Evaluation of foliar-applied biostimulants and nutrients for enhancing drought tolerance in canola (Brassica napus L.)

Due to the adverse effects of climate change, drought has become a global phenomenon that significantly impacts agricultural productivity. This study focused on stimulating tolerance of canola (Brassica napus L.) plants against drought stress by applying different doses of sodium silicate (Na2SiO3), potassium silicate (K2SiO3), melatonin, ascorbic acid, salicylic acid, potassium nitrate (KNO3), and zinc sulfate (ZnSO4). The canola plants were grown under control (80% water holding capacity) and drought-stressed (50%) conditions and were sprayed three times at the three-leaf stage. The experiment was planned by a completely randomized design with six replicates. Morphological (plant height, fresh and dry biomass, and leaf area), physiological traits (chlorophyll content, leaf temperature, relative water content, and electrolyte leakage), and stomata properties were investigated. Drought depressed the growth of the canola plant, decreasing leaf area, plant fresh and dry weight. Plant fresh weight decreased by 35% and leaf area by 25%. Increased electrolyte leakage and stomata number were also observed under drought conditions. Electrolyte leakage was higher in drought (25.7%) than in well-water conditions (11.2%). However, KNO3 treatments effectively mitigated the adverse effects of drought stress on plant growth by regulating chlorophyll content, relative water content, and electrolyte leakage. Principal component and hierarchical cluster analysis have revealed that plant fresh weight, dry weight, leaf area and relative water content were primarily affected by drought, and stomata properties and electrolyte leakage are secondarily critical traits in the drought tolerance of canola plants. In conclusion, foliar applications of KNO3 should be recommended to induce plant growth in canola under drought stress. Stomata properties, particularly stomata number, may also be considered as selection criteria for drought-tolerant canola plants.