The effects of NaCl on ion content and compatible solutes in Canola (Brassica napus L.) cultivars

This study was conducted to investigate the effect of salinity caused by sodium chloride on the content of ions and understand the level of resistance in four varieties of Canola (Brassica napus L.) namely Talaye, Sarigol, Zarafam, and Opera. The plants were grown in hydroponic solution (1/4 strength Hoagland solution) and greenhouse conditions. When the plants reached the 5-leaf stage, they were treated with concentrations of 0 (control), 50, 75, and 100 mM sodium chloride for 14 days and then harvested. In order to measure the content of nutritional elements of the plant organs, extracts of the desired organs were prepared.
The results of the experiments showed that with increasing salinity, the potassium content of shoots and roots decreased in all 4 cultivars in response to salinity stress, while the content of chlorine and sodium increased. The highest increase of chlorine and sodium elements in response to salinity stress was observed in Talaye and Sarigol cultivars, about 1000 times more than the control plants. The increase in the sodium content of the shoot was higher than that of the root and it increased about 800 times in the roots of Talaye and Sarigol cultivars, it increased about 800 times. Proline and glycine betaine contents of leaves increased in response to salt stress. The amount of soluble sugar in shoot and root also increased in some cultivars, so in the treatment of 100 mM, its amount in the leaves of Talaye variety increased by 43% and in Sarigol by 80%. According to the obtained results, it can be concluded that among the cultivars studied, Opera and Zarafam cultivars have better performance under salt stress than the other two cultivars, and the performance of Sarigol is weaker than other cultivars.

Salinity is a widespread environmental stress that restricts crop production worldwide. Soils in Iran, particularly in arid and semi-arid zones, are affected by salinity stress. Under salt stress, plants show reduced water potential, ion toxicity, and ion uptake interference. Crop plants mainly respond to salinity using various mechanisms, including avoidance or tolerance. Almost all crops are glycophytes and cannot inhibit Na+ and Cl- accumulation in shoots and its out-coming damage to leaf tissues (Munns and Tester, 2008). Tolerant plants produce different osmolytes, such as soluble sugars, proline, glycine betaine, and toxic ion compartmentation. Canola is one of the most important oilseed crops used for edible oil production due to the lower content of saturated fatty acids (Brand et al., 2001). Iran has a potential capacity of up to 5 million tons/year of canola production, while it is not more than 190 thousand tons/year (Zarafshani et al., 2017). McNeilly and Ashraf (2004) concluded that salt-tolerant rapeseeds have lower concentrations of sodium and chlorine in their aerial parts when faced with salinity. Plant species show different capacities of tolerance when exposed to high salinity. This study was conducted to investigate the effect of salinity caused by sodium chloride on the content of ions and to determine salt tolerance ability in four varieties of Canola (Brassica napus L.): Talaye, Sarigol, Zarfam, and Opera. This study aimed to determine the salt tolerance capacity of four different cultivars of rapeseed considering, the content of ions, and the effects of chlorine and sodium ions on physiological and biochemical parameters.

One-week-oldseedlings of the above mentioned four cultivars of canola were transferred to ¼ strength Hoagland solution and grown for five weeks under greenhouse conditions and different NaCl treatments (0, 50, 75, and 100 mM).
100 mg ground dry mass of shoots and roots were added to hot water. Then, each extract (10 ml) was used to determine K+, Na+, and Cl- ions content using flame photometry and chloride analyzer based on titration with silver ions methods. The soluble sugars were measured in the samples extracted by ethanol based on the Dubois et al. (1954) method. The proline contents of leaves and roots were determined using Bates et al. (1973). The extracts of leaves and roots in sulfosalicylic acid (3%) were transferred to toluene after adding acetic acid, phosphoric acid and, ninhydrin reagents to the mixture. Then, the optical density of the upper part of the solution was read at 520 nm using a spectrophotometer. All calculations were made according to a standard curve plotted by known different concentrations of proline. The glycine betaine contents were measured according to Grattan and Grieve (1983). Ground dry mass of leaves and roots were incubated in sulfuric acid and dichloroethane for 1 and 2.5 hours, respectively, and the optical density of the extracts was determined at 360 nm 

In this study, salinity stress has increased the sodium content of shoots and roots in all four cultivars. This increase is more in Talayah and Sari-Gol cultivars than the other two cultivars. Also, the sodium content of the shoots is higher than the roots. In some cultivars of the rapeseed plant, the barrier mechanisms that prevent sodium from entering the roots, less Na accumulation in the aerial organs, and maintaining high ratios of potassium to sodium, especially in young photosynthetic tissues, probably have increased tolerance to salinity (Atlasi Pak, 2016). However, in all cultivars, the potassium contents of root and shoot significantly decreased under different salinity. Other researchers also considered the increase of sodium absorption in the roots and its competition with potassium absorption under salt stress as the cause of the decrease in the amount of potassium in the roots (Mittal and Dubey, 1991). The reduction of potassium in the roots and shoots has also been reported in rapeseed under salinity. The researchers stated that the amount of potassium in the shoot is directly related to the growth rate as brassica species that have more ability to retain potassium will be more tolerant to salinity (He and Cramer, 1993). The results of the present research showed that the chlorine content of aerial parts and roots increased with the application of salinity. It was also observed that the amount of chlorine in aerial parts is about 3 times higher than the sodium content of aerial parts. It could be due to the higher inhibitory effects of Cl on plant growth rather than Na in Canola (Teakle and Tyerman 2010). According to the results obtained in this study, changes in osmolytes concentrations showed different patterns. This variation was higher in soluble sugars than the others. However, proline and glycine betaine contents increased with increasing salinity in all cultivars. Various studies showed a positive relationship between the accumulation of osmolytes, such as GB and proline, and stress tolerance in plants. Some studies showed that an increase in the concentration of these two osmolytes under stress is not an adaptive stress response. The accumulation of GB in response to salinity has been found in many crops, including corn (Sorghum bicolor) (Weimberg et al., 1984). 

According to the results, salinity treatment causes changes in the ion content of roots and shoots including a higher content of Na and Cl and a lower content of K. In addition, based on the highest amounts of chlorine in aerial organs in all four cultivars (3 times greater than Na), it can be concluded that the decrease in growth in all cultivars could be related to chlorine toxicity, and sodium has intensified the effects. Proline and glycine betaine contents increased under salt stress in four cultivars. Overall, the results showed that Opera and Zarafam cultivars have higher capacities to tolerate salinity than the others.