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Woody plants faced multiple abiotic stresses in forest plantation that can influence their growth and development. Dalbergia odorifera T. Chen is a vulnerably endangered tree species, and references about its responses to abiotic stresses are very rare in literature. Furthermore, the mechanisms underlying the abiotic stress tolerance in plants induced by exogenous glycine betaine (GB) remains unclear. Indeed, the alternative oxidase (AOX) is one of the major components of antioxidant enzymatic machinery, and there are no studies that focused on the effects of GB on the amount of AOX protein in plants under drought or cold stresses. Thus, the aim of this study was to investigate the effect of exogenous GB on the phenotype, osmoprotectants, photosynthetic pigments, and antioxidant systems in D. odorifera under cold and drought stresses. The layout of the trial was a factorial experiment in a completely randomized design using two factors including abiotic stress (drought and cold) and GB. Moreover, the principal component analysis based on the measured parameters manifested how the selected ROS scavengers were dispersed throughout the treatments. The results showed huge beneficial impacts of GB on the phenotypic traits of D. odorifera; GB also influences positively the antioxidant machinery, photosynthetic pigments, redox-homeostasis, and water status in D. odorifera seedlings under both stresses. Moreover, exogenous GB affects more the AOX pathway in D. odorifera under cold stress than under drought stress.

Salinity has a huge negative impact on plant growth and development by increasing sodium ions accumulation and potassium ions loss that deeply disturbs the plant cell homeostasis and can lead to plant cell death. The imbalance between Na+ and K+ could be solved by applying potassium silicate (K2SiO3). The glycine betaine (GB) is well-known to play a crucial role against oxidative stress in plants by improving the antioxidant machinery. Thus, this research aimed to apply K2SiO3 (1 mM) and GB (10 mM) alone or in combination against 200 mM NaCl-induced damages in Dalbergia odorifera. The results showed a significant amelioration of negative effects of salt stress on the phenotypic traits, chlorophyll content, net photosynthetic rate, stomatal conductance, transpiration rate, and water use efficiency by applied substances. The contents of saccharides and proline were down-regulated by K2SiO3, GB, and K2SiO3-GB, whereas the proteins content was increased by these treatments. The contents of lipid peroxidation, superoxide anion, hydrogen peroxide were reduced by exogenous substances under stress. The activities of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase) and the accumulation of antioxidants (glutathione and ascorbate) were enhanced by exogenous substances. The K2SiO3-GB combination mostly showed better effects on antioxidant machinery compared to a single treatment.