Salicylic acid and biochar improve drought tolerance in Borago officinalis L. by enhancing antioxidant enzymes, leaf proline and soluble sugars

Water scarcity has become an increasingly important factor contributing to crop yield decline in arid regions. Soil amendments and certain growth regulators are some strategies used to mitigate the negative effects of drought stress. This study aimed to investigate the effects of biochar and salicylic acid on some physiological and biochemical properties of Borago officinalis L. under water deficit conditions. This experiment was conducted as a split-plot factorial experiment based on a randomized complete block design with four replicates. The factors were irrigation regimes according to the percentage of water requirement (100% WR, 75% WR, and 50% WR) in the main plots, biochar application (0, 5, and 10 ton ha-1) and foliar application of salicylic acid (0, 0.5 mM) in the subplots. This study was conducted in the research field of Shahrood University of Technology, Shahrood, Iran, in two cropping years, 2017 and 2018. The results showed that the total amount of soluble sugars decreased with an increase in the irrigation regime; however, the leaf antioxidant enzymes (SOD, CAT, and POX) and leaf proline concentration significantly increased in the 75% WR irrigation regime and slightly decreased in the 50% WR irrigation regime in both years. Applying salicylic acid (0.5 mM) significantly increased the leaf proline by 50% and 55% in irrigation regimes of 100% WR and 75% WR, respectively, in both years. It also modified the total amount of soluble sugars in both years. In addition, salicylic acid also stimulated leaf antioxidant enzymes under 100%WR under 75% WR and 50% WR irrigation regimes in both years. Biochar application reduced leaf proline concentration by 27% and 28% (10 ton ha-1) under 100% WR and 75% WR irrigation regimes, respectively. An improvement of all antioxidant enzymes via biochar application was found in both years. Application of 5 ton ha -1 biochar mitigated relative water content in 75% WR. Our results clearly indicated that applying exogenous salicylic acid in combination with biochar could be a promising approach to improve plant stress tolerance mechanisms under water deficit conditions.