SALINITY EFFECTS ON YIELD , YIELD COMPONENTS AND NUTRIENT IONS IN RAPESEED GENOTYPES

Soil salinity is a major restriction to crop production in many areas of the world. Seven spring types of rapeseed genotypes were evaluated in four salinity levels of irrigation water including 0, 4, 8 and 12 dsm. A factorial experiment based on completely randomized design with 3 replications was considered for evaluation of 28 treatments at green house condition. Significant mean square of salinity levels was observed for plant height, pods per plant, 1000seed weight, seed yield, calcium (Ca), potassium (K) and sodium (Na), indicating significant differences of salinity levels for these traits. The genotypes had significant differences for all the studied traits except Ca. Significant positive correlations were detected among plant height and seed yield and other yield associated traits including number of pods per plant, 1000-seed weight and K, therefore the genotypes with high plant height in saline environment will have high seed yield and yield associated traits. The genotype L18 with high mean values of 1000-seed weight and seed yield was more tolerant to salinity than the other genotypes.


Introduction
High salinity is a common abiotic stress factor that seriously affects crop production in some parts of the world, particularly in arid and semi-arid regions.Saline environments affect the plant growth in different ways such as a reduction of water uptake, an accretion of ions to toxic levels, and a reduction of nutrient accessibility.In some extensive reviews concerning strategies of overcoming the salinity problem, two primary lines of action were emphasized: reclamation of saltaffected soils by chemical amendments, and alternatively, the saline soils can be used to grow salt-tolerant plants (Ashraf and McNeilly, 2004).High concentrations of salts cause ion imbalance and hyperosmotic stress in plants.As a consequence of these primary effects, a secondary stress such as oxidative damage often will occur.High salt stress disrupts homeostasis in water potential and ion distribution.This disruption of homeostasis occurs at both the cellular and the whole plant levels (Tunuturk et al., 2011).Severe changes in ions and water homeostasis lead to molecular damage, growth apprehend and even death (Zhu, 2001).Tolerance of oilseed brassicas to salt stress is a complex trait, which is greatly modified by cultural, climatic and biological factors (Minhas et al., 1990;Kumar, 1995;Mahmoodzadeh, 2008).The amphitetraploids Brassica species including Brassica napus, B. carinata and B. juncea are more tolerant to salinity and sodicity than their respective diploid progenitors such as B. campestris, B. nigra and B. oleracea (Kumar, 1995).High salt concentration in root affects the growth and yield of many important crops.Salinity may reduce the crop yield by upsetting water and nutritional balance of plant (Francois, 1994;Islam et al., 2001).Water availability and nutrient uptake by plant roots is limited because of high osmotic potential and toxicity of sodium (Na) and chlorine (Cl) ions (Kumar, 1995).Saline soils and saline irrigation waters present potential hazards to canola production.Germination failures on saline soils are often the results of high salt concentrations in the seed planting zone because of upward movement of soil solution and subsequent evaporation at the soil surface.The most common adverse effect of salinity on the crop of Brassica is the reduction in plant height, size and yield as well as deterioration of the product quality (Zamani et al., 2010).There are differences in sensitivity to salinity among canola cultivars.Significant variation in seed germination and other growth stages among canola cultivars grown under salinity condition is widely reported by Puppala et al. (1999), Bybordi (2010), Zamani et al. (2010), andTunuturk et al. (2011).Calcium (Ca) and potassium (K) ameliorate the adverse effects of salinity on plants (Volkamar et al., 1998;Munnus, 2002;Amador et al., 2007).Salinity impairs the uptake of Ca by plants, possibly by displacing it from the cell membrane or in some way affecting membrane function (Lauchli, 1990;Rameeh et al., 2004).Gorham (1993) claimed that all plants discriminate to some extent between Na and K. Na can be substituted for K for uptake, and it is believed that similar mechanisms of uptake may operate for both ions (He and Cramer, 1992;Schorder et al., 1994;Porcelli et al., 1995).High levels of K in young expanding tissue are associated with salt tolerance in many plant species (Mer et al., 2000;Ashaf and McNeilly, 2004;Bandeh-Hagh et al., 2008).Closely allied to salt exclusion and its relationship to salt tolerance is the regulation of ion selectivity, in particular the role of Ca/Na and K/Na discrimination in salt tolerance (Sharma and Gill, 1994;Volkamar et al., 1998).He and Cramer (1992) reported that Ca could play a regulatory role in the responses of Brassica species to saline environments.Thakral et al. (1998) reported positive non-significant correlation between seed yield and K/Na in stress environment in B. juncea.These researchers also pointed out that in response to increasing salt stress K/Na decreased while chlorophyll, proline and protein contents increased.Das et al. (1994) claimed that increase in NaCl concentration was associated with increased Na and Cl influx and K efflux in B. campestris.Porcelli et al. (1995) reported when soil salinity and also sodium adsorption ratios (SAR) increased, K/Na and Ca/Na ratios in plants and also K-Na and Ca-Na selectivity decreased.Furthermore, its reduction in seed yield under saline environments occurs in genotypes with lower Na and higher K accumulation in leaves together with increased primary branch production (Sharma and Gill, 1994).
Due to variation of canola cultivars to salinity in different growth stages, the objective of the present study was to investigate the effect of salinity on the yield and yield associated traits of rapeseed genotypes and also to study the nutrient (K, Na and Ca) contents in the leaves and shoots under salinity stress and their relation to seed yield in order to obtain suitable criteria for salinity tolerance.

