are suitable for salinity tolerance. Calcium ameliorates the adverse effects of salinity on plants (Volkamar et al., 1998; Lauchli, 1990) therefore, the genotypes in which amounts of Ca2+ increased in a saline environment can be considered suitable for salinity tolerance. Although shoot dry weight has positive correlation with plant Ca2+ content but it is not significant.
Table 3: | Means of shoot dry weight, root length, plant height, shoot fresh weight and shoot ions, including Na+, K+, Ca2+, K+/Na+ and Ca2+/Na+ , SDW/SFW, RL/SH in a saline environment |
On the basis of Significant positive correlation between shoot dry weight with root length, shoot fresh weight and SDW/SFW these characteristics and RL/SH and its significant negative correlation with shoot dry weight K+ contents can be considered as indirect selection criteria for improving shoot dry weight in a saline environment. Shoot dry weight has not significant correlation with other traits.
Sairam and Tyazi (2004) also reported that Salinity stress response is multigenic, as a number of processes involved in the tolerance mechanism are affected, such as various compatible solutes/osmolytes, polyamines, reactive oxygen species and antioxidant defense mechanism, ion transport and compartmentalization of injurious ions. Various genes/cDNAs encoding proteins involved in the above mentioned processes have been identified and isolated. For this reason recognition of dependent and principal effects of all characteristics on the independent characteristic estimated by factor analysis method. However the result of factor analysis based on minimum eigenvalue by means of principle component analysis extraction method and rotation method of Varimax with Kaiser Normalization revealed four factors for eleven studied traits in a saline environment. Factor one was detected as morphological trait in which RL/SH, SDW/SFW, root length had a high coefficients factor loading, respectively (Table 5). Factor two was detected as nutritional, factor three and four were detected as morphological and nutritional factors dependent on their high coefficients factor loading simultaneously. The results of factor analysis indicated that selection for morphological traits, specifically; that selection based on K+, Ca2+ and K+/Na+ should be more efficient than other traits. Cluster classification of genotypes on the basis of value of four principal for each genotype (Table 6) by means of principal component analysis of correlation matrix distinguished genotypes SahelxBelizovar, Siokra and SindosexKoker from other genotypes in salinity tolerance (Fig. 1).
On the basis of factor analysis, it was suggested that in the first factor as a morphological factor some traits including shoot dry weight, root length, SDW/SFW and RL/SH have high coefficients factor loading could be used for selection criteria and also in second factor as a nutritional factor shoot dry weight Na+, K+/Na+ and Ca2+/Na+ content that have high coefficients factor loading could be used for selection criteria on the basis of their correlation with shoot dry weight.
Table 4: | Geometric mean productivity (GMP), stress tolerance index (STI) and tolerance index (TOL) estimates for shoot dry weight , shoot dry weight/shoot fresh weight (SDW/SFW) and shoot dry weight ions, including Ca2+, K+, Na+, Ca2+/Na+ and K+/Na+ |
Fig. 1: | Cluster classification of genotypes on the basis of value of four principal for each genotype by using ward method |
Table 5: | Estimated factor loadings for eleven studied traits in Gossypium hirsutum cultivars and their F1 in a saline environment factor analysis |
Table 6: | Values of the four principal components for the all of genotypes with 85.278% cumulative variance |
Although Salinity tolerance in early stages of plant growth may not necessarily be a good indicator of salt tolerance for adult plants. However, some traits including shoot dry weight and its relation and correlation with root length, SDW/SFW and RL/SH, shoot dry weight Na+, K+/Na+ and Ca2+/Na+ content as a criteria could be used in early stage of growth.