Exogenous calcium improves growth and physiological responses of pistachio rootstocks against excess boron under salinity

Abstract

Although boron (B) is necessary for plant growth and development, excessive concentrations of it can be harmful. Toxicity due to boron can occur naturally in the soils of arid and semiarid areas. Soluble forms of Ca were applied as foliar sprays at 1.1 and 2.2 g L−1 on three pistachio rootstocks, namely Pistacia vera L. ‘Ghazvini’, P. vera L. ‘Badami-Riz’ and P. atlantica subsp. Atlantica. Effects of the foliar sprays were assessed on growth and physiological responses of the three rootstocks while an excess of B was added to the irrigation water under saline soil (ECe ≅12 dS/m). Four-month plants were irrigated with water containing B as boric acid (20 and 40 mg L−1). Calcium treatments occurred either as aminochelate or chloride, and were sprayed on the foliage once a week. The experiment lasted for 12 wk and terminated when 50% of the plants showed symptoms of B toxicity. Excess B retarded growth and affected physiological parameters including plant height, leaf number, leaf area, leaf dry weight, stem diameter, shoot dry weight, root length, root dry weight, leaf proline content, SPAD value and RWC. Boron concentrations increased in the plant tissues, due to the boric acid in the irrigation water, while foliar sprays of calcium reduced B content in the leaves, stems and roots. Applications of Ca-Chelate and CaCl2 (2.2 g L−1 each) significantly improved growth and physiological responses of all rootstocks. The Ca-Chelate was more effective than CaCl2 in ameliorating B toxicity. In terms of rootstock tolerance to B toxicity, ‘Ghazvini’ and ‘Atlantica’ were more tolerant than ‘Badami’. The results indicate that foliar sprays of Ca are effective in mitigating excess boron symptoms in pistachio rootstocks grown under saline conditions.