Greenhouse experiment
This experiment was carried out at Agriculture and Natural Resources Research Center of Mazandran, Sari, Iran in 2010.Seven diverse rapeseed genotypes including three breeding lines (L 14 , L 18 and L 111 ) and four cultivars (RGS003, Zarfam, Hyola401 and PF7045/91) were evaluated at four salinity levels of irrigation water including 0, 4, 8 and 12 dsm -1 under greenhouse conditions during 2010-11.A factorial experiment based on completely randomized design (CRD) with 3 replications was considered for evaluation of 28 treatments.The salt solution was prepared by taking NaCl: CaCl 2 in the ratio of 1:1 and the electrical conductivity of different salinity levels was adjusted by a direct reading conductivity meter.Soil analysis results are shown in Table 1.The soil belongs to the non-saline soil with a neutral reaction with the medium level of lime.Levels of nutrients, soil organic matter levels in the medium and other nutrients, including potassium, phosphorus, iron, manganese and copper are desirable.In each plot 10 seeds were planted in separate 8-liter pots and five plants were maintained for evaluating.Electrical conductivities of the saline treatments were increased to the desired levels by incremental additions of the salts over 10-day period to avoid osmotic shock to the seedlings.Plants in all pots were irrigated until saturation, with the excess solution allowed to drain into collection pans.All pots were maintained under farm conditions and also they were isolated from raining.The studied traits were plant height, number of pods per plant, 1000-seed weight, seed yield and ion concentrations in shoot including Ca, K and Na.

Laboratory experiment
For ions extractions, plant samples were ground by mill and then dried in an oven at 500ºC for 2 hours.After that, 5 ml of 2M HCl was added in each plant sample for digestion and then they were filtered and diluted by distilled water.The final volume of each sample was 100ml.An amount of K and Na of each final sample was measured by flame photometer and Ca was measured by atomic absorption (Isaac and Kerber, 1971).Pearson correlation was detected for all the traits.All studied traits were analyzed upon factorial experiments based on completely randomized design, whereas means comparisons were performed upon the least significant difference test (Gomez and Gomez, 1984).

Results and Discussion
Analysis of variance Significant differences of salinity levels were observed for plant height, pods per plant, 1000-seed weight, seed yield, Ca, K and Na, indicating influence of salinity levels for these traits (Table 2).Significant mean square of genotypes for all studied traits except for Ca indicated the significant differences among genotypes.Interaction effects of salinity levels × genotypes were significant for all traits except the plant height, indicating the different trend of variations among the genotypes at different salinity levels.Similarly, significant variations of impact of increasing salinity were reported for rapeseed cultivars, including interaction of salinity-cultivars for plant height, and other yield associated traits (Bybordi, 2010).

