Introduction: Soil salinity in arid and semi-arid regions is one of the important factors that limits crop production, and affects plant yield through various mechanisms. Chlorophyll destruction and reduction of leaf area due to high sodium concentration leads to a decrease in photosynthesis. The application of humic acid and amino acid with hormonal effects and adjustment of nutrients has been suggested to reduce salinity stress. The experiment aimed to investigate the effect of plant growth biostimulants and salinity levels on the growth indices of two canola cultivars in greenhouse condition.

Materials and Methods: This study was carried out as a factorial study based on a completely randomized design with three replications. The three experimental factors included: 1) salinity factor in three levels (0.571, 4, and 8 dS m-1), 2) canola cultivars in two levels (Hyola 50 and Dalgan), and 3) types of plant growth biostimulants in four levels (amino acid, humic acid, a combination of these two biostimulants, and a control treatment with no biostimulants application). Before the flowering stage (8 weeks after planting), the plants were harvested and some morphological traits including plant dry weight, root dry weight, and the concentration of calcium, sodium, and potassium in the shoot were measured.

Results and Discussion:

Based on the results, the cultivar Hyola 50 (8.35 g plant-1) had a significantly higher shoot dry weight than the cultivar Dalgan (7.58 g plant-1). The reduction in shoot dry weight at a salinity level of 8 dS m-1 was 19.3% compared to the treatment of 0.571 dS m-1. At salinity levels of 0.571 and 4 dS m-1, there was no difference between the effects of biostimulant treatments compared to the control (without biostimulants). But at a salinity level of 8 dS m-1, the shoot dry weight of the humic acid treatment (7.26 g plant-1) was significantly higher than the control treatment (16.6 g plant-1) (17.9% difference). The effect of humic acid application on leaf area (748 cm2 plant-1) was greater than that of amino acid foliar application (670 cm2 plant-1). With an increase in salinity level to 8 dS m-1, the dry weight of roots decreased. The highest dry weight of the root in the Dalgan cultivar at 8 dS m-1 was obtained in the combined treatment (0.607 g plant-1) and for the cultivar Hyola 50 in the combined treatment and amino acid (0.607 g plant-1). There was a significant difference in root dry weight between salinity of 4 dS m-1 (0.703 g plant-1) and 8 dS m-1 (0.543 g plant-1). The root dry weight of the plant was more affected by salinity stress than the shoot dry weight. In both canola cultivars, at a salinity level of 8 dS m-1, the chlorophyll index (SPAD) increased compared to a salinity level of 0.571 dS m-1 (4.6% increase in Dalgan cultivar and 9% in Hyola 50 cultivar). The shoot potassium concentration of the Dalgan cultivar (11.6%) was significantly higher than that of the Hyola 50 cultivar (92.5%), but the amount of uptake was higher in Hyola 50 cultivar (492.0 versus 459.0 g plant-1). At 8 dS m-1 salinity, the furthest potassium to sodium concentration ratio of the Dalgan cultivar was obtained with amino acid and humic acid treatment (4.87) and with the Hyola 50 cultivar with the humic acid treatment (4.21). At 8 dS m-1 salinity, the calcium to sodium concentration ratio of the Dalgan cultivar was obtained from the amino acid and humic acid treatment (12) and for the Hyola 50 cultivar from the humic acid, amino acid, and combined treatment (11). At a salinity level of 8 dS m-1, combined treatment of biostimulant in both Dalgan (1236 meq l-1) and Hyola 50 (250 meq l-1) cultivar had the highest amount of soil-soluble chlorine. This treatment may have reduced the absorption of chlorine ions by the plant. Shoot dry weight had the highest correlation with root dry weight and leaf area (0.58**). Shoot dry weight had the negative correlation with plant sodium concentration (-0.27**). The negative effect of soil-soluble sodium concentration on root dry weight (-0.39**) was greater than on shoot dry weight (-0.27**).

Conclusion: The positive effect of the application of growth biostimulants was observed through improving potassium absorption and improving the ratio of calcium and potassium to sodium concentrations in the shoot. In saline conditions (8 dS/m), the recommended phosphorus fertilizer for Dalgan variety must be 23.6%, and for the Hyola 50 variety, 10% more phosphorus fertilizer than in non-saline conditions. The application of Hyola 50 and humic acid as fertigation in saline conditions is recommended for canola.