Abscisic Acid, Paclobutrazol, and Salicylic Acid Alleviate Salt Stress in Populus talassica × Populus euphratica by Modulating Plant Root Architecture, Photosynthesis, and the Antioxidant Defense System

Abstract

The exogenous plant growth regulators (PGRs) represent a useful strategy for reducing the adverse effects of salt stress in plants. In order to investigate the regulatory effect of exogenous PGRs on the salt tolerance of Populus talassica × Populus euphratica seedlings, in this study, the effects of different foliar spray concentrations of ABA (5 mg·L−1, 15 mg·L−1, 25 mg·L−1), PP333 (300 mg·L−1, 900 mg·L−1, 1500 mg·L−1), and SA (40 mg·L−1, 120 mg·L−1, 200 mg·L−1) on P. talassica × P. euphratica seedlings under salt stress (soil salt concentration of 2%) were determined. The results showed that the dry weight, total root length, root surface area, root volume, total Chl content, and photosynthetic parameters of P. talassica × P. euphratica seedlings significantly decreased under salt stress and increased their contents of malondialdehyde (MDA), hydrogen peroxide (H2O2), superoxide anion (O2−), free proline (Pro), superoxide dismutase (SOD), and peroxidase (POD). However, ABA, PP333, and SA can mitigate the adverse effects of salt stress on these indicators. Especially, the 15 mg·L−1 ABA, 900 mg·L−1 PP333, and 120 mg·L−1 SA treatments had the best effect on alleviating salt stress, with significant increases in dry weight, root parameters, total Chl content, and the photosynthetic parameters of the P. talassica × P. euphratica seedlings, improving their photosynthetic characteristics; meanwhile, increased Pro content and enzyme activity and decreased MDA, H2O2, and O2− content protected the integrity of membrane system and enhanced the salt tolerance of the seedlings. SA resulted in a better ameliorative effect on salt stress compared to ABA and PP333.