Means comparison of the salinity levels
The result of means comparison of salinity levels for the traits is presented in Table 3. Due to increasing salinity levels from 0 to 12 dsm -1 , plant height was decreased from 68.68 cm to 50.66 cm.Number of pods per plant was decreased at high salinity levels.1000-seed weight was less affected than the number of pods per plant by increasing of salinity levels.Although at high salinity levels seed yield of the genotypes decreased, at the second salinity level the most of genotypes had higher seed yield than at the first salinity level.Similarly, in previous studies (Islam et al., 2001;Mahmoodzadeh, 2008) it was reported that due to increasing salinity levels, yield and yield associated traits were reduced.Although at high salinity levels, level of Ca increased, there were not significant differences at different salinity levels.The mean value of K was found to decrease gradually at high salinity levels.In contrast to K, mean values of Na increased at high salinity levels, whereas at the fourth salinity level it was about ten times higher, comparing with the control.Earlier researchers (Volkamar et al., 1998;Munnus, 2002;Amador et al., 2007) reported that Ca and K ameliorate the adverse effects of Na on different plant traits.Salinity impairs Ca uptake in plants, possibly by displacing it from the cell membrane or affecting the membrane function (Lauchli, 1990;Rameeh et al., 2004).18.86b 10.38a S 1 : 0 dsm -1 , S 2 : 4 dsm -1 , S 3 : 8 dsm -1 and S 4 : 12 dsm -1 .Means, in each column, followed by at least one letter in common are not significantly different at the 1% level of probability -using LSD test.

Means comparison of the genotypes
The genotypes exhibited significant differences for plant height at different salinity levels (Table 4).The genotypes Hyola401, L 18 , L 111 and Sarigol with high plant height were classified in the same group.The genotypes expressed significant differences for number of pods per plant, and L 18 with 35.09 pods per plant had the highest value for this trait.The highest amount of 1000-seed weight was determined for L 14 , Hyola401 and RGS003.A significant positive correlation was detected between this trait and seed yield, therefore, most of the genotypes with a high amount of 1000-seed weight had high seed yield.Although most of genotypes were classified in the same group for level of Ca, the RGS003 and L 14 with 70.97 and 74.41 mg g -1 of Ca differed from the other tested genotypes.The highest mean values of K were obtained for Sarigol, L 111 and Zarfam, while the lowest levels of Na were determined for Zarfam.In earlier studies (Puppala et al., 1999;Zamani et al., 2010;Tunuturk et al., 2011).A significant genetic variability among rapeseed cultivars for salinity tolerance was reported at different growth stages related to nutrients uptake in saline environment.

Means comparison of the interaction effects of genotypes × salinity levels
Means comparison of interaction effects of genotypes × salinity levels for the studied traits is presented in Table 5. Water availability and nutrient uptake by plant roots are limited because of high osmotic potential and toxicity of Na and Cl ions (Kumar, 1995).Therefore, due to increasing salinity concentration, most of the yield associated traits were decreased.Although at high salinity levels plant height of all the genotypes decreased, the trend of its reduction was different depending on susceptibility of genotypes to salinity.RGS003 had the lowest plant height at the fourth salinity level comparing to control.Means value of yield components and seed yield of all the genotypes were decreased at high salinity levels but their increments of reduction were different for some of genotypes.Regarding number of pods per plant, Hyola401 was less susceptible than the other genotypes.L 18 with high mean values of 1000-seed weight and seed yield was more tolerant to salinity than the other genotypes.
Table 5. Means comparison of interaction effects of genotypes × salinity levels for plant height, number of seeds per pods, 1000-seed weight, seed yield and shoot ions including Ca, K and Na.Significant positive correlations were detected among plant height and seed yield and other yield associated traits including number of pods per plant, 1000seed weight and K (Table 6), therefore, the genotypes with considerable plant height expressed in saline environment will have high seed yield and yield associated traits.A significant positive correlation was also detected between 1000seed weight and Ca, therefore, this nutrient had an important effect on 1000-seed weight.A non-significant positive correlation was exhibited between seed yield and K, hence the genotypes with high amount of K will have high seed yield.High levels of K in young expanding tissue are associated with salt tolerance in many plant species (Mer et al., 2000;Ashaf and McNeilly, 2004;Bandeh-Hagh et al., 2008).There is a significant negative correlation between K and Na, indicating that Na can be substituted for K in saline environment.

Table 1 .
Some of physicochemical properties of soil sample.

Table 2 .
Analysis of variance for plant height, number of seeds per pods, 1000-seed weight, seed yield and shoot ions including Ca, K and Na.

Table 3 .
Means comparison of salinity levels for plant height, number of seeds per pods, 1000-seed weight, seed yield and shoot ions including Ca, K and Na.

Table 4 .
Means comparison of genotypes for plant height, number of seeds per pods, 1000-seed weight, seed yield and shoot ions including Ca, K and Na.

Table 6 .
Correlation among studied traits in rapeseed genotypes at different salinity levels.